bug: yamux sends SYN/ACK/FIN/RST as TYPE_DATA instead of TYPE_WINDOW_UPDATE

[asabya]

Summary

The yamux stream muxer sends SYN, ACK, FIN, and RST control frames with TYPE_DATA (0) instead of TYPE_WINDOW_UPDATE (1). This violates the yamux spec and breaks interop with go-yamux, which expects control frames as TYPE_WINDOW_UPDATE.

Additionally, the length field in SYN/ACK frames should carry the window size (not 0), matching go-yamux behavior.

Impact

• Go interop broken: Go peers reject or misinterpret control frames sent as TYPE_DATA
• Frame interleaving: Without write serialization, concurrent writes can corrupt frames on the wire

Root Cause

In libp2p/stream_muxer/yamux/yamux.py:

• open_stream() sends SYN with TYPE_DATA — should be TYPE_WINDOW_UPDATE with length=DEFAULT_WINDOW_SIZE
• close() sends FIN with TYPE_DATA — should be TYPE_WINDOW_UPDATE
• reset() sends RST with TYPE_DATA — should be TYPE_WINDOW_UPDATE
• ACK responses use TYPE_DATA — should be TYPE_WINDOW_UPDATE with length=DEFAULT_WINDOW_SIZE
• No write lock — concurrent frame writes can interleave

Fix

A working fix is available at asabya/py-libp2p#1:

• Changed all control frames to TYPE_WINDOW_UPDATE
• Added _write_lock (trio.Lock) to serialize all frame writes
• SYN/ACK now include DEFAULT_WINDOW_SIZE in the length field

Ref: go-yamux stream.go, session.go, const.go

[asabya]

This PR #1269 resolves this issue