language: Move l10n-related classes from /cache to /language

* Make the code easier to own and maintain by being part of a topical
  area of expertise rather than a generic catch-all bucket.
  This also helps Gerrit team dashboards, [[mw:Git/Reviewers]]
  subscription patterns, more meaningful component prefix for
  [[mw:Commit_message_guidelines]].

* Prep for possible CODEOWNERS adoption.

* Prep for T166010 to give us better PSR-4 namespaces that we won't
  have to break/migrate again shortly after for the above.

Bug: T225756
Change-Id: I62c701d574d2a79365b1c5350f92bef47770e3ce

19 files changed, 3835 insertions, 0 deletions

diff --git a/includes/language/Hook/LocalisationCacheRecacheFallbackHook.php b/includes/language/Hook/LocalisationCacheRecacheFallbackHook.php
new file mode 100644
index 000000000000..ca699108fa56
--- /dev/null
+++ b/includes/language/Hook/LocalisationCacheRecacheFallbackHook.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace MediaWiki\Hook;
+
+/**
+ * This is a hook handler interface, see docs/Hooks.md.
+ * Use the hook name "LocalisationCacheRecacheFallback" to register handlers implementing this interface.
+ *
+ * @stable to implement
+ * @ingroup Hooks
+ */
+interface LocalisationCacheRecacheFallbackHook {
+	/**
+	 * Called for each language when merging
+	 * fallback data into the cache.
+	 *
+	 * @since 1.35
+	 *
+	 * @param ?mixed $cache The LocalisationCache object
+	 * @param ?mixed $code language code
+	 * @param ?mixed &$alldata The localisation data from core and extensions. Note some keys may
+	 *   be omitted if they won't be merged into the final result.
+	 * @return bool|void True or no return value to continue or false to abort
+	 */
+	public function onLocalisationCacheRecacheFallback( $cache, $code, &$alldata );
+}
diff --git a/includes/language/Hook/LocalisationCacheRecacheHook.php b/includes/language/Hook/LocalisationCacheRecacheHook.php
new file mode 100644
index 000000000000..c489694f9907
--- /dev/null
+++ b/includes/language/Hook/LocalisationCacheRecacheHook.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace MediaWiki\Hook;
+
+/**
+ * This is a hook handler interface, see docs/Hooks.md.
+ * Use the hook name "LocalisationCacheRecache" to register handlers implementing this interface.
+ *
+ * @stable to implement
+ * @ingroup Hooks
+ */
+interface LocalisationCacheRecacheHook {
+	/**
+	 * Called when loading the localisation data into
+	 * cache.
+	 *
+	 * @since 1.35
+	 *
+	 * @param ?mixed $cache The LocalisationCache object
+	 * @param ?mixed $code language code
+	 * @param ?mixed &$alldata The localisation data from core and extensions
+	 * @param ?mixed $unused Used to be $purgeBlobs, removed in 1.34
+	 * @return bool|void True or no return value to continue or false to abort
+	 */
+	public function onLocalisationCacheRecache( $cache, $code, &$alldata, $unused );
+}
diff --git a/includes/language/Hook/MessageCacheReplaceHook.php b/includes/language/Hook/MessageCacheReplaceHook.php
new file mode 100644
index 000000000000..7d555320fd46
--- /dev/null
+++ b/includes/language/Hook/MessageCacheReplaceHook.php
@@ -0,0 +1,25 @@
+<?php
+
+namespace MediaWiki\Cache\Hook;
+
+use Title;
+
+/**
+ * This is a hook handler interface, see docs/Hooks.md.
+ * Use the hook name "MessageCacheReplace" to register handlers implementing this interface.
+ *
+ * @stable to implement
+ * @ingroup Hooks
+ */
+interface MessageCacheReplaceHook {
+	/**
+	 * This hook is called when a message page is changed. Use this hook to update caches.
+	 *
+	 * @since 1.35
+	 *
+	 * @param Title $title Name of the page changed
+	 * @param string $text New contents of the page
+	 * @return bool|void True or no return value to continue or false to abort
+	 */
+	public function onMessageCacheReplace( $title, $text );
+}
diff --git a/includes/language/Hook/MessageCache__getHook.php b/includes/language/Hook/MessageCache__getHook.php
new file mode 100644
index 000000000000..aad00a1af688
--- /dev/null
+++ b/includes/language/Hook/MessageCache__getHook.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace MediaWiki\Cache\Hook;
+
+// phpcs:disable Squiz.Classes.ValidClassName.NotCamelCaps
+/**
+ * This is a hook handler interface, see docs/Hooks.md.
+ * Use the hook name "MessageCache::get" to register handlers implementing this interface.
+ *
+ * @stable to implement
+ * @ingroup Hooks
+ */
+interface MessageCache__getHook {
+	/**
+	 * This hook is called when fetching a message. Use this hook to override the key
+	 * for customisations. Given and returned message key must be formatted with:
+	 * 1) the first letter in lower case according to the content language
+	 * 2) spaces replaced with underscores
+	 *
+	 * @since 1.35
+	 *
+	 * @param string &$key Message key
+	 * @return bool|void True or no return value to continue or false to abort
+	 */
+	public function onMessageCache__get( &$key );
+}
diff --git a/includes/language/Hook/MessagesPreLoadHook.php b/includes/language/Hook/MessagesPreLoadHook.php
new file mode 100644
index 000000000000..e020f1d2b3db
--- /dev/null
+++ b/includes/language/Hook/MessagesPreLoadHook.php
@@ -0,0 +1,24 @@
+<?php
+
+namespace MediaWiki\Cache\Hook;
+
+/**
+ * This is a hook handler interface, see docs/Hooks.md.
+ * Use the hook name "MessagesPreLoad" to register handlers implementing this interface.
+ *
+ * @stable to implement
+ * @ingroup Hooks
+ */
+interface MessagesPreLoadHook {
+	/**
+	 * This hook is called when loading a message from the database.
+	 *
+	 * @since 1.35
+	 *
+	 * @param string $title Title of the message
+	 * @param string &$message Message you want to define
+	 * @param string $code Language code
+	 * @return bool|void True or no return value to continue or false to abort
+	 */
+	public function onMessagesPreLoad( $title, &$message, $code );
+}
diff --git a/includes/language/LCStore.php b/includes/language/LCStore.php
new file mode 100644
index 000000000000..dbd996e537c7
--- /dev/null
+++ b/includes/language/LCStore.php
@@ -0,0 +1,66 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+
+/**
+ * Interface for the persistence layer of LocalisationCache.
+ *
+ * The persistence layer is two-level hierarchical cache. The first level
+ * is the language, the second level is the item or subitem.
+ *
+ * Since the data for a whole language is rebuilt in one operation, it needs
+ * to have a fast and atomic method for deleting or replacing all of the
+ * current data for a given language. The interface reflects this bulk update
+ * operation. Callers writing to the cache must first call startWrite(), then
+ * will call set() a couple of thousand times, then will call finishWrite()
+ * to commit the operation. When finishWrite() is called, the cache is
+ * expected to delete all data previously stored for that language.
+ *
+ * The values stored are PHP variables suitable for serialize(). Implementations
+ * of LCStore are responsible for serializing and unserializing.
+ */
+interface LCStore {
+
+	/**
+	 * Get a value.
+	 * @param string $code Language code
+	 * @param string $key Cache key
+	 */
+	public function get( $code, $key );
+
+	/**
+	 * Start a write transaction.
+	 * @param string $code Language code
+	 */
+	public function startWrite( $code );
+
+	/**
+	 * Finish a write transaction.
+	 */
+	public function finishWrite();
+
+	/**
+	 * Set a key to a given value. startWrite() must be called before this
+	 * is called, and finishWrite() must be called afterwards.
+	 * @param string $key
+	 * @param mixed $value
+	 */
+	public function set( $key, $value );
+
+}
diff --git a/includes/language/LCStoreCDB.php b/includes/language/LCStoreCDB.php
new file mode 100644
index 000000000000..e9cac7c08ba9
--- /dev/null
+++ b/includes/language/LCStoreCDB.php
@@ -0,0 +1,134 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+use Cdb\Exception as CdbException;
+use Cdb\Reader;
+use Cdb\Writer;
+
+/**
+ * LCStore implementation which stores data as a collection of CDB files.
+ *
+ * Profiling indicates that on Linux, this implementation outperforms MySQL if
+ * the directory is on a local filesystem and there is ample kernel cache
+ * space. The performance advantage is greater when the DBA extension is
+ * available than it is with the PHP port.
+ *
+ * See Cdb.php and https://cr.yp.to/cdb.html
+ */
+class LCStoreCDB implements LCStore {
+
+	/** @var Reader[]|false[] */
+	private $readers;
+
+	/** @var Writer|null */
+	private $writer;
+
+	/** @var string|null Current language code */
+	private $currentLang;
+
+	/** @var string Cache directory */
+	private $directory;
+
+	public function __construct( $conf = [] ) {
+		$this->directory = $conf['directory'];
+	}
+
+	public function get( $code, $key ) {
+		if ( !isset( $this->readers[$code] ) ) {
+			$fileName = $this->getFileName( $code );
+
+			$this->readers[$code] = false;
+			if ( is_file( $fileName ) ) {
+				try {
+					$this->readers[$code] = Reader::open( $fileName );
+				} catch ( CdbException $e ) {
+					wfDebug( __METHOD__ . ": unable to open cdb file for reading" );
+				}
+			}
+		}
+
+		if ( !$this->readers[$code] ) {
+			return null;
+		} else {
+			$value = false;
+			try {
+				$value = $this->readers[$code]->get( $key );
+			} catch ( CdbException $e ) {
+				wfDebug( __METHOD__ . ": \Cdb\Exception caught, error message was "
+					. $e->getMessage() );
+			}
+			if ( $value === false ) {
+				return null;
+			}
+
+			return unserialize( $value );
+		}
+	}
+
+	public function startWrite( $code ) {
+		if ( !is_dir( $this->directory ) && !wfMkdirParents( $this->directory, null, __METHOD__ ) ) {
+			throw new MWException( "Unable to create the localisation store " .
+				"directory \"{$this->directory}\"" );
+		}
+
+		// Close reader to stop permission errors on write
+		if ( !empty( $this->readers[$code] ) ) {
+			$this->readers[$code]->close();
+		}
+
+		try {
+			$this->writer = Writer::open( $this->getFileName( $code ) );
+		} catch ( CdbException $e ) {
+			throw new MWException( $e->getMessage() );
+		}
+		$this->currentLang = $code;
+	}
+
+	public function finishWrite() {
+		// Close the writer
+		try {
+			$this->writer->close();
+		} catch ( CdbException $e ) {
+			throw new MWException( $e->getMessage() );
+		}
+		$this->writer = null;
+		unset( $this->readers[$this->currentLang] );
+		$this->currentLang = null;
+	}
+
+	public function set( $key, $value ) {
+		if ( $this->writer === null ) {
+			throw new MWException( __CLASS__ . ': must call startWrite() before calling set()' );
+		}
+		try {
+			$this->writer->set( $key, serialize( $value ) );
+		} catch ( CdbException $e ) {
+			throw new MWException( $e->getMessage() );
+		}
+	}
+
+	protected function getFileName( $code ) {
+		if ( strval( $code ) === '' || strpos( $code, '/' ) !== false ) {
+			throw new MWException( __METHOD__ . ": Invalid language \"$code\"" );
+		}
+
+		return "{$this->directory}/l10n_cache-$code.cdb";
+	}
+
+}
diff --git a/includes/language/LCStoreDB.php b/includes/language/LCStoreDB.php
new file mode 100644
index 000000000000..d6a9866bb711
--- /dev/null
+++ b/includes/language/LCStoreDB.php
@@ -0,0 +1,147 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+
+use Wikimedia\Rdbms\Database;
+use Wikimedia\Rdbms\DBQueryError;
+use Wikimedia\Rdbms\IDatabase;
+use Wikimedia\ScopedCallback;
+
+/**
+ * LCStore implementation which uses the standard DB functions to store data.
+ * This will work on any MediaWiki installation.
+ */
+class LCStoreDB implements LCStore {
+	/** @var string|null Language code */
+	private $code;
+	/** @var array Server configuration map */
+	private $server;
+
+	/** @var array Rows buffered for insertion */
+	private $batch = [];
+
+	/** @var IDatabase|null */
+	private $dbw;
+	/** @var bool Whether a batch of writes were recently written */
+	private $writesDone = false;
+	/** @var bool Whether the DB is read-only or otherwise unavailable for writes */
+	private $readOnly = false;
+
+	public function __construct( $params ) {
+		$this->server = $params['server'] ?? [];
+	}
+
+	public function get( $code, $key ) {
+		if ( $this->server || $this->writesDone ) {
+			// If a server configuration map is specified, always used that connection
+			// for reads and writes. Otherwise, if writes occurred in finishWrite(), make
+			// sure those changes are always visible.
+			$db = $this->getWriteConnection();
+		} else {
+			$db = wfGetDB( DB_REPLICA );
+		}
+
+		$value = $db->selectField(
+			'l10n_cache',
+			'lc_value',
+			[ 'lc_lang' => $code, 'lc_key' => $key ],
+			__METHOD__
+		);
+
+		return ( $value !== false ) ? unserialize( $db->decodeBlob( $value ) ) : null;
+	}
+
+	public function startWrite( $code ) {
+		if ( $this->readOnly ) {
+			return;
+		} elseif ( !$code ) {
+			throw new MWException( __METHOD__ . ": Invalid language \"$code\"" );
+		}
+
+		$dbw = $this->getWriteConnection();
+		$this->readOnly = $dbw->isReadOnly();
+
+		$this->code = $code;
+		$this->batch = [];
+	}
+
+	public function finishWrite() {
+		if ( $this->readOnly ) {
+			return;
+		} elseif ( $this->code === null ) {
+			throw new MWException( __CLASS__ . ': must call startWrite() before finishWrite()' );
+		}
+
+		$scope = Profiler::instance()->getTransactionProfiler()->silenceForScope();
+		$dbw = $this->getWriteConnection();
+		$dbw->startAtomic( __METHOD__ );
+		try {
+			$dbw->delete( 'l10n_cache', [ 'lc_lang' => $this->code ], __METHOD__ );
+			foreach ( array_chunk( $this->batch, 500 ) as $rows ) {
+				$dbw->insert( 'l10n_cache', $rows, __METHOD__ );
+			}
+			$this->writesDone = true;
+		} catch ( DBQueryError $e ) {
+			if ( $dbw->wasReadOnlyError() ) {
+				$this->readOnly = true; // just avoid site down time
+			} else {
+				throw $e;
+			}
+		}
+		$dbw->endAtomic( __METHOD__ );
+		ScopedCallback::consume( $scope );
+
+		$this->code = null;
+		$this->batch = [];
+	}
+
+	public function set( $key, $value ) {
+		if ( $this->readOnly ) {
+			return;
+		} elseif ( $this->code === null ) {
+			throw new MWException( __CLASS__ . ': must call startWrite() before set()' );
+		}
+
+		$dbw = $this->getWriteConnection();
+
+		$this->batch[] = [
+			'lc_lang' => $this->code,
+			'lc_key' => $key,
+			'lc_value' => $dbw->encodeBlob( serialize( $value ) )
+		];
+	}
+
+	/**
+	 * @return IDatabase
+	 */
+	private function getWriteConnection() {
+		if ( !$this->dbw ) {
+			if ( $this->server ) {
+				$this->dbw = Database::factory( $this->server['type'], $this->server );
+				if ( !$this->dbw ) {
+					throw new MWException( __CLASS__ . ': failed to obtain a DB connection' );
+				}
+			} else {
+				$this->dbw = wfGetDB( DB_PRIMARY );
+			}
+		}
+
+		return $this->dbw;
+	}
+}
diff --git a/includes/language/LCStoreNull.php b/includes/language/LCStoreNull.php
new file mode 100644
index 000000000000..62f88ebf2d73
--- /dev/null
+++ b/includes/language/LCStoreNull.php
@@ -0,0 +1,39 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+
+/**
+ * Null store backend, used to avoid DB errors during install
+ */
+class LCStoreNull implements LCStore {
+
+	public function get( $code, $key ) {
+		return null;
+	}
+
+	public function startWrite( $code ) {
+	}
+
+	public function finishWrite() {
+	}
+
+	public function set( $key, $value ) {
+	}
+
+}
diff --git a/includes/language/LCStoreStaticArray.php b/includes/language/LCStoreStaticArray.php
new file mode 100644
index 000000000000..ca7c997d1bcd
--- /dev/null
+++ b/includes/language/LCStoreStaticArray.php
@@ -0,0 +1,166 @@
+<?php
+/**
+ *  Localisation cache storage based on PHP files and static arrays.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+
+use Wikimedia\StaticArrayWriter;
+
+/**
+ * @since 1.26
+ */
+class LCStoreStaticArray implements LCStore {
+	/** @var string|null Current language code. */
+	private $currentLang = null;
+
+	/** @var array Localisation data. */
+	private $data = [];
+
+	/** @var string|null File name. */
+	private $fname = null;
+
+	/** @var string Directory for cache files. */
+	private $directory;
+
+	public function __construct( $conf = [] ) {
+		$this->directory = $conf['directory'];
+	}
+
+	public function startWrite( $code ) {
+		if ( !is_dir( $this->directory ) && !wfMkdirParents( $this->directory, null, __METHOD__ ) ) {
+			throw new MWException( "Unable to create the localisation store " .
