Deadlock in __psynch_mutexwait on calling player.stop()

[banjerluke]

Important preface: I went back to using a pool of AudioPlayer objects that are all instantiated before the engine is started; that fixed random engine-related errors I was getting, and also eliminated the overhead of creating new AudioPlayer objects all the time. As a bonus, I can now easily reuse buffered players for repeated samples! In master, each Channel has its own pool of AudioPlayer/SamplePlayer objects; I am experimenting with using a shared pool of AudioPlayers (in the dev/unified-pool branch) but that has no bearing on this issue...

If you hit "Play Drum Loop" and wait awhile (10-30 seconds usually), eventually it will stop responding. If then I pause execution, it's always inside __psynch_mutexwait:

libsystem_kernel.dylib`__psynch_mutexwait:
    0x1bd7bea3c <+0>:  mov    x16, #0x12d
    0x1bd7bea40 <+4>:  svc    #0x80
->  0x1bd7bea44 <+8>:  b.lo   0x1bd7bea60               ; <+36>
    0x1bd7bea48 <+12>: stp    x29, x30, [sp, #-0x10]!
    0x1bd7bea4c <+16>: mov    x29, sp
    0x1bd7bea50 <+20>: bl     0x1bd7bc9f4               ; cerror_nocancel
    0x1bd7bea54 <+24>: mov    sp, x29
    0x1bd7bea58 <+28>: ldp    x29, x30, [sp], #0x10
    0x1bd7bea5c <+32>: ret    
    0x1bd7bea60 <+36>: ret    

This is originating from [AVAudioPlayerNode stop], which is being called by AudioPlayer.stop() – sometimes because it's called directly, sometimes as part of a call to .load() or .play(). It seems like there's a small chance of that function deadlocking anytime it's called... which is obviously far from ideal!

Happens in both iOS 15 and iOS 14.4 simulators as well as my real iPhone.

[banjerluke]

This is the only related thread I could find on StackOverflow:

https://stackoverflow.com/questions/59080708/calling-stop-on-avaudioplayernode-after-finished-playing-causes-crash

Only I'm already on the main thread (DispatchQueue.main.async)...

[banjerluke]

It appears that this was caused by this:

self.player.play(from: offset, to: nil, at: time, completionCallbackType: .dataRendered)
                                                                          ^^^^^^^^^^^^^

I changed completionCallbackType back to .dataPlayedBack and it doesn't deadlock anymore. There are definitely still problems but I'll open new issue(s) for them.