commit 40b030edbfc2e35c8124a8ca4457b659ccb53ce6
author Henrik Boström <hbos@webrtc.org> Thu Feb 28 08:49:31 2019
committer Commit Bot <commit-bot@chromium.org> Thu Feb 28 12:38:30 2019
tree 7098233361d0307b8c70135fee737e533b7015d7
parent 25fb765367e1ac7481d43cf846947eff327181de

Reland "Fix getStats() freeze bug affecting Chromium but not WebRTC standalone."

This is a reland of 05d43c6f7fe497fed0f2c8714e2042dd07a86df2

The original CL got reverted because Chrome did not support IsQuitting() which
triggered a NOTREACHED() inside of a DCHECK. With
https://chromium-review.googlesource.com/c/chromium/src/+/1491620
it is safe to reland this CL.

The only changes between this and the original patch set is that this is now
rebased on top of https://webrtc-review.googlesource.com/c/src/+/124701, i.e.
rtc::PostMessageWithFunctor() has been replaced by rtc::Thread::PostTask().

Original change's description:
> Fix getStats() freeze bug affecting Chromium but not WebRTC standalone.
>
> PeerConnection::Close() is, per-spec, a blocking operation.
> Unfortunately, PeerConnection is implemented to own resources used by
> the network thread, and Close() - on the signaling thread - destroys
> these resources. As such, tasks run in parallel like getStats() get into
> race conditions with Close() unless synchronized. The mechanism in-place
> is RTCStatsCollector::WaitForPendingRequest(), it waits until the
> network thread is done with the in-parallel stats request.
>
> Prior to this CL, this was implemented by performing
> rtc::Thread::ProcessMessages() in a loop until the network thread had
> posted a task on the signaling thread to say that it was done which
> would then get processed by ProcessMessages(). In WebRTC this works, and
> the test is RTCStatsIntegrationTest.GetsStatsWhileClosingPeerConnection.
>
> But because Chromium's thread wrapper does no support
> ProcessMessages(), calling getStats() followed by close() in Chrome
> resulted in waiting forever (https://crbug.com/850907).
>
> In this CL, the process messages loop is removed. Instead, the shared
> resources are guarded by an rtc::Event. WaitForPendingRequest() still
> blocks the signaling thread, but only while shared resources are in use
> by the network thread. After this CL, calling WaitForPendingRequest() no
> longer has any unexpected side-effects since it no longer processes
> other messages that might have been posted on the thread.
>
> The resource ownership and threading model of WebRTC deserves to be
> revisited, but this fixes a common Chromium crash without redesigning
> PeerConnection, in a way that does not cause more blocking than what
> the other PeerConnection methods are already doing.
>
> Note: An alternative to using rtc::Event is to use resource locks and
> to not perform the stats collection on the network thread if the
> request was cancelled before the start of processing, but this has very
> little benefit in terms of performance: once the network thread starts
> collecting the stats, it would use the lock until collection is
> completed, blocking the signaling thread trying to acquire that lock
> anyway. This defeats the purpose and is a riskier change, since
> cancelling partial collection in this inherently racy edge-case would
> have observable differences from the returned stats, which may cause
> more regressions.
>
> Bug: chromium:850907
> Change-Id: Idceeee0bddc0c9d5518b58a2b263abb2bbf47cff
> Reviewed-on: https://webrtc-review.googlesource.com/c/121567
> Commit-Queue: Henrik Boström <hbos@webrtc.org>
> Reviewed-by: Steve Anton <steveanton@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#26707}

TBR=steveanton@webrtc.org

Bug: chromium:850907
Change-Id: I5be7f69f0de65ff1120e4926fbf904def97ea9c0
Reviewed-on: https://webrtc-review.googlesource.com/c/124781
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26896}