+				"directory \"{$this->directory}\"" );
+		}
+
+		$this->currentLang = $code;
+		$this->fname = $this->directory . '/' . $code . '.l10n.php';
+		$this->data[$code] = [];
+		if ( is_file( $this->fname ) ) {
+			$this->data[$code] = require $this->fname;
+		}
+	}
+
+	public function set( $key, $value ) {
+		$this->data[$this->currentLang][$key] = self::encode( $value );
+	}
+
+	/**
+	 * Determine whether this array contains only scalar values.
+	 *
+	 * @param array $arr
+	 * @return bool
+	 */
+	private static function isValueArray( array $arr ) {
+		foreach ( $arr as $key => $value ) {
+			if ( is_scalar( $value )
+				|| $value === null
+				|| ( is_array( $value ) && self::isValueArray( $value ) )
+			) {
+				continue;
+			}
+			return false;
+		}
+		return true;
+	}
+
+	/**
+	 * Encodes a value into an array format
+	 *
+	 * @param mixed $value
+	 * @return array|mixed
+	 * @throws RuntimeException
+	 */
+	public static function encode( $value ) {
+		if ( is_array( $value ) && self::isValueArray( $value ) ) {
+			// Type: scalar [v]alue.
+			// Optimization: Write large arrays as one value to avoid recursive decoding cost.
+			return [ 'v', $value ];
+		}
+		if ( is_array( $value ) || is_object( $value ) ) {
+			// Type: [s]serialized.
+			// Optimization: Avoid recursive decoding cost. Write arrays with an objects
+			// as one serialised value.
+			return [ 's', serialize( $value ) ];
+		}
+		if ( is_scalar( $value ) || $value === null ) {
+			// Optimization: Reduce file size by not wrapping scalar values.
+			return $value;
+		}
+
+		throw new RuntimeException( 'Cannot encode ' . var_export( $value, true ) );
+	}
+
+	/**
+	 * Decode something that was encoded with encode
+	 *
+	 * @param mixed $encoded
+	 * @return array|mixed
+	 * @throws RuntimeException
+	 */
+	public static function decode( $encoded ) {
+		if ( !is_array( $encoded ) ) {
+			// Unwrapped scalar value
+			return $encoded;
+		}
+
+		list( $type, $data ) = $encoded;
+
+		switch ( $type ) {
+			case 'v':
+				// Value array (1.35+) or unwrapped scalar value (1.32 and earlier)
+				return $data;
+			case 's':
+				return unserialize( $data );
+			case 'a':
+				// Support: MediaWiki 1.34 and earlier (older file format)
+				return array_map( [ __CLASS__, 'decode' ], $data );
+			default:
+				throw new RuntimeException(
+					'Unable to decode ' . var_export( $encoded, true ) );
+		}
+	}
+
+	public function finishWrite() {
+		$writer = new StaticArrayWriter();
+		$out = $writer->create(
+			$this->data[$this->currentLang],
+			'Generated by LCStoreStaticArray.php -- do not edit!'
+		);
+		file_put_contents( $this->fname, $out );
+		// Release the data to manage the memory in rebuildLocalisationCache
+		unset( $this->data[$this->currentLang] );
+		$this->currentLang = null;
+		$this->fname = null;
+	}
+
+	public function get( $code, $key ) {
+		if ( !array_key_exists( $code, $this->data ) ) {
+			$fname = $this->directory . '/' . $code . '.l10n.php';
+			if ( !is_file( $fname ) ) {
+				return null;
+			}
+			$this->data[$code] = require $fname;
+		}
+		$data = $this->data[$code];
+		if ( array_key_exists( $key, $data ) ) {
+			return self::decode( $data[$key] );
+		}
+		return null;
+	}
+}
diff --git a/includes/language/LocalisationCache.php b/includes/language/LocalisationCache.php
new file mode 100644
index 000000000000..92615d9867d9
--- /dev/null
+++ b/includes/language/LocalisationCache.php
@@ -0,0 +1,1125 @@
+<?php
+/**
+ * Cache of the contents of localisation files.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+
+use CLDRPluralRuleParser\Error as CLDRPluralRuleError;
+use CLDRPluralRuleParser\Evaluator;
+use MediaWiki\Config\ServiceOptions;
+use MediaWiki\HookContainer\HookContainer;
+use MediaWiki\HookContainer\HookRunner;
+use MediaWiki\Languages\LanguageNameUtils;
+use MediaWiki\MainConfigNames;
+use Psr\Log\LoggerInterface;
+
+/**
+ * Class for caching the contents of localisation files, Messages*.php
+ * and *.i18n.php.
+ *
+ * An instance of this class is available using MediaWikiServices.
+ *
+ * The values retrieved from here are merged, containing items from extension
+ * files, core messages files and the language fallback sequence (e.g. zh-cn ->
+ * zh-hans -> en ). Some common errors are corrected, for example namespace
+ * names with spaces instead of underscores, but heavyweight processing, such
+ * as grammatical transformation, is done by the caller.
+ */
+class LocalisationCache {
+	public const VERSION = 4;
+
+	/** @var ServiceOptions */
+	private $options;
+
+	/**
+	 * True if recaching should only be done on an explicit call to recache().
+	 * Setting this reduces the overhead of cache freshness checking, which
+	 * requires doing a stat() for every extension i18n file.
+	 */
+	private $manualRecache;
+
+	/**
+	 * The cache data. 3-d array, where the first key is the language code,
+	 * the second key is the item key e.g. 'messages', and the third key is
+	 * an item specific subkey index. Some items are not arrays and so for those
+	 * items, there are no subkeys.
+	 */
+	protected $data = [];
+
+	/**
+	 * The persistent store object. An instance of LCStore.
+	 *
+	 * @var LCStore
+	 */
+	private $store;
+
+	/**
+	 * @var LoggerInterface
+	 */
+	private $logger;
+
+	/** @var HookRunner */
+	private $hookRunner;
+
+	/** @var callable[] See comment for parameter in constructor */
+	private $clearStoreCallbacks;
+
+	/** @var LanguageNameUtils */
+	private $langNameUtils;
+
+	/**
+	 * A 2-d associative array, code/key, where presence indicates that the item
+	 * is loaded. Value arbitrary.
+	 *
+	 * For split items, if set, this indicates that all of the subitems have been
+	 * loaded.
+	 *
+	 */
+	private $loadedItems = [];
+
+	/**
+	 * A 3-d associative array, code/key/subkey, where presence indicates that
+	 * the subitem is loaded. Only used for the split items, i.e. messages.
+	 */
+	private $loadedSubitems = [];
+
+	/**
+	 * An array where presence of a key indicates that that language has been
+	 * initialised. Initialisation includes checking for cache expiry and doing
+	 * any necessary updates.
+	 */
+	private $initialisedLangs = [];
+
+	/**
+	 * An array mapping non-existent pseudo-languages to fallback languages. This
+	 * is filled by initShallowFallback() when data is requested from a language
+	 * that lacks a Messages*.php file.
+	 */
+	private $shallowFallbacks = [];
+
+	/**
+	 * An array where the keys are codes that have been recached by this instance.
+	 */
+	private $recachedLangs = [];
+
+	/**
+	 * All item keys
+	 */
+	public static $allKeys = [
+		'fallback', 'namespaceNames', 'bookstoreList',
+		'magicWords', 'messages', 'rtl',
+		'digitTransformTable', 'separatorTransformTable',
+		'minimumGroupingDigits', 'fallback8bitEncoding',
+		'linkPrefixExtension', 'linkTrail', 'linkPrefixCharset',
+		'namespaceAliases', 'dateFormats', 'datePreferences',
+		'datePreferenceMigrationMap', 'defaultDateFormat',
+		'specialPageAliases', 'imageFiles', 'preloadedMessages',
+		'namespaceGenderAliases', 'digitGroupingPattern', 'pluralRules',
+		'pluralRuleTypes', 'compiledPluralRules',
+	];
+
+	/**
+	 * Keys for items which consist of associative arrays, which may be merged
+	 * by a fallback sequence.
+	 */
+	public static $mergeableMapKeys = [ 'messages', 'namespaceNames',
+		'namespaceAliases', 'dateFormats', 'imageFiles', 'preloadedMessages'
+	];
+
+	/**
+	 * Keys for items which are a numbered array.
+	 */
+	public static $mergeableListKeys = [];
+
+	/**
+	 * Keys for items which contain an array of arrays of equivalent aliases
+	 * for each subitem. The aliases may be merged by a fallback sequence.
+	 */
+	public static $mergeableAliasListKeys = [ 'specialPageAliases' ];
+
+	/**
+	 * Keys for items which contain an associative array, and may be merged if
+	 * the primary value contains the special array key "inherit". That array
+	 * key is removed after the first merge.
+	 */
+	public static $optionalMergeKeys = [ 'bookstoreList' ];
+
+	/**
+	 * Keys for items that are formatted like $magicWords
+	 */
+	public static $magicWordKeys = [ 'magicWords' ];
+
+	/**
+	 * Keys for items where the subitems are stored in the backend separately.
+	 */
+	public static $splitKeys = [ 'messages' ];
+
+	/**
+	 * Keys which are loaded automatically by initLanguage()
+	 */
+	public static $preloadedKeys = [ 'dateFormats', 'namespaceNames' ];
+
+	/**
+	 * Associative array of cached plural rules. The key is the language code,
+	 * the value is an array of plural rules for that language.
+	 */
+	private $pluralRules = null;
+
+	/**
+	 * Associative array of cached plural rule types. The key is the language
+	 * code, the value is an array of plural rule types for that language. For
+	 * example, $pluralRuleTypes['ar'] = ['zero', 'one', 'two', 'few', 'many'].
+	 * The index for each rule type matches the index for the rule in
+	 * $pluralRules, thus allowing correlation between the two. The reason we
+	 * don't just use the type names as the keys in $pluralRules is because
+	 * Language::convertPlural applies the rules based on numeric order (or
+	 * explicit numeric parameter), not based on the name of the rule type. For
+	 * example, {{plural:count|wordform1|wordform2|wordform3}}, rather than
+	 * {{plural:count|one=wordform1|two=wordform2|many=wordform3}}.
+	 */
+	private $pluralRuleTypes = null;
+
+	private $mergeableKeys = null;
+
+	/**
+	 * Return a suitable LCStore as specified by the given configuration.
+	 *
+	 * @since 1.34
+	 * @param array $conf In the format of $wgLocalisationCacheConf
+	 * @param string|false|null $fallbackCacheDir In case 'storeDirectory' isn't specified
+	 * @return LCStore
+	 */
+	public static function getStoreFromConf( array $conf, $fallbackCacheDir ): LCStore {
+		$storeArg = [];
+		$storeArg['directory'] =
+			$conf['storeDirectory'] ?: $fallbackCacheDir;
+
+		if ( !empty( $conf['storeClass'] ) ) {
+			$storeClass = $conf['storeClass'];
+		} elseif ( $conf['store'] === 'files' || $conf['store'] === 'file' ||
+			( $conf['store'] === 'detect' && $storeArg['directory'] )
+		) {
+			$storeClass = LCStoreCDB::class;
+		} elseif ( $conf['store'] === 'db' || $conf['store'] === 'detect' ) {
+			$storeClass = LCStoreDB::class;
+			$storeArg['server'] = $conf['storeServer'] ?? [];
+		} elseif ( $conf['store'] === 'array' ) {
+			$storeClass = LCStoreStaticArray::class;
+		} else {
+			throw new MWException(
+				'Please set $wgLocalisationCacheConf[\'store\'] to something sensible.'
+			);
+		}
+
+		return new $storeClass( $storeArg );
+	}
+
+	/**
+	 * @internal For use by ServiceWiring
+	 */
+	public const CONSTRUCTOR_OPTIONS = [
+		// True to treat all files as expired until they are regenerated by this object.
+		'forceRecache',
+		'manualRecache',
+		MainConfigNames::ExtensionMessagesFiles,
+		MainConfigNames::MessagesDirs,
+	];
+
+	/**
+	 * For constructor parameters, see the documentation for the LocalisationCacheConf
+	 * setting in docs/Configuration.md.
+	 *
+	 * Do not construct this directly. Use MediaWikiServices.
+	 *
+	 * @param ServiceOptions $options
+	 * @param LCStore $store What backend to use for storage
+	 * @param LoggerInterface $logger
+	 * @param callable[] $clearStoreCallbacks To be called whenever the cache is cleared. Can be
+	 *   used to clear other caches that depend on this one, such as ResourceLoader's
+	 *   MessageBlobStore.
+	 * @param LanguageNameUtils $langNameUtils
+	 * @param HookContainer $hookContainer
+	 * @throws MWException
+	 */
+	public function __construct(
+		ServiceOptions $options,
+		LCStore $store,
+		LoggerInterface $logger,
+		array $clearStoreCallbacks,
+		LanguageNameUtils $langNameUtils,
+		HookContainer $hookContainer
+	) {
+		$options->assertRequiredOptions( self::CONSTRUCTOR_OPTIONS );
+
+		$this->options = $options;
+		$this->store = $store;
+		$this->logger = $logger;
+		$this->clearStoreCallbacks = $clearStoreCallbacks;
+		$this->langNameUtils = $langNameUtils;
+		$this->hookRunner = new HookRunner( $hookContainer );
+
+		// Keep this separate from $this->options so it can be mutable
+		$this->manualRecache = $options->get( 'manualRecache' );
+	}
+
+	/**
+	 * Returns true if the given key is mergeable, that is, if it is an associative
+	 * array which can be merged through a fallback sequence.
+	 * @param string $key
+	 * @return bool
+	 */
+	public function isMergeableKey( $key ) {
+		if ( $this->mergeableKeys === null ) {
+			$this->mergeableKeys = array_fill_keys( array_merge(
+				self::$mergeableMapKeys,
+				self::$mergeableListKeys,
+				self::$mergeableAliasListKeys,
+				self::$optionalMergeKeys,
+				self::$magicWordKeys
+			), true );
+		}
+
+		return isset( $this->mergeableKeys[$key] );
+	}
+
+	/**
+	 * Get a cache item.
+	 *
+	 * Warning: this may be slow for split items (messages), since it will
+	 * need to fetch all of the subitems from the cache individually.
+	 * @param string $code
+	 * @param string $key
+	 * @return mixed
+	 */
+	public function getItem( $code, $key ) {
+		if ( !isset( $this->loadedItems[$code][$key] ) ) {
+			$this->loadItem( $code, $key );
+		}
+
+		if ( $key === 'fallback' && isset( $this->shallowFallbacks[$code] ) ) {
+			return $this->shallowFallbacks[$code];
+		}
+
+		return $this->data[$code][$key];
+	}
+
+	/**
+	 * Get a subitem, for instance a single message for a given language.
