AVFoundation Mutex interlock on AVAudioPlayerNode.Stop - swift

Question

I am trying to write my first Swift application, and its running great!… in the simulator. The application is generating tones via AVFoundation, and it works well when I start playing the tones on the iPhone, but as soon as i try to stop the AVAudioPlayerNode, then 5 out of 10 times my iPhone application freezes up. Everything is working perfectly in the simulator.
Here are the stopping code:
#IBAction func togglePlay(_ sender: UIButton) {
if tone.isPlaying {
engine.mainMixerNode.volume = 0.0
tone.stop() // This is where is freezes up and tone is a AVAudioPlayerNode
engine.reset() // engine is a AVAudioEngine
sender.setImage(UIImage(named:"icons8-play-100")!, for: UIControl.State())
} else {
tone.preparePlaying()
tone.play()
engine.mainMixerNode.volume = 1.0
sender.setImage(UIImage(named:"icons8-stop-filled-100")!, for: UIControl.State())
}
If i pause the code when it freeze and look at the stack trace, then it looks like it's waiting for some sort of Mutex be be released. But I'm not experienced enough to know how to investigate further on what i'm doing wrong
Here are the stack trace:
VFAudio`-[AVAudioPlayerNode stop]:
0x187b68c34 <+0>: stp x22, x21, [sp, #-0x30]!
0x187b68c38 <+4>: stp x20, x19, [sp, #0x10]
0x187b68c3c <+8>: stp x29, x30, [sp, #0x20]
0x187b68c40 <+12>: add x29, sp, #0x20 ; =0x20
0x187b68c44 <+16>: mov x21, x0
0x187b68c48 <+20>: adrp x8, 177438
0x187b68c4c <+24>: add x8, x8, #0x700 ; =0x700
0x187b68c50 <+28>: ldr x19, [x8]
0x187b68c54 <+32>: cbz x19, 0x187b68c70 ; <+60>
0x187b68c58 <+36>: ldr x8, [x19]
0x187b68c5c <+40>: ldr x8, [x8, #0x10]
0x187b68c60 <+44>: mov x0, x19
0x187b68c64 <+48>: blr x8
0x187b68c68 <+52>: mov x20, x0
0x187b68c6c <+56>: b 0x187b68c74 ; <+64>
0x187b68c70 <+60>: mov w20, #0x0
0x187b68c74 <+64>: adrp x8, 177435
0x187b68c78 <+68>: add x8, x8, #0x9dc ; =0x9dc
0x187b68c7c <+72>: ldrsw x8, [x8]
0x187b68c80 <+76>: ldr x21, [x21, x8]
0x187b68c84 <+80>: ldr x8, [x21]
0x187b68c88 <+84>: ldr x8, [x8, #0x198]
0x187b68c8c <+88>: mov x0, x21
0x187b68c90 <+92>: blr x8
0x187b68c94 <+96>: ldr x0, [x21, #0x8] <— This is where it breaks
libsystem_kernel.dylib`__psynch_mutexwait:
0x181c15134 <+0>: mov x16, #0x12d
0x181c15138 <+4>: svc #0x80
-> 0x181c1513c <+8>: b.lo 0x181c15154 ; <+32>
What would be next step in debugging?

Answer 1

Pay attention to threading in your audio engine setup.
Had the same issue when saved audio frames from the injected equalizer unit:
equalizer.installTap(onBus: 0, ... ) { buffer, _ in
...
status = ExtAudioFileWrite(file, numberOfFrames, audioBufferList)
...
}
Mutex interlock was fixed by forwarding ExtAudioFileWrite call to the main thread