Issue #2936941 by alexpott: Diff can infinitely loop since #2885309

core/lib/Drupal/Component/Diff/Engine/DiffEngine.php

@@ -201,8 +201,9 @@ protected function _diag($xoff, $xlim, $yoff, $ylim, $nchunks) {
           continue;
         }
         $matches = $ymatches[$line];
+        $found_empty = FALSE;
         foreach ($matches as $y) {
-          if (!isset($found_empty)) {
+          if (!$found_empty) {
             if (empty($this->in_seq[$y])) {
               $k = $this->_lcs_pos($y);
               $this::USE_ASSERTS && assert($k > 0);

core/tests/Drupal/Tests/Component/Diff/Engine/DiffEngineTest.php

@@ -86,4 +86,21 @@ public function testDiff($expected, $from, $to) {
     }
   }
 
+  /**
+   * Tests that two files can be successfully diffed.
+   *
+   * @covers ::diff
+   */
+  public function testDiffInfiniteLoop() {
+    $from = explode("\n", file_get_contents(__DIR__ . '/fixtures/file1.txt'));
+    $to = explode("\n", file_get_contents(__DIR__ . '/fixtures/file2.txt'));
+    $diff_engine = new DiffEngine();
+    $diff = $diff_engine->diff($from, $to);
+    $this->assertCount(4, $diff);
+    $this->assertEquals($diff[0], new DiffOpDelete(['    - image.style.max_650x650']));
+    $this->assertEquals($diff[1], new DiffOpCopy(['    - image.style.max_325x325']));
+    $this->assertEquals($diff[2], new DiffOpAdd(['    - image.style.max_650x650', '_core:', '  default_config_hash: 3mjM9p-kQ8syzH7N8T0L9OnCJDSPvHAZoi3q6jcXJKM']));
+    $this->assertEquals($diff[3], new DiffOpCopy(['fallback_image_style: max_325x325', '']));
+  }
+
 }

core/tests/Drupal/Tests/Component/Diff/Engine/fixtures/file1.txt

+    - image.style.max_650x650
+    - image.style.max_325x325
+fallback_image_style: max_325x325

core/tests/Drupal/Tests/Component/Diff/Engine/fixtures/file2.txt

+    - image.style.max_325x325
+    - image.style.max_650x650
+_core:
+  default_config_hash: 3mjM9p-kQ8syzH7N8T0L9OnCJDSPvHAZoi3q6jcXJKM
+fallback_image_style: max_325x325