[lhoward]

Based on #248; the first two commits are that PR, so this diff shows them until it merges. The change here is the last two commits.

MessageFrameWSHandler.makeFrames moves frames and messages between the connection and the handler through two AsyncStream continuations with unbounded buffers: incoming messages are yielded into messagesIn, outgoing frames into framesOut, and a task pumps between them. A client that floods a server whose handler is slow grows the first buffer; a client that stops reading while the handler keeps sending grows the second. On a small ARM board that ends with the process killed. And since yield never waits, a handler has no way to tell when a message has reached the connection.

Change

• AsyncHandoff<Element> (new, internal): an actor that passes elements one at a time. send suspends until next takes the element, so a sender gets no further ahead than its receiver. finish(with:) ends it with a final element or error; a waiting sender or receiver that is cancelled is removed. It is built on withIdentifiableThrowingContinuation, like SocketPool.
• MessageFrameWSHandler uses two handoffs in place of the continuations. The handler's input is AsyncStream { try? await messagesIn.next() }, so a client frame is read only when the handler has taken the previous message; the connection's frames come from AsyncThrowingStream { try await framesOut.next() }, so a message is taken from the handler's stream only when the connection is about to write the previous frame. Back-pressure reaches the client's socket in one direction and the handler's stream in the other. A handler that produces messages on demand, for example with AsyncStream(unfolding:), is paced by the client; one that yields into a continuation keeps its own buffer, as before. WSMessageHandler and WSHandler are unchanged.
• AsyncStream.protocolFrames(from:onTermination:): the frames stream is now pull-based, so there is no continuation whose onTermination cancels the handler's tasks. The closure runs when the stream ends or is released without being read to the end, and the handler cancels its tasks from there. This is the same mechanism as before (the stream's storage going away), one level up.
• On a client's close frame, the echo is sent and the frames finished before the .close message is delivered as the final message of the handler's stream, so the closing handshake no longer waits on the handler.

One existing test changed: responseFrames read both messages before collecting the frames, which relied on the pong being buffered; it now collects the frames concurrently.

Testing

New tests: AsyncHandoffTests (8) cover suspension on both sides, final element and error delivery, and cancellation of a sender or receiver. WSHandlerTests check that a client frame stream is read at most one frame ahead of the handler taking messages, that the handler's stream is pulled at most one message ahead of frames being taken, and that a close frame is echoed and then delivered as the final message. protocolFrames runs onTermination both when read to the end and when released early. The WebSocket suites were run 8 times in a row without a failure.

swift test on Linux (Swift 6.4): 323 tests, the same 3 issues as on main (HTTPDecoderTests.standardizedPath and naughtyPath_IsParsed). Not run on Darwin locally.

🤖 Generated with Claude Code

https://claude.ai/code/session_01T8cahEd4GfsHQ89sNrmP4p

[codecov]

Codecov Report

❌ Patch coverage is 98.38710% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.83%. Comparing base (4d2d822) to head (948301b).

Files with missing lines	Patch %	Lines
FlyingFox/Sources/WebSocket/AsyncHandoff.swift	95.74%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
+ Coverage   93.75%   93.83%   +0.08%     
==========================================
  Files          72       73       +1     
  Lines        3793     3878      +85     
==========================================
+ Hits         3556     3639      +83     
- Misses        237      239       +2     

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