Fix FormData clone boundary inconsistency issue

[helloliuyiming]

Resolves #2303

Fix boundary inconsistency in FormData.clone method

Description:

This PR addresses an issue where cloning FormData objects resulted in different boundaries each time. The problem was caused by allocating a new boundary for each clone without any boundary reuse logic.

Changes made:

• Added an initBoundary parameter to the FormData constructor.
• Implemented boundary reuse logic when cloning FormData objects.
• Updated documentation to explain how initBoundary affects boundaryName.

Technical details:

• The FormData constructor now accepts an optional initBoundary parameter.
• When initBoundary is provided, it overrides the boundaryName configuration.
• The cloning process now reuses the existing boundary when appropriate.

Impact:
This fix ensures consistent behavior when cloning FormData objects and improves efficiency by reusing boundaries where possible. It should resolve issues related to unexpected boundary changes during FormData manipulation.

Testing:

• Added unit tests to verify boundary consistency across cloned FormData objects.
• Manually tested with various use cases to ensure backwards compatibility.

Note to reviewers:
Please pay special attention to the boundary reuse logic and any potential side effects on existing FormData usage.

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

[AlexV525]

Is it possible to directly update the _boundary after the construction, so we don't need an extra argument for the constructor which won't increase the complexity for usages?

[helloliuyiming]

Is it possible to directly update the _boundary after the construction, so we don't need an extra argument for the constructor which won't increase the complexity for usages?

I apologize for the delayed response as I've been quite busy lately.

Regarding your suggested solution - I want to make sure I understand it correctly. Would this be what you had in mind?

  FormData({
    String? initBoundary,
    this.boundaryName = _boundaryName,
    this.camelCaseContentDisposition = false,
  }) {
    _init();
    if(initBoundary != null) {
      _boundary = initBoundary;
    }
  }

[AlexV525]

No. What I mean is:

// Convenience method to clone finalized FormData when retrying requests.
FormData clone() {
  final clone = FormData();
  clone._boundary = _boundary; // <-- Replace the boundary.
  clone.fields.addAll(fields);
  for (final file in files) {
    clone.files.add(MapEntry(file.key, file.value.clone()));
  }
  return clone;
}

[helloliuyiming]

Ah, I see. Your solution is indeed much better than what I proposed. Thank you for the guidance. I have submitted a new one implementing your suggested approach. Please review again, thanks.

[AlexV525]

Please add the CHANGELOG entry and update the PR checklist status.

[helloliuyiming]

done!

[AlexV525]

Thanks for the update. One last nit

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
Overall Coverage	🟢 84.7%	🟢 84.71%	🟢 0.01%

Minimum allowed coverage is 0%, this run produced 84.71%