+	 * @param string $code
+	 * @param string $key
+	 * @param string $subkey
+	 * @return mixed|null
+	 */
+	public function getSubitem( $code, $key, $subkey ) {
+		if ( !isset( $this->loadedSubitems[$code][$key][$subkey] ) &&
+			!isset( $this->loadedItems[$code][$key] )
+		) {
+			$this->loadSubitem( $code, $key, $subkey );
+		}
+
+		return $this->data[$code][$key][$subkey] ?? null;
+	}
+
+	/**
+	 * Get the list of subitem keys for a given item.
+	 *
+	 * This is faster than array_keys($lc->getItem(...)) for the items listed in
+	 * self::$splitKeys.
+	 *
+	 * Will return null if the item is not found, or false if the item is not an
+	 * array.
+	 * @param string $code
+	 * @param string $key
+	 * @return bool|null|string|string[]
+	 */
+	public function getSubitemList( $code, $key ) {
+		if ( in_array( $key, self::$splitKeys ) ) {
+			return $this->getSubitem( $code, 'list', $key );
+		} else {
+			$item = $this->getItem( $code, $key );
+			if ( is_array( $item ) ) {
+				return array_keys( $item );
+			} else {
+				return false;
+			}
+		}
+	}
+
+	/**
+	 * Load an item into the cache.
+	 * @param string $code
+	 * @param string $key
+	 */
+	protected function loadItem( $code, $key ) {
+		if ( !isset( $this->initialisedLangs[$code] ) ) {
+			$this->initLanguage( $code );
+		}
+
+		// Check to see if initLanguage() loaded it for us
+		if ( isset( $this->loadedItems[$code][$key] ) ) {
+			return;
+		}
+
+		if ( isset( $this->shallowFallbacks[$code] ) ) {
+			$this->loadItem( $this->shallowFallbacks[$code], $key );
+
+			return;
+		}
+
+		if ( in_array( $key, self::$splitKeys ) ) {
+			$subkeyList = $this->getSubitem( $code, 'list', $key );
+			foreach ( $subkeyList as $subkey ) {
+				if ( isset( $this->data[$code][$key][$subkey] ) ) {
+					continue;
+				}
+				$this->data[$code][$key][$subkey] = $this->getSubitem( $code, $key, $subkey );
+			}
+		} else {
+			$this->data[$code][$key] = $this->store->get( $code, $key );
+		}
+
+		$this->loadedItems[$code][$key] = true;
+	}
+
+	/**
+	 * Load a subitem into the cache
+	 * @param string $code
+	 * @param string $key
+	 * @param string $subkey
+	 */
+	protected function loadSubitem( $code, $key, $subkey ) {
+		if ( !in_array( $key, self::$splitKeys ) ) {
+			$this->loadItem( $code, $key );
+
+			return;
+		}
+
+		if ( !isset( $this->initialisedLangs[$code] ) ) {
+			$this->initLanguage( $code );
+		}
+
+		// Check to see if initLanguage() loaded it for us
+		if ( isset( $this->loadedItems[$code][$key] ) ||
+			isset( $this->loadedSubitems[$code][$key][$subkey] )
+		) {
+			return;
+		}
+
+		if ( isset( $this->shallowFallbacks[$code] ) ) {
+			$this->loadSubitem( $this->shallowFallbacks[$code], $key, $subkey );
+
+			return;
+		}
+
+		$value = $this->store->get( $code, "$key:$subkey" );
+		$this->data[$code][$key][$subkey] = $value;
+		$this->loadedSubitems[$code][$key][$subkey] = true;
+	}
+
+	/**
+	 * Returns true if the cache identified by $code is missing or expired.
+	 *
+	 * @param string $code
+	 *
+	 * @return bool
+	 */
+	public function isExpired( $code ) {
+		if ( $this->options->get( 'forceRecache' ) && !isset( $this->recachedLangs[$code] ) ) {
+			$this->logger->debug( __METHOD__ . "($code): forced reload" );
+
+			return true;
+		}
+
+		$deps = $this->store->get( $code, 'deps' );
+		$keys = $this->store->get( $code, 'list' );
+		$preload = $this->store->get( $code, 'preload' );
+		// Different keys may expire separately for some stores
+		if ( $deps === null || $keys === null || $preload === null ) {
+			$this->logger->debug( __METHOD__ . "($code): cache missing, need to make one" );
+
+			return true;
+		}
+
+		foreach ( $deps as $dep ) {
+			// Because we're unserializing stuff from cache, we
+			// could receive objects of classes that don't exist
+			// anymore (e.g. uninstalled extensions)
+			// When this happens, always expire the cache
+			if ( !$dep instanceof CacheDependency || $dep->isExpired() ) {
+				$this->logger->debug( __METHOD__ . "($code): cache for $code expired due to " .
+					get_class( $dep ) );
+
+				return true;
+			}
+		}
+
+		return false;
+	}
+
+	/**
+	 * Initialise a language in this object. Rebuild the cache if necessary.
+	 * @param string $code
+	 * @throws MWException
+	 */
+	protected function initLanguage( $code ) {
+		if ( isset( $this->initialisedLangs[$code] ) ) {
+			return;
+		}
+
+		$this->initialisedLangs[$code] = true;
+
+		# If the code is of the wrong form for a Messages*.php file, do a shallow fallback
+		if ( !$this->langNameUtils->isValidBuiltInCode( $code ) ) {
+			$this->initShallowFallback( $code, 'en' );
+
+			return;
+		}
+
+		# Recache the data if necessary
+		if ( !$this->manualRecache && $this->isExpired( $code ) ) {
+			if ( $this->langNameUtils->isSupportedLanguage( $code ) ) {
+				$this->recache( $code );
+			} elseif ( $code === 'en' ) {
+				throw new MWException( 'MessagesEn.php is missing.' );
+			} else {
+				$this->initShallowFallback( $code, 'en' );
+			}
+
+			return;
+		}
+
+		# Preload some stuff
+		$preload = $this->getItem( $code, 'preload' );
+		if ( $preload === null ) {
+			if ( $this->manualRecache ) {
+				// No Messages*.php file. Do shallow fallback to en.
+				if ( $code === 'en' ) {
+					throw new MWException( 'No localisation cache found for English. ' .
+						'Please run maintenance/rebuildLocalisationCache.php.' );
+				}
+				$this->initShallowFallback( $code, 'en' );
+
+				return;
+			} else {
+				throw new MWException( 'Invalid or missing localisation cache.' );
+			}
+		}
+		$this->data[$code] = $preload;
+		foreach ( $preload as $key => $item ) {
+			if ( in_array( $key, self::$splitKeys ) ) {
+				foreach ( $item as $subkey => $subitem ) {
+					$this->loadedSubitems[$code][$key][$subkey] = true;
+				}
+			} else {
+				$this->loadedItems[$code][$key] = true;
+			}
+		}
+	}
+
+	/**
+	 * Create a fallback from one language to another, without creating a
+	 * complete persistent cache.
+	 * @param string $primaryCode
+	 * @param string $fallbackCode
+	 */
+	public function initShallowFallback( $primaryCode, $fallbackCode ) {
+		$this->data[$primaryCode] =& $this->data[$fallbackCode];
+		$this->loadedItems[$primaryCode] =& $this->loadedItems[$fallbackCode];
+		$this->loadedSubitems[$primaryCode] =& $this->loadedSubitems[$fallbackCode];
+		$this->shallowFallbacks[$primaryCode] = $fallbackCode;
+	}
+
+	/**
+	 * Read a PHP file containing localisation data.
+	 * @param string $_fileName
+	 * @param string $_fileType
+	 * @throws MWException
+	 * @return array
+	 */
+	protected function readPHPFile( $_fileName, $_fileType ) {
+		include $_fileName;
+
+		$data = [];
+		if ( $_fileType == 'core' || $_fileType == 'extension' ) {
+			foreach ( self::$allKeys as $key ) {
+				// Not all keys are set in language files, so
+				// check they exist first
+				if ( isset( $$key ) ) {
+					$data[$key] = $$key;
+				}
+			}
+		} elseif ( $_fileType == 'aliases' ) {
+			// @phan-suppress-next-line PhanImpossibleCondition May be set in included file
+			if ( isset( $aliases ) ) {
+				$data['aliases'] = $aliases;
+			}
+		} else {
+			throw new MWException( __METHOD__ . ": Invalid file type: $_fileType" );
+		}
+
+		return $data;
+	}
+
+	/**
+	 * Read a JSON file containing localisation messages.
+	 * @param string $fileName Name of file to read
+	 * @throws MWException If there is a syntax error in the JSON file
+	 * @return array Array with a 'messages' key, or empty array if the file doesn't exist
+	 */
+	public function readJSONFile( $fileName ) {
+		if ( !is_readable( $fileName ) ) {
+			return [];
+		}
+
+		$json = file_get_contents( $fileName );
+		if ( $json === false ) {
+			return [];
+		}
+
+		$data = FormatJson::decode( $json, true );
+		if ( $data === null ) {
+			throw new MWException( __METHOD__ . ": Invalid JSON file: $fileName" );
+		}
+
+		// Remove keys starting with '@', they're reserved for metadata and non-message data
+		foreach ( $data as $key => $unused ) {
+			if ( $key === '' || $key[0] === '@' ) {
+				unset( $data[$key] );
+			}
+		}
+
+		// The JSON format only supports messages, none of the other variables, so wrap the data
+		return [ 'messages' => $data ];
+	}
+
+	/**
+	 * Get the compiled plural rules for a given language from the XML files.
+	 * @since 1.20
+	 * @param string $code
+	 * @return array|null
+	 */
+	public function getCompiledPluralRules( $code ) {
+		$rules = $this->getPluralRules( $code );
+		if ( $rules === null ) {
+			return null;
+		}
+		try {
+			$compiledRules = Evaluator::compile( $rules );
+		} catch ( CLDRPluralRuleError $e ) {
+			$this->logger->debug( $e->getMessage() );
+
+			return [];
+		}
+
+		return $compiledRules;
+	}
+
+	/**
+	 * Get the plural rules for a given language from the XML files.
+	 * Cached.
+	 * @since 1.20
+	 * @param string $code
+	 * @return array|null
+	 */
+	public function getPluralRules( $code ) {
+		if ( $this->pluralRules === null ) {
+			$this->loadPluralFiles();
+		}
+		return $this->pluralRules[$code] ?? null;
+	}
+
+	/**
+	 * Get the plural rule types for a given language from the XML files.
+	 * Cached.
+	 * @since 1.22
+	 * @param string $code
+	 * @return array|null
+	 */
+	public function getPluralRuleTypes( $code ) {
+		if ( $this->pluralRuleTypes === null ) {
+			$this->loadPluralFiles();
+		}
+		return $this->pluralRuleTypes[$code] ?? null;
+	}
+
+	/**
+	 * Load the plural XML files.
+	 */
+	protected function loadPluralFiles() {
+		foreach ( $this->getPluralFiles() as $fileName ) {
+			$this->loadPluralFile( $fileName );
+		}
+	}
+
+	private function getPluralFiles(): array {
+		global $IP;
+		return [
+			// Load CLDR plural rules
+			"$IP/languages/data/plurals.xml",
+			// Override or extend with MW-specific rules
+			"$IP/languages/data/plurals-mediawiki.xml",
+		];
+	}
+
+	/**
+	 * Load a plural XML file with the given filename, compile the relevant
+	 * rules, and save the compiled rules in a process-local cache.
+	 *
+	 * @param string $fileName
+	 * @throws MWException
+	 */
+	protected function loadPluralFile( $fileName ) {
+		// Use file_get_contents instead of DOMDocument::load (T58439)
+		$xml = file_get_contents( $fileName );
+		if ( !$xml ) {
+			throw new MWException( "Unable to read plurals file $fileName" );
+		}
+		$doc = new DOMDocument;
+		$doc->loadXML( $xml );
+		$rulesets = $doc->getElementsByTagName( "pluralRules" );
+		foreach ( $rulesets as $ruleset ) {
+			$codes = $ruleset->getAttribute( 'locales' );
+			$rules = [];
+			$ruleTypes = [];
+			$ruleElements = $ruleset->getElementsByTagName( "pluralRule" );
+			foreach ( $ruleElements as $elt ) {
+				$ruleType = $elt->getAttribute( 'count' );
+				if ( $ruleType === 'other' ) {
+					// Don't record "other" rules, which have an empty condition
+					continue;
+				}
+				$rules[] = $elt->nodeValue;
+				$ruleTypes[] = $ruleType;
+			}
+			foreach ( explode( ' ', $codes ) as $code ) {
+				$this->pluralRules[$code] = $rules;
+				$this->pluralRuleTypes[$code] = $ruleTypes;
+			}
+		}
+	}
+
+	/**
+	 * Read the data from the source files for a given language, and register
+	 * the relevant dependencies in the $deps array. If the localisation
+	 * exists, the data array is returned, otherwise false is returned.
+	 *
+	 * @param string $code
+	 * @param array &$deps
+	 * @return array
+	 */
+	protected function readSourceFilesAndRegisterDeps( $code, &$deps ) {
+		// This reads in the PHP i18n file with non-messages l10n data
+		$fileName = $this->langNameUtils->getMessagesFileName( $code );
+		if ( !is_file( $fileName ) ) {
+			$data = [];
+		} else {
+			$deps[] = new FileDependency( $fileName );
+			$data = $this->readPHPFile( $fileName, 'core' );
+		}
+
+		// Load CLDR plural rules for JavaScript
+		$data['pluralRules'] = $this->getPluralRules( $code );
+		// And for PHP
+		$data['compiledPluralRules'] = $this->getCompiledPluralRules( $code );
+		// Load plural rule types
+		$data['pluralRuleTypes'] = $this->getPluralRuleTypes( $code );
+
+		foreach ( $this->getPluralFiles() as $fileName ) {
+			$deps[] = new FileDependency( $fileName );
+		}
+
+		return $data;
+	}
+
+	/**
+	 * Merge two localisation values, a primary and a fallback, overwriting the
+	 * primary value in place.
+	 * @param string $key
+	 * @param mixed &$value
+	 * @param mixed $fallbackValue
+	 */
+	protected function mergeItem( $key, &$value, $fallbackValue ) {
+		if ( $value !== null ) {
+			if ( $fallbackValue !== null ) {
+				if ( in_array( $key, self::$mergeableMapKeys ) ) {
+					$value += $fallbackValue;
+				} elseif ( in_array( $key, self::$mergeableListKeys ) ) {
+					$value = array_unique( array_merge( $fallbackValue, $value ) );
+				} elseif ( in_array( $key, self::$mergeableAliasListKeys ) ) {
+					$value = array_merge_recursive( $value, $fallbackValue );
+				} elseif ( in_array( $key, self::$optionalMergeKeys ) ) {
+					if ( !empty( $value['inherit'] ) ) {
+						$value = array_merge( $fallbackValue, $value );
+					}
+
+					unset( $value['inherit'] );
+				} elseif ( in_array( $key, self::$magicWordKeys ) ) {
+					$this->mergeMagicWords( $value, $fallbackValue );
+				}
+			}
+		} else {
+			$value = $fallbackValue;
+		}
+	}
+
+	/**
+	 * @param mixed &$value
+	 * @param mixed $fallbackValue
+	 */
+	protected function mergeMagicWords( &$value, $fallbackValue ) {
+		foreach ( $fallbackValue as $magicName => $fallbackInfo ) {
+			if ( !isset( $value[$magicName] ) ) {
+				$value[$magicName] = $fallbackInfo;
+			} else {
+				$oldSynonyms = array_slice( $fallbackInfo, 1 );
+				$newSynonyms = array_slice( $value[$magicName], 1 );
+				$synonyms = array_values( array_unique( array_merge(
+					$newSynonyms, $oldSynonyms ) ) );
+				$value[$magicName] = array_merge( [ $fallbackInfo[0] ], $synonyms );
+			}
+		}
+	}
+
+	/**
+	 * Given an array mapping language code to localisation value, such as is
+	 * found in extension *.i18n.php files, iterate through a fallback sequence
+	 * to merge the given data with an existing primary value.
