Diffs with different line endings leads to Invalid $mode 3 specified

Simple fix, correctly fixes the failed test and fixes our issues on a client project.

Left a question on the MR

Restoring status.

Thanks for working on this. It seems my initial implementation fell short on a number of things. Sorry for that.

The good thing is that we are adding test coverage for cases that were totally untested in the prior implementation - a more reliable pattern against possible future regressions.

In a couple of similar issues earlier, 🐛 DiffOpOutputBuilder does not correctly match the 10.0.x implementation and produces unexpected output Fixed and 🐛 DiffOpOutputBuilder does not correctly match the 10.0.x implementation and produces unexpected output (part 2) Needs review , @alexpott also added the test case to the @legacy test for DiffEngine, so to show that the change is consistent with earlier behavior. I think it should be done here, too. NW for that.

Added test coverage to DiffEngineTest

OK, so now we have the regression fixed.

Since this is making a 'change' out of two exactly same lines that only differ by the EOL (being UNIX-like or WINDOWS-like), I was wondering: this is technically correct, but for a human it would make no difference. So I checked usage of Diff, and I find one only in core, where the strings are certainly stripped of the '\n' (but I am unsure about the '\r'), BEFORE they are sent for diffing. So this one might be an edge case. I think that would deserve some comments somewhere re. how the array of strings passed to Diff should be preprocessed for EOL markings.

From ConfigManager:

  /**
   * {@inheritdoc}
   */
  public function diff(StorageInterface $source_storage, StorageInterface $target_storage, $source_name, $target_name = NULL, $collection = StorageInterface::DEFAULT_COLLECTION) {
    if ($collection != StorageInterface::DEFAULT_COLLECTION) {
      $source_storage = $source_storage->createCollection($collection);
      $target_storage = $target_storage->createCollection($collection);
    }
    if (!isset($target_name)) {
      $target_name = $source_name;
    }
    // The output should show configuration object differences formatted as YAML.
    // But the configuration is not necessarily stored in files. Therefore, they
    // need to be read and parsed, and lastly, dumped into YAML strings.
    $source_data = explode("\n", Yaml::encode($source_storage->read($source_name)));
    $target_data = explode("\n", Yaml::encode($target_storage->read($target_name)));

    // Check for new or removed files.
    if ($source_data === ['false']) {
      // Added file.
      // Cast the result of t() to a string, as the diff engine doesn't know
      // about objects.
      $source_data = [(string) $this->t('File added')];
    }
    if ($target_data === ['false']) {
      // Deleted file.
      // Cast the result of t() to a string, as the diff engine doesn't know
      // about objects.
      $target_data = [(string) $this->t('File removed')];
    }

    return new Diff($source_data, $target_data);
  }

Also, I think additional test cases should be provided for DiffFormatterTest, that is currently only testing array of strings free from any EOL markings.