Don't treat configured but absent engine as executable

E.g. if you have $wgExternalDiffEngine = 'wikidiff2' but after
a PHP update you no longer have the module you still shouldn't attempt
to shell out to some nonexistent wikidiff2.

Bug: T74030
Change-Id: I745cd1cb2e152f4fbb95c8f782d70117f8c844f1

diff --git a/includes/diff/DifferenceEngine.php b/includes/diff/DifferenceEngine.php
index 622137a..1fa1970 100644 (file)
--- a/includes/diff/DifferenceEngine.php
+++ b/includes/diff/DifferenceEngine.php
@@ -852,15 +852,16 @@ class DifferenceEngine extends ContextSource {
                        $wgExternalDiffEngine = false;
                }
 
-               if ( $wgExternalDiffEngine == 'wikidiff2' && function_exists( 'wikidiff2_do_diff' ) ) {
-                       # Better external diff engine, the 2 may some day be dropped
-                       # This one does the escaping and segmenting itself
-                       $text = wikidiff2_do_diff( $otext, $ntext, 2 );
-                       $text .= $this->debug( 'wikidiff2' );
-
-                       return $text;
-               }
-               if ( $wgExternalDiffEngine != 'wikidiff3' && $wgExternalDiffEngine !== false ) {
+               if ( $wgExternalDiffEngine == 'wikidiff2' ) {
+                       if ( function_exists( 'wikidiff2_do_diff' ) ) {
+                               # Better external diff engine, the 2 may some day be dropped
+                               # This one does the escaping and segmenting itself
+                               $text = wikidiff2_do_diff( $otext, $ntext, 2 );
+                               $text .= $this->debug( 'wikidiff2' );
+
+                               return $text;
+                       }
+               } elseif ( $wgExternalDiffEngine != 'wikidiff3' && $wgExternalDiffEngine !== false ) {
                        # Diff via the shell
                        $tmpDir = wfTempDir();
                        $tempName1 = tempnam( $tmpDir, 'diff_' );