+	 *
+	 * Returns true if any data from the extension array was used, false
+	 * otherwise.
+	 * @param array $codeSequence
+	 * @param string $key
+	 * @param mixed &$value
+	 * @param mixed $fallbackValue
+	 * @return bool
+	 */
+	protected function mergeExtensionItem( $codeSequence, $key, &$value, $fallbackValue ) {
+		$used = false;
+		foreach ( $codeSequence as $code ) {
+			if ( isset( $fallbackValue[$code] ) ) {
+				$this->mergeItem( $key, $value, $fallbackValue[$code] );
+				$used = true;
+			}
+		}
+
+		return $used;
+	}
+
+	/**
+	 * Gets the combined list of messages dirs from
+	 * core and extensions
+	 *
+	 * @since 1.25
+	 * @return array
+	 */
+	public function getMessagesDirs() {
+		global $IP;
+
+		return [
+			'core' => "$IP/languages/i18n",
+			'exif' => "$IP/languages/i18n/exif",
+			'api' => "$IP/includes/api/i18n",
+			'rest' => "$IP/includes/Rest/i18n",
+			'oojs-ui' => "$IP/resources/lib/ooui/i18n",
+			'paramvalidator' => "$IP/includes/libs/ParamValidator/i18n",
+		] + $this->options->get( MainConfigNames::MessagesDirs );
+	}
+
+	/**
+	 * Load localisation data for a given language for both core and extensions
+	 * and save it to the persistent cache store and the process cache
+	 * @param string $code
+	 * @throws MWException
+	 */
+	public function recache( $code ) {
+		if ( !$code ) {
+			throw new MWException( "Invalid language code requested" );
+		}
+		$this->recachedLangs[ $code ] = true;
+
+		# Initial values
+		$initialData = array_fill_keys( self::$allKeys, null );
+		$coreData = $initialData;
+		$deps = [];
+
+		# Load the primary localisation from the source file
+		$data = $this->readSourceFilesAndRegisterDeps( $code, $deps );
+		$this->logger->debug( __METHOD__ . ": got localisation for $code from source" );
+
+		# Merge primary localisation
+		foreach ( $data as $key => $value ) {
+			$this->mergeItem( $key, $coreData[ $key ], $value );
+		}
+
+		# Fill in the fallback if it's not there already
+		// @phan-suppress-next-line PhanSuspiciousValueComparison
+		if ( ( $coreData['fallback'] === null || $coreData['fallback'] === false ) && $code === 'en' ) {
+			$coreData['fallback'] = false;
+			$coreData['originalFallbackSequence'] = $coreData['fallbackSequence'] = [];
+		} else {
+			if ( $coreData['fallback'] !== null ) {
+				$coreData['fallbackSequence'] = array_map( 'trim', explode( ',', $coreData['fallback'] ) );
+			} else {
+				$coreData['fallbackSequence'] = [];
+			}
+			$len = count( $coreData['fallbackSequence'] );
+
+			# Before we add the 'en' fallback for messages, keep a copy of
+			# the original fallback sequence
+			$coreData['originalFallbackSequence'] = $coreData['fallbackSequence'];
+
+			# Ensure that the sequence ends at 'en' for messages
+			if ( !$len || $coreData['fallbackSequence'][$len - 1] !== 'en' ) {
+				$coreData['fallbackSequence'][] = 'en';
+			}
+		}
+
+		$codeSequence = array_merge( [ $code ], $coreData['fallbackSequence'] );
+		$messageDirs = $this->getMessagesDirs();
+
+		# Load non-JSON localisation data for extensions
+		$extensionData = array_fill_keys( $codeSequence, $initialData );
+		foreach ( $this->options->get( MainConfigNames::ExtensionMessagesFiles ) as $extension => $fileName ) {
+			if ( isset( $messageDirs[$extension] ) ) {
+				# This extension has JSON message data; skip the PHP shim
+				continue;
+			}
+
+			$data = $this->readPHPFile( $fileName, 'extension' );
+			$used = false;
+
+			foreach ( $data as $key => $item ) {
+				foreach ( $codeSequence as $csCode ) {
+					if ( isset( $item[$csCode] ) ) {
+						$this->mergeItem( $key, $extensionData[$csCode][$key], $item[$csCode] );
+						$used = true;
+					}
+				}
+			}
+
+			if ( $used ) {
+				$deps[] = new FileDependency( $fileName );
+			}
+		}
+
+		# Load the localisation data for each fallback, then merge it into the full array
+		$allData = $initialData;
+		foreach ( $codeSequence as $csCode ) {
+			$csData = $initialData;
+
+			# Load core messages and the extension localisations.
+			foreach ( $messageDirs as $dirs ) {
+				foreach ( (array)$dirs as $dir ) {
+					$fileName = "$dir/$csCode.json";
+					$data = $this->readJSONFile( $fileName );
+
+					foreach ( $data as $key => $item ) {
+						$this->mergeItem( $key, $csData[$key], $item );
+					}
+
+					$deps[] = new FileDependency( $fileName );
+				}
+			}
+
+			# Merge non-JSON extension data
+			if ( isset( $extensionData[$csCode] ) ) {
+				foreach ( $extensionData[$csCode] as $key => $item ) {
+					$this->mergeItem( $key, $csData[$key], $item );
+				}
+			}
+
+			if ( $csCode === $code ) {
+				# Merge core data into extension data
+				foreach ( $coreData as $key => $item ) {
+					$this->mergeItem( $key, $csData[$key], $item );
+				}
+			} else {
+				# Load the secondary localisation from the source file to
+				# avoid infinite cycles on cyclic fallbacks
+				$fbData = $this->readSourceFilesAndRegisterDeps( $csCode, $deps );
+				# Only merge the keys that make sense to merge
+				foreach ( self::$allKeys as $key ) {
+					if ( !isset( $fbData[ $key ] ) ) {
+						continue;
+					}
+
+					if ( ( $coreData[ $key ] ) === null || $this->isMergeableKey( $key ) ) {
+						$this->mergeItem( $key, $csData[ $key ], $fbData[ $key ] );
+					}
+				}
+			}
+
+			# Allow extensions an opportunity to adjust the data for this
+			# fallback
+			$this->hookRunner->onLocalisationCacheRecacheFallback( $this, $csCode, $csData );
+
+			# Merge the data for this fallback into the final array
+			if ( $csCode === $code ) {
+				$allData = $csData;
+			} else {
+				foreach ( self::$allKeys as $key ) {
+					if ( !isset( $csData[$key] ) ) {
+						continue;
+					}
+
+					// @phan-suppress-next-line PhanTypeArraySuspiciousNullable
+					if ( $allData[$key] === null || $this->isMergeableKey( $key ) ) {
+						$this->mergeItem( $key, $allData[$key], $csData[$key] );
+					}
+				}
+			}
+		}
+
+		# Add cache dependencies for any referenced globals
+		$deps['wgExtensionMessagesFiles'] = new GlobalDependency( 'wgExtensionMessagesFiles' );
+		// The 'MessagesDirs' config setting is used in LocalisationCache::getMessagesDirs().
+		// We use the key 'wgMessagesDirs' for historical reasons.
+		$deps['wgMessagesDirs'] = new MainConfigDependency( MainConfigNames::MessagesDirs );
+		$deps['version'] = new ConstantDependency( 'LocalisationCache::VERSION' );
+
+		# Add dependencies to the cache entry
+		$allData['deps'] = $deps;
+
+		# Replace spaces with underscores in namespace names
+		$allData['namespaceNames'] = str_replace( ' ', '_', $allData['namespaceNames'] );
+
+		# And do the same for special page aliases. $page is an array.
+		foreach ( $allData['specialPageAliases'] as &$page ) {
+			$page = str_replace( ' ', '_', $page );
+		}
+		# Decouple the reference to prevent accidental damage
+		unset( $page );
+
+		# If there were no plural rules, return an empty array
+		if ( $allData['pluralRules'] === null ) {
+			$allData['pluralRules'] = [];
+		}
+		if ( $allData['compiledPluralRules'] === null ) {
+			$allData['compiledPluralRules'] = [];
+		}
+		# If there were no plural rule types, return an empty array
+		if ( $allData['pluralRuleTypes'] === null ) {
+			$allData['pluralRuleTypes'] = [];
+		}
+
+		# Set the list keys
+		$allData['list'] = [];
+		foreach ( self::$splitKeys as $key ) {
+			$allData['list'][$key] = array_keys( $allData[$key] );
+		}
+		# Run hooks
+		$unused = true; // Used to be $purgeBlobs, removed in 1.34
+		$this->hookRunner->onLocalisationCacheRecache( $this, $code, $allData, $unused );
+
+		// @phan-suppress-next-line PhanTypePossiblyInvalidDimOffset False positive
+		if ( $allData['namespaceNames'] === null ) {
+			throw new MWException( __METHOD__ . ': Localisation data failed validation check! ' .
+				'Check that your languages/messages/MessagesEn.php file is intact.' );
+		}
+
+		# Set the preload key
+		$allData['preload'] = $this->buildPreload( $allData );
+
+		# Save to the process cache and register the items loaded
+		$this->data[$code] = $allData;
+		foreach ( $allData as $key => $item ) {
+			$this->loadedItems[$code][$key] = true;
+		}
+
+		# Save to the persistent cache
+		$this->store->startWrite( $code );
+		foreach ( $allData as $key => $value ) {
+			if ( in_array( $key, self::$splitKeys ) ) {
+				foreach ( $value as $subkey => $subvalue ) {
+					$this->store->set( "$key:$subkey", $subvalue );
+				}
+			} else {
+				$this->store->set( $key, $value );
+			}
+		}
+		$this->store->finishWrite();
+
+		# Clear out the MessageBlobStore
+		# HACK: If using a null (i.e. disabled) storage backend, we
+		# can't write to the MessageBlobStore either
+		if ( !$this->store instanceof LCStoreNull ) {
+			foreach ( $this->clearStoreCallbacks as $callback ) {
+				$callback();
+			}
+		}
+	}
+
+	/**
+	 * Build the preload item from the given pre-cache data.
+	 *
+	 * The preload item will be loaded automatically, improving performance
+	 * for the commonly-requested items it contains.
+	 * @param array $data
+	 * @return array
+	 */
+	protected function buildPreload( $data ) {
+		$preload = [ 'messages' => [] ];
+		foreach ( self::$preloadedKeys as $key ) {
+			$preload[$key] = $data[$key];
+		}
+
+		foreach ( $data['preloadedMessages'] as $subkey ) {
+			$subitem = $data['messages'][$subkey] ?? null;
+			$preload['messages'][$subkey] = $subitem;
+		}
+
+		return $preload;
+	}
+
+	/**
+	 * Unload the data for a given language from the object cache.
+	 * Reduces memory usage.
+	 * @param string $code
+	 */
+	public function unload( $code ) {
+		unset( $this->data[$code] );
+		unset( $this->loadedItems[$code] );
+		unset( $this->loadedSubitems[$code] );
+		unset( $this->initialisedLangs[$code] );
+		unset( $this->shallowFallbacks[$code] );
+
+		foreach ( $this->shallowFallbacks as $shallowCode => $fbCode ) {
+			if ( $fbCode === $code ) {
+				$this->unload( $shallowCode );
+			}
+		}
+	}
+
+	/**
+	 * Unload all data
+	 */
+	public function unloadAll() {
+		foreach ( $this->initialisedLangs as $lang => $unused ) {
+			$this->unload( $lang );
+		}
+	}
+
+	/**
+	 * Disable the storage backend
+	 */
+	public function disableBackend() {
+		$this->store = new LCStoreNull;
+		$this->manualRecache = false;
+	}
+}
diff --git a/includes/language/LocalisationCacheBulkLoad.php b/includes/language/LocalisationCacheBulkLoad.php
new file mode 100644
index 000000000000..5c962d3a040e
--- /dev/null
+++ b/includes/language/LocalisationCacheBulkLoad.php
@@ -0,0 +1,126 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+
+/**
+ * A localisation cache optimised for loading large amounts of data for many
+ * languages. Used by rebuildLocalisationCache.php.
+ */
+class LocalisationCacheBulkLoad extends LocalisationCache {
+
+	/**
+	 * A cache of the contents of data files.
+	 * Core files are serialized to avoid using ~1GB of RAM during a recache.
+	 */
+	private $fileCache = [];
+
+	/**
+	 * Most recently used languages. Uses the linked-list aspect of PHP hashtables
+	 * to keep the most recently used language codes at the end of the array, and
+	 * the language codes that are ready to be deleted at the beginning.
+	 */
+	private $mruLangs = [];
+
+	/**
+	 * Maximum number of languages that may be loaded into $this->data
+	 */
+	private $maxLoadedLangs = 10;
+
+	/**
+	 * @param string $fileName
+	 * @param string $fileType
+	 * @return array|mixed
+	 */
+	protected function readPHPFile( $fileName, $fileType ) {
+		$serialize = $fileType === 'core';
+		if ( !isset( $this->fileCache[$fileName][$fileType] ) ) {
+			$data = parent::readPHPFile( $fileName, $fileType );
+
+			if ( $serialize ) {
+				$encData = serialize( $data );
+			} else {
+				$encData = $data;
+			}
+
+			$this->fileCache[$fileName][$fileType] = $encData;
+
+			return $data;
+		} elseif ( $serialize ) {
+			return unserialize( $this->fileCache[$fileName][$fileType] );
+		} else {
+			return $this->fileCache[$fileName][$fileType];
+		}
+	}
+
+	/**
+	 * @param string $code
+	 * @param string $key
+	 * @return mixed
+	 */
+	public function getItem( $code, $key ) {
+		unset( $this->mruLangs[$code] );
+		$this->mruLangs[$code] = true;
+
+		return parent::getItem( $code, $key );
+	}
+
+	/**
+	 * @param string $code
+	 * @param string $key
+	 * @param string $subkey
+	 * @return mixed
+	 */
+	public function getSubitem( $code, $key, $subkey ) {
+		unset( $this->mruLangs[$code] );
+		$this->mruLangs[$code] = true;
+
+		return parent::getSubitem( $code, $key, $subkey );
+	}
+
+	/**
+	 * @param string $code
+	 */
+	public function recache( $code ) {
+		parent::recache( $code );
+		unset( $this->mruLangs[$code] );
+		$this->mruLangs[$code] = true;
+		$this->trimCache();
+	}
+
+	/**
+	 * @param string $code
+	 */
+	public function unload( $code ) {
+		unset( $this->mruLangs[$code] );
+		parent::unload( $code );
+	}
+
+	/**
+	 * Unload cached languages until there are less than $this->maxLoadedLangs
+	 */
+	protected function trimCache() {
+		while ( count( $this->data ) > $this->maxLoadedLangs && count( $this->mruLangs ) ) {
+			reset( $this->mruLangs );
+			$code = key( $this->mruLangs );
+			wfDebug( __METHOD__ . ": unloading $code" );
+			$this->unload( $code );
+		}
+	}
+
+}
diff --git a/includes/language/MessageCache.php b/includes/language/MessageCache.php
new file mode 100644
index 000000000000..ced9810caa76
--- /dev/null
+++ b/includes/language/MessageCache.php
@@ -0,0 +1,1495 @@
+<?php
+/**
+ * Localisation messages cache.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Cache
+ */
+
+use MediaWiki\HookContainer\HookContainer;
+use MediaWiki\HookContainer\HookRunner;
+use MediaWiki\Languages\LanguageConverterFactory;
+use MediaWiki\Languages\LanguageFactory;
+use MediaWiki\Languages\LanguageFallback;
+use MediaWiki\Languages\LanguageNameUtils;
+use MediaWiki\Linker\LinkTarget;
+use MediaWiki\Logger\LoggerFactory;
+use MediaWiki\MainConfigNames;
+use MediaWiki\MediaWikiServices;
+use MediaWiki\Page\PageReference;
+use MediaWiki\Page\PageReferenceValue;
+use MediaWiki\Revision\SlotRecord;
+use Psr\Log\LoggerAwareInterface;
+use Psr\Log\LoggerInterface;
+use Wikimedia\Rdbms\Database;
+use Wikimedia\RequestTimeout\TimeoutException;
+use Wikimedia\ScopedCallback;
+
+/**
+ * MediaWiki message cache structure version.
