Drain the callback pipe on stream stop#199
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audioipc2 services each stream over two independent IPC pipes, each on its own thread per side: the control pipe (StreamStop/StreamStopped) and the callback pipe (CallbackReq::Data/CallbackResp::Data). There is no ordering guarantee between them, so a data callback already queued on the callback pipe can be delivered to the client after StreamStopped has come back on the control pipe — i.e. after cubeb_stream_stop() has returned. This violates cubeb's contract that no data callback runs after stop. The fix is to add CallbackReq::Drain / CallbackResp::Drain. After stopping the native stream, the server makes a *synchronous* Drain call on the callback pipe before replying StreamStopped. The client services that pipe on a single thread in receive order, so by the time it answers Drain every earlier data callback has already run to completion and returned. The call must stay blocking: a fire-and-forget Drain would not order anything and would reintroduce the race. This guarantee is specific to data callbacks, which users rely on to synchronize shutdown sequences.Sstate and device change callbacks aren't affected by this fix and aren't a problem.
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audioipc2 services each stream over two independent IPC pipes, each on its own thread per side: the control pipe (StreamStop/StreamStopped) and the callback pipe (CallbackReq::Data/CallbackResp::Data). There is no ordering guarantee between them, so a data callback already queued on the callback pipe can be delivered to the client after StreamStopped has come back on the control pipe — i.e. after cubeb_stream_stop() has returned. This violates cubeb's contract that no data callback runs after stop.
The fix is to add CallbackReq::Drain / CallbackResp::Drain. After stopping the native stream, the server makes a synchronous Drain call on the callback pipe before replying StreamStopped. The client services that pipe on a single thread in receive order, so by the time it answers Drain every earlier data callback has already run to completion and returned. The call must stay blocking: a fire-and-forget Drain would not order anything and would reintroduce the race.
This guarantee is specific to data callbacks, which users rely on to synchronize shutdown sequences.Sstate and device change callbacks aren't affected by this fix and aren't a problem.