Depending on the platform, this can be ridiculously expensive. For example, on Windows, it's basically create a new audio stream _each audio callback_. On OSX, it's doing a few system calls that are rather expensive, but not as expensive as windows (on OSX, this unnecessary cubeb call accounts for around 20% of the total callback time).

Regressed by bug 1378070, patch coming up.

Comment on attachment 8903180 [details] Bug 1395593 - Don't query the maximum channel count in ::PlayAudio using cubeb. https://reviewboard.mozilla.org/r/174964/#review180014 Looks good. It would be even better to have an assert on MediaStreamGraphImpl::AudioChannelCount() that it is not executed in a Real Time thread.

Pushed by paul@paul.cx: https://hg.mozilla.org/integration/autoland/rev/eab0db5216b7 Don't query the maximum channel count in ::PlayAudio using cubeb. r=achronop

Pushed by archaeopteryx@coole-files.de: https://hg.mozilla.org/mozilla-central/rev/34933f6390d5 Don't query the maximum channel count in ::PlayAudio using cubeb. r=achronop a=ill-communication

There was a backout in between: https://hg.mozilla.org/mozilla-central/rev/cc85091ab81c7eb865d2cfb31539702c1bb7a18d The last comment was just the relanding.