+ * Bump this whenever the message cache format has changed.
+ */
+define( 'MSG_CACHE_VERSION', 2 );
+
+/**
+ * Cache of messages that are defined by MediaWiki namespace pages or by hooks
+ *
+ * Performs various MediaWiki namespace-related functions
+ * @ingroup Cache
+ */
+class MessageCache implements LoggerAwareInterface {
+	/**
+	 * The size of the MapCacheLRU which stores message data. The maximum
+	 * number of languages which can be efficiently loaded in a given request.
+	 */
+	public const MAX_REQUEST_LANGUAGES = 10;
+
+	private const FOR_UPDATE = 1; // force message reload
+
+	/** How long to wait for memcached locks */
+	private const WAIT_SEC = 15;
+	/** How long memcached locks last */
+	private const LOCK_TTL = 30;
+
+	/**
+	 * Lifetime for cache, for keys stored in $wanCache, in seconds.
+	 * @var int
+	 */
+	private const WAN_TTL = IExpiringStore::TTL_DAY;
+
+	/** @var LoggerInterface */
+	private $logger;
+
+	/**
+	 * Process cache of loaded messages that are defined in MediaWiki namespace
+	 *
+	 * @var MapCacheLRU Map of (language code => key => " <MESSAGE>" or "!TOO BIG" or "!ERROR")
+	 */
+	protected $cache;
+
+	/**
+	 * Map of (lowercase message key => unused) for all software defined messages
+	 *
+	 * @var array
+	 */
+	private $systemMessageNames;
+
+	/**
+	 * @var bool[] Map of (language code => boolean)
+	 */
+	protected $cacheVolatile = [];
+
+	/**
+	 * Should mean that database cannot be used, but check
+	 * @var bool
+	 */
+	protected $mDisable;
+
+	/**
+	 * Message cache has its own parser which it uses to transform messages
+	 * @var ParserOptions
+	 */
+	protected $mParserOptions;
+	/** @var Parser */
+	protected $mParser;
+
+	/**
+	 * @var bool
+	 */
+	protected $mInParser = false;
+
+	/** @var WANObjectCache */
+	protected $wanCache;
+	/** @var BagOStuff */
+	protected $clusterCache;
+	/** @var BagOStuff */
+	protected $srvCache;
+	/** @var Language */
+	protected $contLang;
+	/** @var string */
+	protected $contLangCode;
+	/** @var ILanguageConverter */
+	protected $contLangConverter;
+	/** @var LanguageFactory */
+	protected $langFactory;
+	/** @var LocalisationCache */
+	protected $localisationCache;
+	/** @var LanguageNameUtils */
+	protected $languageNameUtils;
+	/** @var LanguageFallback */
+	protected $languageFallback;
+	/** @var HookRunner */
+	private $hookRunner;
+
+	/**
+	 * Normalize message key input
+	 *
+	 * @param string $key Input message key to be normalized
+	 * @return string Normalized message key
+	 */
+	public static function normalizeKey( $key ) {
+		$lckey = strtr( $key, ' ', '_' );
+		if ( $lckey === '' ) {
+			// T300792
+			return $lckey;
+		}
+
+		if ( ord( $lckey ) < 128 ) {
+			$lckey[0] = strtolower( $lckey[0] );
+		} else {
+			$lckey = MediaWikiServices::getInstance()->getContentLanguage()->lcfirst( $lckey );
+		}
+
+		return $lckey;
+	}
+
+	/**
+	 * @internal For use by ServiceWiring
+	 * @param WANObjectCache $wanCache
+	 * @param BagOStuff $clusterCache
+	 * @param BagOStuff $serverCache
+	 * @param Language $contLang Content language of site
+	 * @param LanguageConverterFactory $langConverterFactory
+	 * @param LoggerInterface $logger
+	 * @param array $options
+	 *  - useDB (bool): Whether to allow message overrides from "MediaWiki:" pages.
+	 *    Default: true.
+	 * @param LanguageFactory $langFactory
+	 * @param LocalisationCache $localisationCache
+	 * @param LanguageNameUtils $languageNameUtils
+	 * @param LanguageFallback $languageFallback
+	 * @param HookContainer $hookContainer
+	 */
+	public function __construct(
+		WANObjectCache $wanCache,
+		BagOStuff $clusterCache,
+		BagOStuff $serverCache,
+		Language $contLang,
+		LanguageConverterFactory $langConverterFactory,
+		LoggerInterface $logger,
+		array $options,
+		LanguageFactory $langFactory,
+		LocalisationCache $localisationCache,
+		LanguageNameUtils $languageNameUtils,
+		LanguageFallback $languageFallback,
+		HookContainer $hookContainer
+	) {
+		$this->wanCache = $wanCache;
+		$this->clusterCache = $clusterCache;
+		$this->srvCache = $serverCache;
+		$this->contLang = $contLang;
+		$this->contLangConverter = $langConverterFactory->getLanguageConverter( $contLang );
+		$this->contLangCode = $contLang->getCode();
+		$this->logger = $logger;
+		$this->langFactory = $langFactory;
+		$this->localisationCache = $localisationCache;
+		$this->languageNameUtils = $languageNameUtils;
+		$this->languageFallback = $languageFallback;
+		$this->hookRunner = new HookRunner( $hookContainer );
+
+		// limit size
+		$this->cache = new MapCacheLRU( self::MAX_REQUEST_LANGUAGES );
+
+		$this->mDisable = !( $options['useDB'] ?? true );
+	}
+
+	public function setLogger( LoggerInterface $logger ) {
+		$this->logger = $logger;
+	}
+
+	/**
+	 * ParserOptions is lazy initialised.
+	 *
+	 * @return ParserOptions
+	 */
+	private function getParserOptions() {
+		if ( !$this->mParserOptions ) {
+			$context = RequestContext::getMain();
+			$user = $context->getUser();
+			if ( !$user->isSafeToLoad() ) {
+				// It isn't safe to use the context user yet, so don't try to get a
+				// ParserOptions for it. And don't cache this ParserOptions
+				// either.
+				$po = ParserOptions::newFromAnon();
+				$po->setAllowUnsafeRawHtml( false );
+				return $po;
+			}
+
+			$this->mParserOptions = ParserOptions::newFromContext( $context );
+			// Messages may take parameters that could come
+			// from malicious sources. As a precaution, disable
+			// the <html> parser tag when parsing messages.
+			$this->mParserOptions->setAllowUnsafeRawHtml( false );
+		}
+
+		return $this->mParserOptions;
+	}
+
+	/**
+	 * Try to load the cache from APC.
+	 *
+	 * @param string $code Optional language code, see documentation of load().
+	 * @return array|false The cache array, or false if not in cache.
+	 */
+	protected function getLocalCache( $code ) {
+		$cacheKey = $this->srvCache->makeKey( __CLASS__, $code );
+
+		return $this->srvCache->get( $cacheKey );
+	}
+
+	/**
+	 * Save the cache to APC.
+	 *
+	 * @param string $code
+	 * @param array $cache The cache array
+	 */
+	protected function saveToLocalCache( $code, $cache ) {
+		$cacheKey = $this->srvCache->makeKey( __CLASS__, $code );
+		$this->srvCache->set( $cacheKey, $cache );
+	}
+
+	/**
+	 * Loads messages from caches or from database in this order:
+	 * (1) local message cache (if $wgUseLocalMessageCache is enabled)
+	 * (2) memcached
+	 * (3) from the database.
+	 *
+	 * When successfully loading from (2) or (3), all higher level caches are
+	 * updated for the newest version.
+	 *
+	 * Nothing is loaded if member variable mDisable is true, either manually
+	 * set by calling code or if message loading fails (is this possible?).
+	 *
+	 * Returns true if cache is already populated or it was successfully populated,
+	 * or false if populating empty cache fails. Also returns true if MessageCache
+	 * is disabled.
+	 *
+	 * @param string $code Language to which load messages
+	 * @param int|null $mode Use MessageCache::FOR_UPDATE to skip process cache [optional]
+	 * @throws InvalidArgumentException
+	 * @return bool
+	 */
+	protected function load( $code, $mode = null ) {
+		if ( !is_string( $code ) ) {
+			throw new InvalidArgumentException( "Missing language code" );
+		}
+
+		# Don't do double loading...
+		if ( $this->isLanguageLoaded( $code ) && $mode != self::FOR_UPDATE ) {
+			return true;
+		}
+
+		# 8 lines of code just to say (once) that message cache is disabled
+		if ( $this->mDisable ) {
+			static $shownDisabled = false;
+			if ( !$shownDisabled ) {
+				$this->logger->debug( __METHOD__ . ': disabled' );
+				$shownDisabled = true;
+			}
+
+			return true;
+		}
+
+		# Loading code starts
+		$success = false; # Keep track of success
+		$staleCache = false; # a cache array with expired data, or false if none has been loaded
+		$where = []; # Debug info, delayed to avoid spamming debug log too much
+
+		# Hash of the contents is stored in memcache, to detect if data-center cache
+		# or local cache goes out of date (e.g. due to replace() on some other server)
+		list( $hash, $hashVolatile ) = $this->getValidationHash( $code );
+		$this->cacheVolatile[$code] = $hashVolatile;
+
+		# Try the local cache and check against the cluster hash key...
+		$cache = $this->getLocalCache( $code );
+		if ( !$cache ) {
+			$where[] = 'local cache is empty';
+		} elseif ( !isset( $cache['HASH'] ) || $cache['HASH'] !== $hash ) {
+			$where[] = 'local cache has the wrong hash';
+			$staleCache = $cache;
+		} elseif ( $this->isCacheExpired( $cache ) ) {
+			$where[] = 'local cache is expired';
+			$staleCache = $cache;
+		} elseif ( $hashVolatile ) {
+			$where[] = 'local cache validation key is expired/volatile';
+			$staleCache = $cache;
+		} else {
+			$where[] = 'got from local cache';
+			$this->cache->set( $code, $cache );
+			$success = true;
+		}
+
+		if ( !$success ) {
+			$cacheKey = $this->clusterCache->makeKey( 'messages', $code );
+			# Try the global cache. If it is empty, try to acquire a lock. If
+			# the lock can't be acquired, wait for the other thread to finish
+			# and then try the global cache a second time.
+			for ( $failedAttempts = 0; $failedAttempts <= 1; $failedAttempts++ ) {
+				if ( $hashVolatile && $staleCache ) {
+					# Do not bother fetching the whole cache blob to avoid I/O.
+					# Instead, just try to get the non-blocking $statusKey lock
+					# below, and use the local stale value if it was not acquired.
+					$where[] = 'global cache is presumed expired';
+				} else {
+					$cache = $this->clusterCache->get( $cacheKey );
+					if ( !$cache ) {
+						$where[] = 'global cache is empty';
+					} elseif ( $this->isCacheExpired( $cache ) ) {
+						$where[] = 'global cache is expired';
+						$staleCache = $cache;
+					} elseif ( $hashVolatile ) {
+						# DB results are replica DB lag prone until the holdoff TTL passes.
+						# By then, updates should be reflected in loadFromDBWithLock().
+						# One thread regenerates the cache while others use old values.
+						$where[] = 'global cache is expired/volatile';
+						$staleCache = $cache;
+					} else {
+						$where[] = 'got from global cache';
+						$this->cache->set( $code, $cache );
+						$this->saveToCaches( $cache, 'local-only', $code );
+						$success = true;
+					}
+				}
+
+				if ( $success ) {
+					# Done, no need to retry
+					break;
+				}
+
+				# We need to call loadFromDB. Limit the concurrency to one process.
+				# This prevents the site from going down when the cache expires.
+				# Note that the DB slam protection lock here is non-blocking.
+				$loadStatus = $this->loadFromDBWithLock( $code, $where, $mode );
+				if ( $loadStatus === true ) {
+					$success = true;
+					break;
+				} elseif ( $staleCache ) {
+					# Use the stale cache while some other thread constructs the new one
+					$where[] = 'using stale cache';
+					$this->cache->set( $code, $staleCache );
+					$success = true;
+					break;
+				} elseif ( $failedAttempts > 0 ) {
+					# Already blocked once, so avoid another lock/unlock cycle.
+					# This case will typically be hit if memcached is down, or if
+					# loadFromDB() takes longer than LOCK_WAIT.
+					$where[] = "could not acquire status key.";
+					break;
+				} elseif ( $loadStatus === 'cantacquire' ) {
+					# Wait for the other thread to finish, then retry. Normally,
+					# the memcached get() will then yield the other thread's result.
+					$where[] = 'waited for other thread to complete';
+					$this->getReentrantScopedLock( $cacheKey );
+				} else {
+					# Disable cache; $loadStatus is 'disabled'
+					break;
+				}
+			}
+		}
+
+		if ( !$success ) {
+			$where[] = 'loading FAILED - cache is disabled';
+			$this->mDisable = true;
+			$this->cache->set( $code, [] );
+			$this->logger->error( __METHOD__ . ": Failed to load $code" );
+			# This used to throw an exception, but that led to nasty side effects like
+			# the whole wiki being instantly down if the memcached server died
+		}
+
+		if ( !$this->isLanguageLoaded( $code ) ) {
+			throw new LogicException( "Process cache for '$code' should be set by now." );
+		}
+
+		$info = implode( ', ', $where );
+		$this->logger->debug( __METHOD__ . ": Loading $code... $info" );
+
+		return $success;
+	}
+
+	/**
+	 * @param string $code
+	 * @param string[] &$where List of debug comments
+	 * @param int|null $mode Use MessageCache::FOR_UPDATE to use DB_PRIMARY
+	 * @return true|string True on success or one of ("cantacquire", "disabled")
+	 */
+	protected function loadFromDBWithLock( $code, array &$where, $mode = null ) {
+		# If cache updates on all levels fail, give up on message overrides.
+		# This is to avoid easy site outages; see $saveSuccess comments below.
+		$statusKey = $this->clusterCache->makeKey( 'messages', $code, 'status' );
+		$status = $this->clusterCache->get( $statusKey );
+		if ( $status === 'error' ) {
+			$where[] = "could not load; method is still globally disabled";
+			return 'disabled';
+		}
+
+		# Now let's regenerate
+		$where[] = 'loading from database';
+
+		# Lock the cache to prevent conflicting writes.
+		# This lock is non-blocking so stale cache can quickly be used.
+		# Note that load() will call a blocking getReentrantScopedLock()
+		# after this if it really need to wait for any current thread.
+		$cacheKey = $this->clusterCache->makeKey( 'messages', $code );
+		$scopedLock = $this->getReentrantScopedLock( $cacheKey, 0 );
+		if ( !$scopedLock ) {
+			$where[] = 'could not acquire main lock';
+			return 'cantacquire';
+		}
+
+		$cache = $this->loadFromDB( $code, $mode );
+		$this->cache->set( $code, $cache );
+		$saveSuccess = $this->saveToCaches( $cache, 'all', $code );
+
+		if ( !$saveSuccess ) {
+			/**
+			 * Cache save has failed.
+			 *
+			 * There are two main scenarios where this could be a problem:
+			 * - The cache is more than the maximum size (typically 1MB compressed).
+			 * - Memcached has no space remaining in the relevant slab class. This is
+			 *   unlikely with recent versions of memcached.
