Fix bug where appending to a nested block list before an outdented sibling failed

[slashblog]

• This change addresses the following issue I raised earlier. The original issue has more details (including the cause of the issue and steps to reproduce), but briefly adding an item to a list causes the following exception to be thrown: "Failed to produce valid YAML after modification.". This change fixes this issue.

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Code Review

This pull request updates the block list appending logic in yaml_edit to trim trailing whitespace when finding the next newline index. The reviewer identified a performance issue where creating a substring on every append operation leads to O(N) time and memory complexity, as well as a potential RangeError crash if the index becomes negative. An optimized O(1) auxiliary space solution was suggested to scan backwards instead.

[sigurdm]

Thanks for investigating and finding the cause of #2481!

Instead of manually backtracking whitespace characters with a while loop, package:yaml_edit already provides an internal helper getContentSensitiveEnd(YamlNode) in lib/src/utils.dart for this exact scenario (see how _addToBlockMap in lib/src/map_mutations.dart uses it).

We can simplify _appendToBlockList in lib/src/list_mutations.dart to:

  if (list.isNotEmpty) {
    final lastValueSpanEnd = getContentSensitiveEnd(list.nodes.last);
    final nextNewLineIndex = yaml.indexOf('\n', lastValueSpanEnd);
    if (nextNewLineIndex == -1) {
      formattedValue = getLineEnding(yaml) + formattedValue;
    } else {
      offset = nextNewLineIndex + 1;
    }
  }

Could you also:

• Add a regression test for this in test/append_test.dart.
• Add a note to CHANGELOG.md.

[slashblog]

@sigurdm Done changes as per your comment.

[sigurdm]

Please update the PR title to say what the change does, rather than repeat the issue number

[slashblog]

@sigurdm I have changed both PR title and Changelog entry. Please do the needful.