Rethink diff limits

Now, instead of "if your changed paragraphs are larger than 10Kb, you're
screwed":
* Instead of relying on overall length, estimate complexity after splitting to words
  and taking any equal head and tail out of equation.
* Estimate based on words changed, which better reflects the actual complexity
  of generating a diff.
* New limit is determined scientifically, i.e. "above that number XDebug starts
  complaining about recursion limits reached in Vagrant".

Caveat: if new limits are hit, the consequences are more widespread as all adjacent
changed paragraphs are displayed without word level diffs, as opposed to only the
paragraph that's too long being affected. However, the new limit is much higher and
in wikitext you're supposed to put empty lines between paragraphs anyway, negating
this problem.

Bug: T128697
Change-Id: I4e91c7c40f5afdd116b847a859b8517522302489

1 files changed, 30 insertions, 0 deletions

diff --git a/includes/diff/ComplexityException.php b/includes/diff/ComplexityException.php
new file mode 100644
index 000000000000..10ca964ac236
--- /dev/null
+++ b/includes/diff/ComplexityException.php
@@ -0,0 +1,30 @@
+<?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
+ * @ingroup DifferenceEngine
+ */
+
+namespace MediaWiki\Diff;
+
+use Exception;
+
+class ComplexityException extends Exception {
+	public function __construct() {
+		parent::__construct( 'Diff is too complex to generate' );
+	}
+}