[Bug 39814] Choppy audio with pulseaudio after PULSE_LATENCY_MSEC workaround was removed in Debian packages

[wine-bugs at winehq.org]

https://bugs.winehq.org/show_bug.cgi?id=39814

--- Comment #12 from Andrew Eikum <aeikum at codeweavers.com> ---
Thanks for the nice bug report!

This actually looks like a bug in Reaper that the large buffer size was
covering up.

Reaper asks for a buffer of length 0x1a0ab REFERENCE_TIMEs, which is 0.0106667
seconds. I'm not sure how this number is derived, but it at least uses the
result of GetDevicePeriod.

0034:trace:pulse:AudioClient_Initialize (0x17d450)->(0, 40000, 1a0ab, 0,
0x192a0fc, (null))

And PA honors this, giving us a buffer of the same size (expressed here in
bytes at 8bps and 48 kHz):

0034:trace:pulse:dump_attr tlength: 4104

Then Reaper grabs the buffer size and fills the whole buffer:

0034:trace:pulse:AudioClient_GetBufferSize (0x17d450)->(0xc83e910)
0034:trace:pulse:AudioRenderClient_GetBuffer (0x17d450)->(512, 0xc83e8c4)

But, next time it tries to write:

0034:trace:pulse:AudioClient_GetCurrentPadding 0x17d450 Pad: 11 ms (512)
0034:trace:pulse:AudioRenderClient_GetBuffer (0x17d450)->(512, 0xc83e8c4)
0034:warn:pulse:AudioRenderClient_GetBuffer Wanted to write 512, but only 1
available

winepulse is telling the application "512 samples of our 512 sample buffer are
occupied," but then Repear tries to write 512 samples into the full buffer!
That can't work. They're mistakenly using the size of the data in the buffer as
the amount of free space in the buffer.

You see the same bug with each GetBuffer call after that until the buffer is
less than half full, at which point the request succeeds. But that's just
~0.005 seconds of data in the buffer, so we hear underruns.

I tested Reaper out with PULSE_LATENCY_MSEC=60 and saw the same broken
behavior. The reason it sounds better is because the period size is larger, so
the requested buffer size is much larger, which means the half-full buffer
doesn't underrun.

So, this is an application bug exposed by our support for very low buffer
sizes. I suspect in practice Windows devices don't claim the low latency that
winepulse does, or somehow otherwise mask this bug.

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