+			 *
+			 * Either way, if there is a local cache, nothing bad will happen. If there
+			 * is no local cache, disabling the message cache for all requests avoids
+			 * incurring a loadFromDB() overhead on every request, and thus saves the
+			 * wiki from complete downtime under moderate traffic conditions.
+			 */
+			if ( $this->srvCache instanceof EmptyBagOStuff ) {
+				$this->clusterCache->set( $statusKey, 'error', 60 * 5 );
+				$where[] = 'could not save cache, disabled globally for 5 minutes';
+			} else {
+				$where[] = "could not save global cache";
+			}
+		}
+
+		return true;
+	}
+
+	/**
+	 * Loads cacheable messages from the database. Messages bigger than
+	 * $wgMaxMsgCacheEntrySize are assigned a special value, and are loaded
+	 * on-demand from the database later.
+	 *
+	 * @param string $code Language code
+	 * @param int|null $mode Use MessageCache::FOR_UPDATE to skip process cache
+	 * @return array Loaded messages for storing in caches
+	 */
+	protected function loadFromDB( $code, $mode = null ) {
+		$maxMsgCacheEntrySize = MediaWikiServices::getInstance()->getMainConfig()
+			->get( MainConfigNames::MaxMsgCacheEntrySize );
+		$adaptiveMessageCache = MediaWikiServices::getInstance()->getMainConfig()
+			->get( MainConfigNames::AdaptiveMessageCache );
+		// (T164666) The query here performs really poorly on WMF's
+		// contributions replicas. We don't have a way to say "any group except
+		// contributions", so for the moment let's specify 'api'.
+		// @todo: Get rid of this hack.
+		$dbr = wfGetDB( ( $mode == self::FOR_UPDATE ) ? DB_PRIMARY : DB_REPLICA, 'api' );
+
+		$cache = [];
+
+		$mostused = []; // list of "<cased message key>/<code>"
+		if ( $adaptiveMessageCache && $code !== $this->contLangCode ) {
+			if ( !$this->cache->has( $this->contLangCode ) ) {
+				$this->load( $this->contLangCode );
+			}
+			$mostused = array_keys( $this->cache->get( $this->contLangCode ) );
+			foreach ( $mostused as $key => $value ) {
+				$mostused[$key] = "$value/$code";
+			}
+		}
+
+		// Common conditions
+		$conds = [
+			'page_is_redirect' => 0,
+			'page_namespace' => NS_MEDIAWIKI,
+		];
+		if ( count( $mostused ) ) {
+			$conds['page_title'] = $mostused;
+		} elseif ( $code !== $this->contLangCode ) {
+			$conds[] = 'page_title' . $dbr->buildLike( $dbr->anyString(), '/', $code );
+		} else {
+			# Effectively disallows use of '/' character in NS_MEDIAWIKI for uses
+			# other than language code.
+			$conds[] = 'page_title NOT' .
+				$dbr->buildLike( $dbr->anyString(), '/', $dbr->anyString() );
+		}
+
+		// Set the stubs for oversized software-defined messages in the main cache map
+		$res = $dbr->select(
+			'page',
+			[ 'page_title', 'page_latest' ],
+			array_merge( $conds, [ 'page_len > ' . intval( $maxMsgCacheEntrySize ) ] ),
+			__METHOD__ . "($code)-big"
+		);
+		foreach ( $res as $row ) {
+			// Include entries/stubs for all keys in $mostused in adaptive mode
+			if ( $adaptiveMessageCache || $this->isMainCacheable( $row->page_title )
+			) {
+				$cache[$row->page_title] = '!TOO BIG';
+			}
+			// At least include revision ID so page changes are reflected in the hash
+			$cache['EXCESSIVE'][$row->page_title] = $row->page_latest;
+		}
+
+		// Can not inject the RevisionStore as it would break the installer since
+		// it instantiates MessageCache before the DB.
+		$revisionStore = MediaWikiServices::getInstance()->getRevisionStore();
+		// Set the text for small software-defined messages in the main cache map
+		$revQuery = $revisionStore->getQueryInfo( [ 'page' ] );
+
+		// T231196: MySQL/MariaDB (10.1.37) can sometimes irrationally decide that querying `actor` then
+		// `revision` then `page` is somehow better than starting with `page`. Tell it not to reorder the
+		// query (and also reorder it ourselves because as generated by RevisionStore it'll have
+		// `revision` first rather than `page`).
+		$revQuery['joins']['revision'] = $revQuery['joins']['page'];
+		unset( $revQuery['joins']['page'] );
+		// It isn't actually necessary to reorder $revQuery['tables'] as Database does the right thing
+		// when join conditions are given for all joins, but Gergő is wary of relying on that so pull
+		// `page` to the start.
+		$revQuery['tables'] = array_merge(
+			[ 'page' ],
+			array_diff( $revQuery['tables'], [ 'page' ] )
+		);
+
+		$res = $dbr->select(
+			$revQuery['tables'],
+			$revQuery['fields'],
+			array_merge( $conds, [
+				'page_len <= ' . intval( $maxMsgCacheEntrySize ),
+				'page_latest = rev_id' // get the latest revision only
+			] ),
+			__METHOD__ . "($code)-small",
+			[ 'STRAIGHT_JOIN' ],
+			$revQuery['joins']
+		);
+		$result = $revisionStore->newRevisionsFromBatch( $res, [
+			'slots' => [ SlotRecord::MAIN ],
+			'content' => true
+		] );
+		$revisions = $result->isOK() ? $result->getValue() : [];
+		foreach ( $res as $row ) {
+			// Include entries/stubs for all keys in $mostused in adaptive mode
+			if ( $adaptiveMessageCache || $this->isMainCacheable( $row->page_title )
+			) {
+				try {
+					$rev = $revisions[$row->rev_id] ?? null;
+					$content = $rev ? $rev->getContent( SlotRecord::MAIN ) : null;
+					$text = $this->getMessageTextFromContent( $content );
+				} catch ( TimeoutException $e ) {
+					throw $e;
+				} catch ( Exception $ex ) {
+					$text = false;
+				}
+
+				if ( !is_string( $text ) ) {
+					$entry = '!ERROR';
+					$this->logger->error(
+						__METHOD__
+						. ": failed to load message page text for {$row->page_title} ($code)"
+					);
+				} else {
+					$entry = ' ' . $text;
+				}
+				$cache[$row->page_title] = $entry;
+			} else {
+				// T193271: cache object gets too big and slow to generate.
+				// At least include revision ID so page changes are reflected in the hash.
+				$cache['EXCESSIVE'][$row->page_title] = $row->page_latest;
+			}
+		}
+
+		$cache['VERSION'] = MSG_CACHE_VERSION;
+		ksort( $cache );
+
+		# Hash for validating local cache (APC). No need to take into account
+		# messages larger than $wgMaxMsgCacheEntrySize, since those are only
+		# stored and fetched from memcache.
+		$cache['HASH'] = md5( serialize( $cache ) );
+		$cache['EXPIRY'] = wfTimestamp( TS_MW, time() + self::WAN_TTL );
+		unset( $cache['EXCESSIVE'] ); // only needed for hash
+
+		return $cache;
+	}
+
+	/**
+	 * Whether the language was loaded and its data is still in the process cache.
+	 *
+	 * @param string $lang
+	 * @return bool
+	 */
+	private function isLanguageLoaded( $lang ) {
+		// It is important that this only returns true if the cache was fully
+		// populated by load(), so that callers can assume all cache keys exist.
+		// It is possible for $this->cache to be only partially populated through
+		// methods like MessageCache::replace(), which must not make this method
+		// return true (T208897). And this method must cease to return true
+		// if the language was evicted by MapCacheLRU (T230690).
+		return $this->cache->hasField( $lang, 'VERSION' );
+	}
+
+	/**
+	 * Can the given DB key be added to the main cache blob? To reduce the
+	 * impact of abuse of the MediaWiki namespace by {{int:}} and CentralNotice,
+	 * this is only true if the page overrides a predefined message.
+	 *
+	 * @param string $name Message name (possibly with /code suffix)
+	 * @param string|null $code The language code. If this is null, message
+	 *   presence will be bulk loaded for the content language. Otherwise,
+	 *   presence will be detected by loading the specified message.
+	 * @return bool
+	 */
+	private function isMainCacheable( $name, $code = null ) {
+		// Convert first letter to lowercase, and strip /code suffix
+		$name = $this->contLang->lcfirst( $name );
+		// Include common conversion table pages. This also avoids problems with
+		// Installer::parse() bailing out due to disallowed DB queries (T207979).
+		if ( strpos( $name, 'conversiontable/' ) === 0 ) {
+			return true;
+		}
+		$msg = preg_replace( '/\/[a-z0-9-]{2,}$/', '', $name );
+
+		if ( $code === null ) {
+			// Bulk load
+			if ( $this->systemMessageNames === null ) {
+				$this->systemMessageNames = array_fill_keys(
+					$this->localisationCache->getSubitemList( $this->contLangCode, 'messages' ),
+					true );
+			}
+			return isset( $this->systemMessageNames[$msg] );
+		} else {
+			// Use individual subitem
+			return $this->localisationCache->getSubitem( $code, 'messages', $msg ) !== null;
+		}
+	}
+
+	/**
+	 * Updates cache as necessary when message page is changed
+	 *
+	 * @param string $title Message cache key with initial uppercase letter
+	 * @param string|false $text New contents of the page (false if deleted)
+	 */
+	public function replace( $title, $text ) {
+		if ( $this->mDisable ) {
+			return;
+		}
+
+		list( $msg, $code ) = $this->figureMessage( $title );
+		if ( strpos( $title, '/' ) !== false && $code === $this->contLangCode ) {
+			// Content language overrides do not use the /<code> suffix
+			return;
+		}
+
+		// (a) Update the process cache with the new message text
+		if ( $text === false ) {
+			// Page deleted
+			$this->cache->setField( $code, $title, '!NONEXISTENT' );
+		} else {
+			// Ignore $wgMaxMsgCacheEntrySize so the process cache is up to date
+			$this->cache->setField( $code, $title, ' ' . $text );
+		}
+
+		// (b) Update the shared caches in a deferred update with a fresh DB snapshot
+		DeferredUpdates::addUpdate(
+			new MessageCacheUpdate( $code, $title, $msg ),
+			DeferredUpdates::PRESEND
+		);
+	}
+
+	/**
+	 * @param string $code
+	 * @param array[] $replacements List of (title, message key) pairs
+	 * @throws MWException
+	 */
+	public function refreshAndReplaceInternal( $code, array $replacements ) {
+		$maxMsgCacheEntrySize = MediaWikiServices::getInstance()->getMainConfig()
+			->get( MainConfigNames::MaxMsgCacheEntrySize );
+
+		// Allow one caller at a time to avoid race conditions
+		$scopedLock = $this->getReentrantScopedLock(
+			$this->clusterCache->makeKey( 'messages', $code )
+		);
+		if ( !$scopedLock ) {
+			foreach ( $replacements as list( $title ) ) {
+				$this->logger->error(
+					__METHOD__ . ': could not acquire lock to update {title} ({code})',
+					[ 'title' => $title, 'code' => $code ] );
+			}
+
+			return;
+		}
+
+		// Load the existing cache to update it in the local DC cache.
+		// The other DCs will see a hash mismatch.
+		if ( $this->load( $code, self::FOR_UPDATE ) ) {
+			$cache = $this->cache->get( $code );
+		} else {
+			// Err? Fall back to loading from the database.
+			$cache = $this->loadFromDB( $code, self::FOR_UPDATE );
+		}
+		// Check if individual cache keys should exist and update cache accordingly
+		$newTextByTitle = []; // map of (title => content)
+		$newBigTitles = []; // map of (title => latest revision ID), like EXCESSIVE in loadFromDB()
+		// Can not inject the WikiPageFactory as it would break the installer since
+		// it instantiates MessageCache before the DB.
+		$wikiPageFactory = MediaWikiServices::getInstance()->getWikiPageFactory();
+		foreach ( $replacements as list( $title ) ) {
+			$page = $wikiPageFactory->newFromTitle( Title::makeTitle( NS_MEDIAWIKI, $title ) );
+			$page->loadPageData( $page::READ_LATEST );
+			$text = $this->getMessageTextFromContent( $page->getContent() );
+			// Remember the text for the blob store update later on
+			$newTextByTitle[$title] = $text ?? '';
+			// Note that if $text is false, then $cache should have a !NONEXISTANT entry
+			if ( !is_string( $text ) ) {
+				$cache[$title] = '!NONEXISTENT';
+			} elseif ( strlen( $text ) > $maxMsgCacheEntrySize ) {
+				$cache[$title] = '!TOO BIG';
+				$newBigTitles[$title] = $page->getLatest();
+			} else {
+				$cache[$title] = ' ' . $text;
+			}
+		}
+		// Update HASH for the new key. Incorporates various administrative keys,
+		// including the old HASH (and thereby the EXCESSIVE value from loadFromDB()
+		// and previous replace() calls), but that doesn't really matter since we
+		// only ever compare it for equality with a copy saved by saveToCaches().
+		$cache['HASH'] = md5( serialize( $cache + [ 'EXCESSIVE' => $newBigTitles ] ) );
+		// Update the too-big WAN cache entries now that we have the new HASH
+		foreach ( $newBigTitles as $title => $id ) {
+			// Match logic of loadCachedMessagePageEntry()
+			$this->wanCache->set(
+				$this->bigMessageCacheKey( $cache['HASH'], $title ),
+				' ' . $newTextByTitle[$title],
+				self::WAN_TTL
+			);
+		}
+		// Mark this cache as definitely being "latest" (non-volatile) so
+		// load() calls do not try to refresh the cache with replica DB data
+		$cache['LATEST'] = time();
+		// Update the process cache
+		$this->cache->set( $code, $cache );
+		// Pre-emptively update the local datacenter cache so things like edit filter and
+		// prevented changes are reflected immediately; these often use MediaWiki: pages.
+		// The datacenter handling replace() calls should be the same one handling edits
+		// as they require HTTP POST.
+		$this->saveToCaches( $cache, 'all', $code );
+		// Release the lock now that the cache is saved
+		ScopedCallback::consume( $scopedLock );
+
+		// Relay the purge. Touching this check key expires cache contents
+		// and local cache (APC) validation hash across all datacenters.
+		$this->wanCache->touchCheckKey( $this->getCheckKey( $code ) );
+
+		// Purge the messages in the message blob store and fire any hook handlers
+		$blobStore = MediaWikiServices::getInstance()->getResourceLoader()->getMessageBlobStore();
+		foreach ( $replacements as list( $title, $msg ) ) {
+			$blobStore->updateMessage( $this->contLang->lcfirst( $msg ) );
+			$this->hookRunner->onMessageCacheReplace( $title, $newTextByTitle[$title] );
+		}
+	}
+
+	/**
+	 * Is the given cache array expired due to time passing or a version change?
+	 *
+	 * @param array $cache
+	 * @return bool
+	 */
+	protected function isCacheExpired( $cache ) {
+		if ( !isset( $cache['VERSION'] ) || !isset( $cache['EXPIRY'] ) ) {
+			return true;
+		}
+		if ( $cache['VERSION'] != MSG_CACHE_VERSION ) {
+			return true;
+		}
+		if ( wfTimestampNow() >= $cache['EXPIRY'] ) {
+			return true;
+		}
+
+		return false;
+	}
+
+	/**
+	 * Shortcut to update caches.
+	 *
+	 * @param array $cache Cached messages with a version.
+	 * @param string $dest Either "local-only" to save to local caches only
+	 *   or "all" to save to all caches.
