Respect FLAG_FIN & FLAG_RST in TYPE_WINDOW_UPDATE frames

[vegas503]

What was wrong?

Issue #931

How was it fixed?

Added handling of FIN & RST flags in WINDOW_UPDATE frames.

To-Do

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

[paschal533]

Hi @vegas503, The fix looks correct overall. Would you be able to add some unit tests for this? Specifically testing WINDOW_UPDATE frames with FLAG_FIN and FLAG_RST.
One question: have you tested this with any real yamux implementations to make sure the behavior matches expectations?
Overall this looks like a solid fix for a real spec compliance issue. Once we address the testing and potential refactoring, I think this will be ready to merge. Well done @vegas503 🙌

[seetadev]

Thanks @vegas503 for identifying and fixing this important spec compliance issue, and @paschal533 for the thoughtful review and feedback 🙏

The added handling of FIN & RST flags in WINDOW_UPDATE frames is a solid improvement, and the suggestions around unit testing and validation against real yamux implementations will help ensure correctness and stability.

Great collaboration here — once the tests and cleanup are in place, this will be a strong addition to the codebase.

CCing @acul71 , @lla-dane and @sumanjeet0012 for peer review and feedback.

[acul71]

PR Review: Respect FLAG_FIN & FLAG_RST in TYPE_WINDOW_UPDATE frames (#932)

Summary

This PR addresses a spec compliance issue in the yamux implementation where FLAG_FIN and FLAG_RST flags in TYPE_WINDOW_UPDATE frames were not being properly handled.

Spec Verification: The yamux specification explicitly states:

• 0x4 FIN - Performs a half-close of a stream. May be sent with a data message or window update.
• 0x8 RST - Reset a stream immediately. May be sent with a data or window update message.
• "To close a stream, either side sends a data or window update frame along with the FIN flag."

Changes Made

• Added handling for FLAG_FIN in TYPE_WINDOW_UPDATE frames (lines 664-672)
• Added handling for FLAG_RST in TYPE_WINDOW_UPDATE frames (line 579)
• Both flags now properly trigger stream state changes (recv_closed for FIN, reset for RST)

Code Quality

✅ Good: The fix follows existing patterns in the codebase
✅ Good: Proper logging and stream state management
✅ Good: Thread-safe implementation with appropriate locks

Concerns

⚠️ Missing: No unit tests for WINDOW_UPDATE frames with FLAG_FIN/FLAG_RST (existing tests only cover basic WINDOW_UPDATE without flags)
⚠️ Missing: No validation against real yamux implementations
⚠️ Missing: Documentation updates for the changes

Recommendation

Approve with conditions: The fix appears correct and addresses a real spec compliance issue. However, unit tests should be added before merging to ensure the behavior works as expected and to prevent regressions.

Follow-up Actions

1. Add unit tests for WINDOW_UPDATE frames with FLAG_FIN and FLAG_RST
2. Test against real yamux implementations for validation
3. Update relevant documentation
4. Clean up commit history as mentioned in the PR description

[acul71]

@vegas503 what's the status of this PR?