[FileBackend] Stat caching improvements.

Callers often tend to end up calling getFileStat(), at least indirectly,
or in various successive function on the same path. This created RTTs
when the file didn't exist since negatives were not cached. This change
does the following:

* Cache definitive negatives (404s) in the process cache.
  Nothing is cached on failure (like network problems).
* Ignore process cache entries after a brief time period
  so long running scripts do not have overly stale entries.

Change-Id: I356bd9f48281e3c7e7a273778b2aca59c521a0c7

1 files changed, 13 insertions, 7 deletions

diff --git a/includes/filebackend/FileBackendStore.php b/includes/filebackend/FileBackendStore.php
index 97e49a5a0925..43e6cb3ed84a 100644
--- a/includes/filebackend/FileBackendStore.php
+++ b/includes/filebackend/FileBackendStore.php
@@ -48,6 +48,8 @@ abstract class FileBackendStore extends FileBackend {
 
 	protected $maxFileSize = 4294967296; // integer bytes (4GiB)
 
+	const CACHE_TTL = 10; // integer; TTL in seconds for process cache entries
+
 	/**
 	 * @see FileBackend::__construct()
 	 *
@@ -601,14 +603,15 @@ abstract class FileBackendStore extends FileBackend {
 		wfProfileIn( __METHOD__ );
 		wfProfileIn( __METHOD__ . '-' . $this->name );
 		$latest = !empty( $params['latest'] ); // use latest data?
-		if ( !$this->cheapCache->has( $path, 'stat' ) ) {
+		if ( !$this->cheapCache->has( $path, 'stat', self::CACHE_TTL ) ) {
 			$this->primeFileCache( array( $path ) ); // check persistent cache
 		}
-		if ( $this->cheapCache->has( $path, 'stat' ) ) {
+		if ( $this->cheapCache->has( $path, 'stat', self::CACHE_TTL ) ) {
 			$stat = $this->cheapCache->get( $path, 'stat' );
 			// If we want the latest data, check that this cached
-			// value was in fact fetched with the latest available data.
-			if ( !$latest || $stat['latest'] ) {
+			// value was in fact fetched with the latest available data
+			// (the process cache is ignored if it contains a negative).
+			if ( !$latest || ( is_array( $stat ) && $stat['latest'] ) ) {
 				wfProfileOut( __METHOD__ . '-' . $this->name );
 				wfProfileOut( __METHOD__ );
 				return $stat;
@@ -619,7 +622,7 @@ abstract class FileBackendStore extends FileBackend {
 		$stat = $this->doGetFileStat( $params );
 		wfProfileOut( __METHOD__ . '-miss-' . $this->name );
 		wfProfileOut( __METHOD__ . '-miss' );
-		if ( is_array( $stat ) ) { // don't cache negatives
+		if ( is_array( $stat ) ) { // file exists
 			$stat['latest'] = $latest;
 			$this->cheapCache->set( $path, 'stat', $stat );
 			$this->setFileCache( $path, $stat ); // update persistent cache
@@ -627,8 +630,11 @@ abstract class FileBackendStore extends FileBackend {
 				$this->cheapCache->set( $path, 'sha1',
 					array( 'hash' => $stat['sha1'], 'latest' => $latest ) );
 			}
-		} else {
+		} elseif ( $stat === false ) { // file does not exist
+			$this->cheapCache->set( $path, 'stat', false );
 			wfDebug( __METHOD__ . ": File $path does not exist.\n" );
+		} else { // an error occurred
+			wfDebug( __METHOD__ . ": Could not stat file $path.\n" );
 		}
 		wfProfileOut( __METHOD__ . '-' . $this->name );
 		wfProfileOut( __METHOD__ );
@@ -682,7 +688,7 @@ abstract class FileBackendStore extends FileBackend {
 		wfProfileIn( __METHOD__ );
 		wfProfileIn( __METHOD__ . '-' . $this->name );
 		$latest = !empty( $params['latest'] ); // use latest data?
-		if ( $this->cheapCache->has( $path, 'sha1' ) ) {
+		if ( $this->cheapCache->has( $path, 'sha1', self::CACHE_TTL ) ) {
 			$stat = $this->cheapCache->get( $path, 'sha1' );
 			// If we want the latest data, check that this cached
 			// value was in fact fetched with the latest available data.