+	 * @param string|false $code Language code (default: false)
+	 * @return bool
+	 */
+	protected function saveToCaches( array $cache, $dest, $code = false ) {
+		if ( $dest === 'all' ) {
+			$cacheKey = $this->clusterCache->makeKey( 'messages', $code );
+			$success = $this->clusterCache->set( $cacheKey, $cache );
+			$this->setValidationHash( $code, $cache );
+		} else {
+			$success = true;
+		}
+
+		$this->saveToLocalCache( $code, $cache );
+
+		return $success;
+	}
+
+	/**
+	 * Get the md5 used to validate the local APC cache
+	 *
+	 * @param string $code
+	 * @return array (hash or false, bool expiry/volatility status)
+	 */
+	protected function getValidationHash( $code ) {
+		$curTTL = null;
+		$value = $this->wanCache->get(
+			$this->wanCache->makeKey( 'messages', $code, 'hash', 'v1' ),
+			$curTTL,
+			[ $this->getCheckKey( $code ) ]
+		);
+
+		if ( $value ) {
+			$hash = $value['hash'];
+			if ( ( time() - $value['latest'] ) < WANObjectCache::TTL_MINUTE ) {
+				// Cache was recently updated via replace() and should be up-to-date.
+				// That method is only called in the primary datacenter and uses FOR_UPDATE.
+				// Also, it is unlikely that the current datacenter is *now* secondary one.
+				$expired = false;
+			} else {
+				// See if the "check" key was bumped after the hash was generated
+				$expired = ( $curTTL < 0 );
+			}
+		} else {
+			// No hash found at all; cache must regenerate to be safe
+			$hash = false;
+			$expired = true;
+		}
+
+		return [ $hash, $expired ];
+	}
+
+	/**
+	 * Set the md5 used to validate the local disk cache
+	 *
+	 * If $cache has a 'LATEST' UNIX timestamp key, then the hash will not
+	 * be treated as "volatile" by getValidationHash() for the next few seconds.
+	 * This is triggered when $cache is generated using FOR_UPDATE mode.
+	 *
+	 * @param string $code
+	 * @param array $cache Cached messages with a version
+	 */
+	protected function setValidationHash( $code, array $cache ) {
+		$this->wanCache->set(
+			$this->wanCache->makeKey( 'messages', $code, 'hash', 'v1' ),
+			[
+				'hash' => $cache['HASH'],
+				'latest' => $cache['LATEST'] ?? 0
+			],
+			WANObjectCache::TTL_INDEFINITE
+		);
+	}
+
+	/**
+	 * @param string $key A language message cache key that stores blobs
+	 * @param int $timeout Wait timeout in seconds
+	 * @return null|ScopedCallback
+	 */
+	protected function getReentrantScopedLock( $key, $timeout = self::WAIT_SEC ) {
+		return $this->clusterCache->getScopedLock( $key, $timeout, self::LOCK_TTL, __METHOD__ );
+	}
+
+	/**
+	 * Get a message from either the content language or the user language.
+	 *
+	 * First, assemble a list of languages to attempt getting the message from. This
+	 * chain begins with the requested language and its fallbacks and then continues with
+	 * the content language and its fallbacks. For each language in the chain, the following
+	 * process will occur (in this order):
+	 *  1. If a language-specific override, i.e., [[MW:msg/lang]], is available, use that.
+	 *     Note: for the content language, there is no /lang subpage.
+	 *  2. Fetch from the static CDB cache.
+	 *  3. If available, check the database for fallback language overrides.
+	 *
+	 * This process provides a number of guarantees. When changing this code, make sure all
+	 * of these guarantees are preserved.
+	 *  * If the requested language is *not* the content language, then the CDB cache for that
+	 *    specific language will take precedence over the root database page ([[MW:msg]]).
+	 *  * Fallbacks will be just that: fallbacks. A fallback language will never be reached if
+	 *    the message is available *anywhere* in the language for which it is a fallback.
+	 *
+	 * @param string $key The message key
+	 * @param bool $useDB If true, look for the message in the DB, false
+	 *   to use only the compiled l10n cache.
+	 * @param bool|string|Language $langcode Code of the language to get the message for.
+	 *   - If string and a valid code, will create a standard language object
+	 *   - If string but not a valid code, will create a basic language object
+	 *   - If boolean and false, create object from the current users language
+	 *   - If boolean and true, create object from the wikis content language
+	 *   - If language object, use it as given
+	 *
+	 * @throws MWException When given an invalid key
+	 * @return string|false False if the message doesn't exist, otherwise the
+	 *   message (which can be empty)
+	 */
+	public function get( $key, $useDB = true, $langcode = true ) {
+		if ( is_int( $key ) ) {
+			// Fix numerical strings that somehow become ints
+			// on their way here
+			$key = (string)$key;
+		} elseif ( !is_string( $key ) ) {
+			throw new MWException( 'Non-string key given' );
+		} elseif ( $key === '' ) {
+			// Shortcut: the empty key is always missing
+			return false;
+		}
+
+		// Normalise title-case input (with some inlining)
+		$lckey = self::normalizeKey( $key );
+
+		$this->hookRunner->onMessageCache__get( $lckey );
+
+		// Loop through each language in the fallback list until we find something useful
+		$message = $this->getMessageFromFallbackChain(
+			wfGetLangObj( $langcode ),
+			$lckey,
+			!$this->mDisable && $useDB
+		);
+
+		// If we still have no message, maybe the key was in fact a full key so try that
+		if ( $message === false ) {
+			$parts = explode( '/', $lckey );
+			// We may get calls for things that are http-urls from sidebar
+			// Let's not load nonexistent languages for those
+			// They usually have more than one slash.
+			if ( count( $parts ) == 2 && $parts[1] !== '' ) {
+				$message = $this->localisationCache->getSubitem( $parts[1], 'messages', $parts[0] );
+				if ( $message === null ) {
+					$message = false;
+				}
+			}
+		}
+
+		// Post-processing if the message exists
+		if ( $message !== false ) {
+			// Fix whitespace
+			$message = str_replace(
+				[
+					# Fix for trailing whitespace, removed by textarea
+					'&#32;',
+					# Fix for NBSP, converted to space by firefox
+					'&nbsp;',
+					'&#160;',
+					'&shy;'
+				],
+				[
+					' ',
+					"\u{00A0}",
+					"\u{00A0}",
+					"\u{00AD}"
+				],
+				$message
+			);
+		}
+
+		return $message;
+	}
+
+	/**
+	 * Given a language, try and fetch messages from that language.
+	 *
+	 * Will also consider fallbacks of that language, the site language, and fallbacks for
+	 * the site language.
+	 *
+	 * @see MessageCache::get
+	 * @param Language|StubObject $lang Preferred language
+	 * @param string $lckey Lowercase key for the message (as for localisation cache)
+	 * @param bool $useDB Whether to include messages from the wiki database
+	 * @return string|false The message, or false if not found
+	 */
+	protected function getMessageFromFallbackChain( $lang, $lckey, $useDB ) {
+		$alreadyTried = [];
+
+		// First try the requested language.
+		$message = $this->getMessageForLang( $lang, $lckey, $useDB, $alreadyTried );
+		if ( $message !== false ) {
+			return $message;
+		}
+
+		// Now try checking the site language.
+		$message = $this->getMessageForLang( $this->contLang, $lckey, $useDB, $alreadyTried );
+		return $message;
+	}
+
+	/**
+	 * Given a language, try and fetch messages from that language and its fallbacks.
+	 *
+	 * @see MessageCache::get
+	 * @param Language|StubObject $lang Preferred language
+	 * @param string $lckey Lowercase key for the message (as for localisation cache)
+	 * @param bool $useDB Whether to include messages from the wiki database
+	 * @param bool[] &$alreadyTried Contains true for each language that has been tried already
+	 * @return string|false The message, or false if not found
+	 */
+	private function getMessageForLang( $lang, $lckey, $useDB, &$alreadyTried ) {
+		$langcode = $lang->getCode();
+
+		// Try checking the database for the requested language
+		if ( $useDB ) {
+			$uckey = $this->contLang->ucfirst( $lckey );
+
+			if ( !isset( $alreadyTried[$langcode] ) ) {
+				$message = $this->getMsgFromNamespace(
+					$this->getMessagePageName( $langcode, $uckey ),
+					$langcode
+				);
+				if ( $message !== false ) {
+					return $message;
+				}
+				$alreadyTried[$langcode] = true;
+			}
+		} else {
+			$uckey = null;
+		}
+
+		// Check the CDB cache
+		$message = $lang->getMessage( $lckey );
+		if ( $message !== null ) {
+			return $message;
+		}
+
+		// Try checking the database for all of the fallback languages
+		if ( $useDB ) {
+			$fallbackChain = $this->languageFallback->getAll( $langcode );
+
+			foreach ( $fallbackChain as $code ) {
+				if ( isset( $alreadyTried[$code] ) ) {
+					continue;
+				}
+
+				$message = $this->getMsgFromNamespace(
+					// @phan-suppress-next-line PhanTypeMismatchArgumentNullable uckey is set when used
+					$this->getMessagePageName( $code, $uckey ), $code );
+
+				if ( $message !== false ) {
+					return $message;
+				}
+				$alreadyTried[$code] = true;
+			}
+		}
+
+		return false;
+	}
+
+	/**
+	 * Get the message page name for a given language
+	 *
+	 * @param string $langcode
+	 * @param string $uckey Uppercase key for the message
+	 * @return string The page name
+	 */
+	private function getMessagePageName( $langcode, $uckey ) {
+		if ( $langcode === $this->contLangCode ) {
+			// Messages created in the content language will not have the /lang extension
+			return $uckey;
+		} else {
+			return "$uckey/$langcode";
+		}
+	}
+
+	/**
+	 * Get a message from the MediaWiki namespace, with caching. The key must
+	 * first be converted to two-part lang/msg form if necessary.
+	 *
+	 * Unlike self::get(), this function doesn't resolve fallback chains, and
+	 * some callers require this behavior. LanguageConverter::parseCachedTable()
+	 * and self::get() are some examples in core.
+	 *
+	 * @param string $title Message cache key with initial uppercase letter
+	 * @param string $code Code denoting the language to try
+	 * @return string|false The message, or false if it does not exist or on error
+	 */
+	public function getMsgFromNamespace( $title, $code ) {
+		// Load all MediaWiki page definitions into cache. Note that individual keys
+		// already loaded into cache during this request remain in the cache, which
+		// includes the value of hook-defined messages.
+		$this->load( $code );
+
+		$entry = $this->cache->getField( $code, $title );
+
+		if ( $entry !== null ) {
+			// Message page exists as an override of a software messages
+			if ( substr( $entry, 0, 1 ) === ' ' ) {
+				// The message exists and is not '!TOO BIG' or '!ERROR'
+				return (string)substr( $entry, 1 );
+			} elseif ( $entry === '!NONEXISTENT' ) {
+				// The text might be '-' or missing due to some data loss
+				return false;
+			}
+			// Load the message page, utilizing the individual message cache.
+			// If the page does not exist, there will be no hook handler fallbacks.
+			$entry = $this->loadCachedMessagePageEntry(
+				$title,
+				$code,
+				$this->cache->getField( $code, 'HASH' )
+			);
+		} else {
+			// Message page either does not exist or does not override a software message
+			if ( !$this->isMainCacheable( $title, $code ) ) {
+				// Message page does not override any software-defined message. A custom
+				// message might be defined to have content or settings specific to the wiki.
+				// Load the message page, utilizing the individual message cache as needed.
+				$entry = $this->loadCachedMessagePageEntry(
+					$title,
+					$code,
+					$this->cache->getField( $code, 'HASH' )
+				);
+			}
+			if ( $entry === null || substr( $entry, 0, 1 ) !== ' ' ) {
+				// Message does not have a MediaWiki page definition; try hook handlers
+				$message = false;
+				// @phan-suppress-next-line PhanTypeMismatchArgument Type mismatch on pass-by-ref args
+				$this->hookRunner->onMessagesPreLoad( $title, $message, $code );
+				if ( $message !== false ) {
+					$this->cache->setField( $code, $title, ' ' . $message );
+				} else {
+					$this->cache->setField( $code, $title, '!NONEXISTENT' );
+				}
+
+				return $message;
+			}
+		}
+
+		if ( $entry !== false && substr( $entry, 0, 1 ) === ' ' ) {
+			if ( $this->cacheVolatile[$code] ) {
+				// Make sure that individual keys respect the WAN cache holdoff period too
+				$this->logger->debug(
+					__METHOD__ . ': loading volatile key \'{titleKey}\'',
+					[ 'titleKey' => $title, 'code' => $code ] );
+			} else {
+				$this->cache->setField( $code, $title, $entry );
+			}
+			// The message exists, so make sure a string is returned
+			return (string)substr( $entry, 1 );
+		}
+
+		$this->cache->setField( $code, $title, '!NONEXISTENT' );
+
+		return false;
+	}
+
+	/**
+	 * @param string $dbKey
+	 * @param string $code
+	 * @param string $hash
+	 * @return string Either " <MESSAGE>" or "!NONEXISTANT"
+	 */
+	private function loadCachedMessagePageEntry( $dbKey, $code, $hash ) {
+		$fname = __METHOD__;
+		return $this->srvCache->getWithSetCallback(
+			$this->srvCache->makeKey( 'messages-big', $hash, $dbKey ),
+			BagOStuff::TTL_HOUR,
+			function () use ( $code, $dbKey, $hash, $fname ) {
+				return $this->wanCache->getWithSetCallback(
+					$this->bigMessageCacheKey( $hash, $dbKey ),
+					self::WAN_TTL,
+					function ( $oldValue, &$ttl, &$setOpts ) use ( $dbKey, $code, $fname ) {
+						// Try loading the message from the database
+						$setOpts += Database::getCacheSetOptions( wfGetDB( DB_REPLICA ) );
+						// Use newKnownCurrent() to avoid querying revision/user tables
+						$title = Title::makeTitle( NS_MEDIAWIKI, $dbKey );
+						// Injecting RevisionStore breaks installer since it
+						// instantiates MessageCache before DB.
+						$revision = MediaWikiServices::getInstance()
+							->getRevisionLookup()
+							->getKnownCurrentRevision( $title );
+						if ( !$revision ) {
+							// The wiki doesn't have a local override page. Cache absence with normal TTL.
+							// When overrides are created, self::replace() takes care of the cache.
+							return '!NONEXISTENT';
+						}
+						$content = $revision->getContent( SlotRecord::MAIN );
+						if ( $content ) {
+							$message = $this->getMessageTextFromContent( $content );
+						} else {
+							$this->logger->warning(
+								$fname . ': failed to load page text for \'{titleKey}\'',
+								[ 'titleKey' => $dbKey, 'code' => $code ]
+							);
+							$message = null;
+						}
+
+						if ( !is_string( $message ) ) {
+							// Revision failed to load Content, or Content is incompatible with wikitext.
+							// Possibly a temporary loading failure.
+							$ttl = 5;
+
+							return '!NONEXISTENT';
+						}
+
+						return ' ' . $message;
+					}
+				);
+			}
+		);
+	}
+
+	/**
+	 * @param string $message
+	 * @param bool $interface
+	 * @param Language|null $language
+	 * @param PageReference|null $page
+	 * @return string
+	 */
+	public function transform( $message, $interface = false, $language = null, PageReference $page = null ) {
+		// Avoid creating parser if nothing to transform
+		if ( strpos( $message, '{{' ) === false ) {
+			return $message;
+		}
+
+		if ( $this->mInParser ) {
+			return $message;
+		}
+
+		$parser = $this->getParser();
+		if ( $parser ) {
+			$popts = $this->getParserOptions();
+			$popts->setInterfaceMessage( $interface );
+			$popts->setTargetLanguage( $language );
+
+			$userlang = $popts->setUserLang( $language );
+			$this->mInParser = true;
+			$message = $parser->transformMsg( $message, $popts, $page );
+			$this->mInParser = false;
+			$popts->setUserLang( $userlang );
+		}
+
+		return $message;
+	}
+
+	/**
+	 * @return Parser
+	 */
+	public function getParser() {
+		if ( !$this->mParser ) {
+			$parser = MediaWikiServices::getInstance()->getParser();
+			# Clone it and store it
+			$this->mParser = clone $parser;
+		}
+
+		return $this->mParser;
+	}
+
+	/**
+	 * @param string $text
+	 * @param PageReference|null $page
+	 * @param bool $linestart Whether or not this is at the start of a line
+	 * @param bool $interface Whether this is an interface message
+	 * @param Language|string|null $language Language code
+	 * @return ParserOutput|string
+	 */
+	public function parse( $text, PageReference $page = null, $linestart = true,
+		$interface = false, $language = null
+	) {
+		global $wgTitle;
+
+		if ( $this->mInParser ) {
+			return htmlspecialchars( $text );
+		}
+
+		$parser = $this->getParser();
+		$popts = $this->getParserOptions();
+		$popts->setInterfaceMessage( $interface );
+
+		if ( is_string( $language ) ) {
+			$language = $this->langFactory->getLanguage( $language );
+		}
+		$popts->setTargetLanguage( $language );
+
+		if ( !$page ) {
+			$logger = LoggerFactory::getInstance( 'GlobalTitleFail' );
+			$logger->info(
+				__METHOD__ . ' called with no title set.',
+				[ 'exception' => new Exception ]
+			);
+			$page = $wgTitle;
+		}
+		// Sometimes $wgTitle isn't set either...
