Add $wgDiffEngine

The immediate use case is for testing, where some tests need to use the
PHP implementation even when wikidiff2 is installed.

Bug: T237049
Change-Id: I41dc4c0933429065d7638f518ec31f0a056afc41

RELEASE-NOTES-1.35

@@ -29,8 +29,9 @@ For notes on 1.34.x and older releases, see HISTORY.
 …
 
 ==== New configuration ====
-* …
-
+* $wgDiffEngine can be used to specify the difference engine to use, rather
+  than choosing the first of $wgExternalDiffEngine, wikidiff2, or php that
+  is usable.
 
 ==== Changed configuration ====
 * $wgResourceLoaderMaxage (T235314) - This configuration array controls the

includes/DefaultSettings.php

@@ -8480,10 +8480,23 @@ $wgUpdateRowsPerQuery = 100;
  */
 
 /**
- * Name of the external diff engine to use. Supported values:
- * * string: path to an external diff executable
- * * false: wikidiff2 PHP extension if installed, otherwise the default PHP implementation
- * * 'wikidiff', 'wikidiff2', and 'wikidiff3' are treated as false for backwards compatibility
+ * Specify the difference engine to use.
+ *
+ * Supported values:
+ * - 'external': Use an external diff engine, which must be specified via $wgExternalDiffEngine
+ * - 'wikidiff2': Use the wikidiff2 PHP extension
+ * - 'php': PHP implementations included in MediaWiki
+ *
+ * The default (null) is to use the first engine that's available.
+ *
+ * @since 1.35
+ * @var string|null
+ */
+$wgDiffEngine = null;
+
+/**
+ * Name of the external diff engine to use.
+ * @var string|false Path to an external diff executable
  */
 $wgExternalDiffEngine = false;
 

includes/content/ContentHandler.php

@@ -651,7 +651,7 @@ abstract class ContentHandler {
 		$slotDiffRenderer->setLanguage( $contentLanguage );
 
 		$engine = DifferenceEngine::getEngine();
-		if ( $engine === false ) {
+		if ( $engine === 'php' ) {
 			$slotDiffRenderer->setEngine( TextSlotDiffRenderer::ENGINE_PHP );
 		} elseif ( $engine === 'wikidiff2' ) {
 			$slotDiffRenderer->setEngine( TextSlotDiffRenderer::ENGINE_WIKIDIFF2 );

includes/diff/DifferenceEngine.php

@@ -1232,7 +1232,7 @@ class DifferenceEngine extends ContextSource {
 		$engine = $this->getEngine();
 		$params = [
 			'diff',
-			$engine,
+			$engine === 'php' ? false : $engine, // Back compat
 			self::DIFF_VERSION,
 			"old-{$this->mOldid}",
 			"rev-{$this->mNewid}"
@@ -1342,32 +1342,59 @@ class DifferenceEngine extends ContextSource {
 	}
 
 	/**
-	 * Process ExternalDiffEngine config and get a sane, usable engine
+	 * Process DiffEngine config and get a sane, usable engine
 	 *
-	 * @return bool|string 'wikidiff2', path to an executable, or false
+	 * @return string 'wikidiff2', 'php', or path to an executable
 	 * @internal For use by this class and TextSlotDiffRenderer only.
 	 */
 	public static function getEngine() {
+		$diffEngine = MediaWikiServices::getInstance()->getMainConfig()
+			->get( 'DiffEngine' );
 		$externalDiffEngine = MediaWikiServices::getInstance()->getMainConfig()
 			->get( 'ExternalDiffEngine' );
 
-		if ( $externalDiffEngine ) {
-			if ( is_string( $externalDiffEngine ) ) {
-				if ( is_executable( $externalDiffEngine ) ) {
-					return $externalDiffEngine;
-				}
-				wfDebug( 'ExternalDiffEngine config points to a non-executable, ignoring' );
-			} else {
-				wfWarn( 'ExternalDiffEngine config is set to a non-string value, ignoring' );
+		if ( $diffEngine === null ) {
+			$engines = [ 'external', 'wikidiff2', 'php' ];
+		} else {
+			$engines = [ $diffEngine ];
+		}
+
+		$failureReason = null;
+		foreach ( $engines as $engine ) {
+			switch ( $engine ) {
+				case 'external':
+					if ( is_string( $externalDiffEngine ) ) {
+						if ( is_executable( $externalDiffEngine ) ) {
+							return $externalDiffEngine;
+						}
+						$failureReason = 'ExternalDiffEngine config points to a non-executable';
+						if ( $diffEngine === null ) {
+							wfDebug( "$failureReason, ignoring" );
+						}
+					} else {
+						$failureReason = 'ExternalDiffEngine config is set to a non-string value';
+						if ( $diffEngine === null && $externalDiffEngine ) {
+							wfWarn( "$failureReason, ignoring" );
+						}
+					}
+					break;
+
+				case 'wikidiff2':
+					if ( function_exists( 'wikidiff2_do_diff' ) ) {
+						return 'wikidiff2';
+					}
+					$failureReason = 'wikidiff2 is not available';
+					break;
+
+				case 'php':
+					// Always available.
+					return 'php';
+
+				default:
+					throw new DomainException( 'Invalid value for $wgDiffEngine: ' . $engine );
 			}
 		}
-
-		if ( function_exists( 'wikidiff2_do_diff' ) ) {
-			return 'wikidiff2';
-		}
-
-		// Native PHP
-		return false;
+		throw new UnexpectedValueException( "Cannot use diff engine '$engine': $failureReason" );
 	}
 
 	/**
@@ -1421,7 +1448,7 @@ class DifferenceEngine extends ContextSource {
 		$engine = self::getEngine();
 		if ( $engine === 'wikidiff2' ) {
 			return $this->debug( 'wikidiff2' );
-		} elseif ( $engine === false ) {
+		} elseif ( $engine === 'php' ) {
 			return $this->debug( 'native PHP' );
 		} else {
 			return $this->debug( "external $engine" );

tests/phpunit/includes/api/ApiComparePagesTest.php

@@ -121,6 +121,8 @@ class ApiComparePagesTest extends ApiTestCase {
 	 * @dataProvider provideDiff
 	 */
 	public function testDiff( $params, $expect, $exceptionCode = false, $sysop = false ) {
+		$this->setMwGlobals( [ 'wgDiffEngine' => 'php' ] );
+
 		$this->doReplacements( $params );
 
 		$params += [

tests/phpunit/includes/diff/DifferenceEngineTest.php

@@ -32,6 +32,8 @@ class DifferenceEngineTest extends MediaWikiTestCase {
 		if ( !self::$revisions ) {
 			self::$revisions = $this->doEdits();
 		}
+
+		$this->setMwGlobals( [ 'wgDiffEngine' => 'php' ] );
 	}
 
 	/**

tests/phpunit/integration/includes/diff/DifferenceEngineSlotDiffRendererIntegrationTest.php

@@ -9,7 +9,10 @@ class DifferenceEngineSlotDiffRendererIntegrationTest extends \MediaWikiIntegrat
 	 * @covers DifferenceEngineSlotDiffRenderer::getExtraCacheKeys
 	 */
 	public function testGetExtraCacheKeys_noExternalDiffEngineConfigured() {
-		$this->setMwGlobals( [ 'wgExternalDiffEngine' => null ] );
+		$this->setMwGlobals( [
+			'wgDiffEngine' => null,
+			'wgExternalDiffEngine' => null,
+		] );
 
 		$differenceEngine = new CustomDifferenceEngine();
 		$slotDiffRenderer = new DifferenceEngineSlotDiffRenderer( $differenceEngine );