From 47b5d95f12c9288dbeabaae46dd8a369b5797f5c Mon Sep 17 00:00:00 2001
From: =?utf8?q?Gerg=C5=91=20Tisza?= <tgr.huwiki@gmail.com>
Date: Mon, 13 Aug 2018 18:53:59 +0200
Subject: [PATCH] Create DifferenceEngine before calling GetDifferenceEngine

This way the hook is able to consider what kind of DifferenceEngine
was created by default, or even wrap/decorate it.

Change-Id: Ibb6b1b087a2dda445be2b5e8c7518c7d169b5192
---
 docs/hooks.txt                      |  6 ++----
 includes/content/ContentHandler.php | 20 ++++++++++++--------
 2 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/docs/hooks.txt b/docs/hooks.txt
index 219c51f30e..9a634ad35d 100644
--- a/docs/hooks.txt
+++ b/docs/hooks.txt
@@ -1670,15 +1670,13 @@ $title: Title object that we need to get a sortkey for
 &$sortkey: Sortkey to use.
 
 'GetDifferenceEngine': Called when getting a new difference engine interface
-object Return false for valid object in $differenceEngine or true for the
-default difference engine.
+object.  Can be used to decorate or replace the default difference engine.
 $context: IContextSource context to be used for diff
 $old: Revision ID to show and diff with
 $new: Either a revision ID or one of the strings 'cur', 'prev' or 'next'
 $refreshCache: If set, refreshes the diff cache
 $unhide: If set, allow viewing deleted revs
-&$differenceEngine: output parameter, difference engine object to be used for
-  diff
+&$differenceEngine: The difference engine object to be used for the diff
 
 'GetDoubleUnderscoreIDs': Modify the list of behavior switch (double
 underscore) magic words. Called by MagicWord.
diff --git a/includes/content/ContentHandler.php b/includes/content/ContentHandler.php
index 004e38a5ef..0a451b2fa3 100644
--- a/includes/content/ContentHandler.php
+++ b/includes/content/ContentHandler.php
@@ -614,6 +614,14 @@ abstract class ContentHandler {
 	/**
 	 * Factory for creating an appropriate DifferenceEngine for this content model.
 	 *
+	 * The DifferenceEngine subclass to use is selected in getDiffEngineClass(). The
+	 * GetDifferenceEngine hook will receive the DifferenceEngine object and can replace or
+	 * wrap it.
+	 * (Note that in older versions of MediaWiki the hook documentation instructed extensions
+	 * to return false from the hook; you should not rely on always being able to decorate
+	 * the DifferenceEngine instance from the hook. If the owner of the content type wants to
+	 * decorare the instance, overriding this method is a safer approach.)
+	 *
 	 * @since 1.21
 	 *
 	 * @param IContextSource $context Context to use, anything else will be ignored.
@@ -629,15 +637,11 @@ abstract class ContentHandler {
 		$rcid = 0, // FIXME: Deprecated, no longer used
 		$refreshCache = false, $unhide = false
 	) {
-		// hook: get difference engine
-		$differenceEngine = null;
-		if ( !Hooks::run( 'GetDifferenceEngine',
-			[ $context, $old, $new, $refreshCache, $unhide, &$differenceEngine ]
-		) ) {
-			return $differenceEngine;
-		}
 		$diffEngineClass = $this->getDiffEngineClass();
-		return new $diffEngineClass( $context, $old, $new, $rcid, $refreshCache, $unhide );
+		$differenceEngine = new $diffEngineClass( $context, $old, $new, $rcid, $refreshCache, $unhide );
+		Hooks::run( 'GetDifferenceEngine', [ $context, $old, $new, $refreshCache, $unhide,
+			&$differenceEngine ] );
+		return $differenceEngine;
 	}
 
 	/**
-- 
2.20.1