+		if ( !$page ) {
+			# It's not uncommon having a null $wgTitle in scripts. See r80898
+			# Create a ghost title in such case
+			$page = PageReferenceValue::localReference(
+				NS_SPECIAL,
+				'Badtitle/title not set in ' . __METHOD__
+			);
+		}
+
+		$this->mInParser = true;
+		$res = $parser->parse( $text, $page, $popts, $linestart );
+		$this->mInParser = false;
+
+		return $res;
+	}
+
+	public function disable() {
+		$this->mDisable = true;
+	}
+
+	public function enable() {
+		$this->mDisable = false;
+	}
+
+	/**
+	 * Whether DB/cache usage is disabled for determining messages
+	 *
+	 * If so, this typically indicates either:
+	 *   - a) load() failed to find a cached copy nor query the DB
+	 *   - b) we are in a special context or error mode that cannot use the DB
+	 * If the DB is ignored, any derived HTML output or cached objects may be wrong.
+	 * To avoid long-term cache pollution, TTLs can be adjusted accordingly.
+	 *
+	 * @return bool
+	 * @since 1.27
+	 */
+	public function isDisabled() {
+		return $this->mDisable;
+	}
+
+	/**
+	 * Clear all stored messages in global and local cache
+	 *
+	 * Mainly used after a mass rebuild
+	 */
+	public function clear() {
+		$langs = $this->languageNameUtils->getLanguageNames();
+		foreach ( array_keys( $langs ) as $code ) {
+			$this->wanCache->touchCheckKey( $this->getCheckKey( $code ) );
+		}
+		$this->cache->clear();
+	}
+
+	/**
+	 * @param string $key
+	 * @return array
+	 */
+	public function figureMessage( $key ) {
+		$pieces = explode( '/', $key );
+		if ( count( $pieces ) < 2 ) {
+			return [ $key, $this->contLangCode ];
+		}
+
+		$lang = array_pop( $pieces );
+		if ( !$this->languageNameUtils->getLanguageName(
+			$lang,
+			LanguageNameUtils::AUTONYMS,
+			LanguageNameUtils::DEFINED
+		) ) {
+			return [ $key, $this->contLangCode ];
+		}
+
+		$message = implode( '/', $pieces );
+
+		return [ $message, $lang ];
+	}
+
+	/**
+	 * Get all message keys stored in the message cache for a given language.
+	 * If $code is the content language code, this will return all message keys
+	 * for which MediaWiki:msgkey exists. If $code is another language code, this
+	 * will ONLY return message keys for which MediaWiki:msgkey/$code exists.
+	 * @param string $code Language code
+	 * @return string[]|null Array of message keys
+	 */
+	public function getAllMessageKeys( $code ) {
+		$this->load( $code );
+		if ( !$this->cache->has( $code ) ) {
+			// Apparently load() failed
+			return null;
+		}
+		// Remove administrative keys
+		$cache = $this->cache->get( $code );
+		unset( $cache['VERSION'] );
+		unset( $cache['EXPIRY'] );
+		unset( $cache['EXCESSIVE'] );
+		// Remove any !NONEXISTENT keys
+		$cache = array_diff( $cache, [ '!NONEXISTENT' ] );
+
+		// Keys may appear with a capital first letter. lcfirst them.
+		return array_map( [ $this->contLang, 'lcfirst' ], array_keys( $cache ) );
+	}
+
+	/**
+	 * Purge message caches when a MediaWiki: page is created, updated, or deleted
+	 *
+	 * @param LinkTarget $linkTarget Message page title
+	 * @param Content|null $content New content for edit/create, null on deletion
+	 * @since 1.29
+	 */
+	public function updateMessageOverride( LinkTarget $linkTarget, Content $content = null ) {
+		$msgText = $this->getMessageTextFromContent( $content );
+		if ( $msgText === null ) {
+			$msgText = false; // treat as not existing
+		}
+
+		$this->replace( $linkTarget->getDBkey(), $msgText );
+
+		if ( $this->contLangConverter->hasVariants() ) {
+			$this->contLangConverter->updateConversionTable( $linkTarget );
+		}
+	}
+
+	/**
+	 * @param string $code Language code
+	 * @return string WAN cache key usable as a "check key" against language page edits
+	 */
+	public function getCheckKey( $code ) {
+		return $this->wanCache->makeKey( 'messages', $code );
+	}
+
+	/**
+	 * @param Content|null $content Content or null if the message page does not exist
+	 * @return string|false|null Returns false if $content is null and null on error
+	 */
+	private function getMessageTextFromContent( Content $content = null ) {
+		// @TODO: could skip pseudo-messages like js/css here, based on content model
+		if ( $content ) {
+			// Message page exists...
+			// XXX: Is this the right way to turn a Content object into a message?
+			// NOTE: $content is typically either WikitextContent, JavaScriptContent or
+			//       CssContent. MessageContent is *not* used for storing messages, it's
+			//       only used for wrapping them when needed.
+			$msgText = $content->getWikitextForTransclusion();
+			if ( $msgText === false || $msgText === null ) {
+				// This might be due to some kind of misconfiguration...
+				$msgText = null;
+				$this->logger->warning(
+					__METHOD__ . ": message content doesn't provide wikitext "
+					. "(content model: " . $content->getModel() . ")" );
+			}
+		} else {
+			// Message page does not exist...
+			$msgText = false;
+		}
+
+		return $msgText;
+	}
+
+	/**
+	 * @param string $hash Hash for this version of the entire key/value overrides map
+	 * @param string $title Message cache key with initial uppercase letter
+	 * @return string
+	 */
+	private function bigMessageCacheKey( $hash, $title ) {
+		return $this->wanCache->makeKey( 'messages-big', $hash, $title );
+	}
+}
diff --git a/includes/language/dependency/CacheDependency.php b/includes/language/dependency/CacheDependency.php
new file mode 100644
index 000000000000..9bc71ea845be
--- /dev/null
+++ b/includes/language/dependency/CacheDependency.php
@@ -0,0 +1,40 @@
+<?php
+/**
+ * Data caching with dependencies.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Cache
+ */
+
+/**
+ * @stable to extend
+ * @ingroup Cache
+ */
+abstract class CacheDependency {
+	/**
+	 * Returns true if the dependency is expired, false otherwise
+	 */
+	abstract public function isExpired();
+
+	/**
+	 * Hook to perform any expensive pre-serialize loading of dependency values.
+	 * @stable to override
+	 */
+	public function loadDependencyValues() {
+	}
+}
diff --git a/includes/language/dependency/ConstantDependency.php b/includes/language/dependency/ConstantDependency.php
new file mode 100644
index 000000000000..1f62093640f4
--- /dev/null
+++ b/includes/language/dependency/ConstantDependency.php
@@ -0,0 +1,42 @@
+<?php
+/**
+ * Data caching with dependencies.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Cache
+ */
+
+/**
+ * @ingroup Cache
+ */
+class ConstantDependency extends CacheDependency {
+	private $name;
+	private $value;
+
+	public function __construct( $name ) {
+		$this->name = $name;
+		$this->value = constant( $name );
+	}
+
+	/**
+	 * @return bool
+	 */
+	public function isExpired() {
+		return constant( $this->name ) != $this->value;
+	}
+}
diff --git a/includes/language/dependency/DependencyWrapper.php b/includes/language/dependency/DependencyWrapper.php
new file mode 100644
index 000000000000..789a24a0b095
--- /dev/null
+++ b/includes/language/dependency/DependencyWrapper.php
@@ -0,0 +1,130 @@
+<?php
+/**
+ * Data caching with dependencies.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Cache
+ */
+
+/**
+ * This class stores an arbitrary value along with its dependencies.
+ * Users should typically only use DependencyWrapper::getValueFromCache(),
+ * rather than instantiating one of these objects directly.
+ * @ingroup Cache
+ */
+class DependencyWrapper {
+	private $value;
+	/** @var CacheDependency[] */
+	private $deps;
+
+	/**
+	 * @param mixed $value The user-supplied value
+	 * @param CacheDependency|CacheDependency[] $deps A dependency or dependency
+	 *   array. All dependencies must be objects implementing CacheDependency.
+	 */
+	public function __construct( $value = false, $deps = [] ) {
+		$this->value = $value;
+
+		if ( !is_array( $deps ) ) {
+			$deps = [ $deps ];
+		}
+
+		$this->deps = $deps;
+	}
+
+	/**
+	 * Returns true if any of the dependencies have expired
+	 *
+	 * @return bool
+	 */
+	public function isExpired() {
+		foreach ( $this->deps as $dep ) {
+			if ( $dep->isExpired() ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
+	/**
+	 * Initialise dependency values in preparation for storing. This must be
+	 * called before serialization.
+	 */
+	public function initialiseDeps() {
+		foreach ( $this->deps as $dep ) {
+			$dep->loadDependencyValues();
+		}
+	}
+
+	/**
+	 * Get the user-defined value
+	 * @return bool|mixed
+	 */
+	public function getValue() {
+		return $this->value;
+	}
+
+	/**
+	 * Store the wrapper to a cache
+	 *
+	 * @param BagOStuff $cache
+	 * @param string $key
+	 * @param int $expiry
+	 */
+	public function storeToCache( $cache, $key, $expiry = 0 ) {
+		$this->initialiseDeps();
+		$cache->set( $key, $this, $expiry );
+	}
+
+	/**
+	 * Attempt to get a value from the cache. If the value is expired or missing,
+	 * it will be generated with the callback function (if present), and the newly
+	 * calculated value will be stored to the cache in a wrapper.
+	 *
+	 * @param BagOStuff $cache
+	 * @param string $key The cache key
+	 * @param int $expiry The expiry timestamp or interval in seconds
+	 * @param bool|callable $callback The callback for generating the value, or false
+	 * @param array $callbackParams The function parameters for the callback
+	 * @param array $deps The dependencies to store on a cache miss. Note: these
+	 *    are not the dependencies used on a cache hit! Cache hits use the stored
+	 *    dependency array.
+	 *
+	 * @return mixed The value, or null if it was not present in the cache and no
+	 *    callback was defined.
+	 */
+	public static function getValueFromCache( $cache, $key, $expiry = 0, $callback = false,
+		$callbackParams = [], $deps = []
+	) {
+		$obj = $cache->get( $key );
+
+		if ( is_object( $obj ) && $obj instanceof DependencyWrapper && !$obj->isExpired() ) {
+			$value = $obj->value;
+		} elseif ( $callback ) {
+			$value = $callback( ...$callbackParams );
+			# Cache the newly-generated value
+			$wrapper = new DependencyWrapper( $value, $deps );
+			$wrapper->storeToCache( $cache, $key, $expiry );
+		} else {
+			$value = null;
+		}
+
+		return $value;
+	}
+}
diff --git a/includes/language/dependency/FileDependency.php b/includes/language/dependency/FileDependency.php
new file mode 100644
index 000000000000..1cc2379ff668
--- /dev/null
+++ b/includes/language/dependency/FileDependency.php
@@ -0,0 +1,101 @@
+<?php
+/**
+ * Data caching with dependencies.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Cache
+ */
+
+use Wikimedia\AtEase\AtEase;
+
+/**
+ * @newable
+ * @ingroup Cache
+ */
+class FileDependency extends CacheDependency {
+	private $filename;
+	private $timestamp;
+
+	/**
+	 * Create a file dependency
+	 *
+	 * @stable to call
+	 *
+	 * @param string $filename The name of the file, preferably fully qualified
+	 * @param null|bool|int $timestamp The unix last modified timestamp, or false if the
+	 *        file does not exist. If omitted, the timestamp will be loaded from
+	 *        the file.
+	 *
+	 * A dependency on a nonexistent file will be triggered when the file is
+	 * created. A dependency on an existing file will be triggered when the
+	 * file is changed.
+	 */
+	public function __construct( $filename, $timestamp = null ) {
+		$this->filename = $filename;
+		$this->timestamp = $timestamp;
+	}
+
+	/**
+	 * @return array
+	 */
+	public function __sleep() {
+		$this->loadDependencyValues();
+
+		return [ 'filename', 'timestamp' ];
+	}
+
+	public function loadDependencyValues() {
+		if ( $this->timestamp === null ) {
+			AtEase::suppressWarnings();
+			# Dependency on a non-existent file stores "false"
+			# This is a valid concept!
+			$this->timestamp = filemtime( $this->filename );
+			AtEase::restoreWarnings();
+		}
+	}
+
+	/**
+	 * @return bool
+	 */
+	public function isExpired() {
+		AtEase::suppressWarnings();
+		$lastmod = filemtime( $this->filename );
+		AtEase::restoreWarnings();
+		if ( $lastmod === false ) {
+			if ( $this->timestamp === false ) {
+				# Still nonexistent
+				return false;
+			}
+
+			# Deleted
+			wfDebug( "Dependency triggered: {$this->filename} deleted." );
+
+			return true;
+		}
+
+		if ( $lastmod > $this->timestamp ) {
+			# Modified or created
+			wfDebug( "Dependency triggered: {$this->filename} changed." );
+
+			return true;
+		}
+
+		# Not modified
+		return false;
+	}
+}
diff --git a/includes/language/dependency/GlobalDependency.php b/includes/language/dependency/GlobalDependency.php
new file mode 100644
index 000000000000..15a3e90b113d
--- /dev/null
+++ b/includes/language/dependency/GlobalDependency.php
@@ -0,0 +1,46 @@
+<?php
+/**
+ * Data caching with dependencies.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Cache
+ */
+
+/**
+ * @ingroup Cache
+ */
+class GlobalDependency extends CacheDependency {
+	private $name;
+	private $value;
+
+	public function __construct( $name ) {
+		$this->name = $name;
+		$this->value = $GLOBALS[$name];
+	}
+
+	/**
+	 * @return bool
+	 */
+	public function isExpired() {
+		if ( !isset( $GLOBALS[$this->name] ) ) {
+			return true;
+		}
+
+		return $GLOBALS[$this->name] != $this->value;
+	}
+}
diff --git a/includes/language/dependency/MainConfigDependency.php b/includes/language/dependency/MainConfigDependency.php
new file mode 100644
index 000000000000..4eab7268eee6
--- /dev/null
+++ b/includes/language/dependency/MainConfigDependency.php
@@ -0,0 +1,51 @@
+<?php
+/**
+ * Data caching with dependencies.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Cache
+ */
+use MediaWiki\MediaWikiServices;
+
+/**
+ * @ingroup Cache
+ */
+class MainConfigDependency extends CacheDependency {
+	private $name;
+	private $value;
+
+	public function __construct( $name ) {
+		$this->name = $name;
+		$this->value = $this->getConfig()->get( $this->name );
+	}
+
+	private function getConfig() {
+		return MediaWikiServices::getInstance()->getMainConfig();
+	}
+
+	/**
+	 * @return bool
+	 */
+	public function isExpired() {
+		if ( !$this->getConfig()->has( $this->name ) ) {
+			return true;
+		}
+
+		return $this->getConfig()->get( $this->name ) != $this->value;
+	}
+}