How can I track down interference between the flutter libraries audo_recorder and stereo - flutter

Question

I'm building an app with Flutter that allows me to record audio and then play it back later. I'm using the package audio_recorder 0.4.0 for recording the audio and the package stereo 1.0.0 for playing back the audio.
When I record using the iOS simulator, everything works fine. But when I run on the physical device (iPhone 6 Plus), I always get a 28 byte file at the end of the recording session - basically just a header to the m4a file.
I've tracked this down to some kind of interference between the two libraries. I can record on the physical device just fine if I use the sample app included with audio_recorder 0.4.0. But all I have to do is add the line stereo: 1.0.0 to the dependencies in pubspec.yml of that same sample project and suddenly I start getting the 28 byte files on the physical device at the end of a recording session instead of a normal size m4a file.
I have a two-part question:
What kind of interference could the stereo library be causing to make this kind of failure in the audio_recorder library? I suspected channel interference, but my attempt to isolate that didn't yield anything.
How can I track down what is causing this?
I have examined the console output from both sessions. When it is failing, I see several (8) of these lines in the the console output right after attempting to start recording:
mediaserverd 1111: AudioConverterNew returned -50
Others have suggested that this error occurs when there is no microphone available.
I also see this suspicious difference between the outputs.
When recording is broken:
default 11:36:57.098825 -0700 mediaserverd 4414: { "action":"set_play_state", "session":{"ID":"0x3315820","PID":14591,"name":"Runner"}, "details":"entry" }
default 11:36:57.100484 -0700 mediaserverd -CMSessionMgr- cmsSetIsPlaying: CMSession: 'sid:0x3315820, Runner(14591), 'prim'' with [MediaPlayback/Default] [NonMixable] [System Audio] starting playing.
default 11:36:57.104699 -0700 mediaserverd CAReportingClient.mm:328:-[CAReportingClient sendMessage:category:type:reporters:]: message {
Volume = "0.75";
VolumeCategory = "Audio/Video";
VolumeOperationProcess = "com.monkela.audioRecordSample";
VolumeOperationTime = 576708;
VolumePlayingState = 1;
}: (
857014545
)
default 11:36:57.105879 -0700 mediaserverd -CMVAEndptMgr- vaemSetDeviceVolume: CMSession: Setting cmsvolume = 0.750 for device ID: VirtualAudioDevice_Default
default 11:36:57.107295 -0700 mediaserverd VirtualAudio_Device.cpp:2046:SetPropertyData: [volm/outp/0] on device [ type: vdef; id: 354; addr: 0x1014e1ba0; hidden: 0; VA strms: { o/883/0x106fc5980 }; agg dev: [ id: 872; addr: 0x106f1a460; uid: "VAD [vdef] AggDev 52"; virt strms: { }; phys devs: { [ id: 292; addr: 0x104b0fdb0; uid: "Speaker"; streams: { i/294/0x104b11cd0, o/293/0x104b12790 } ] } ] ]: 0.750000.
default 11:36:57.108123 -0700 mediaserverd OutputVolumeControl_HAL_Common.cpp:662:CommitVolume: Setting Volume = -14.500000 dB, Final HW Volume = 0.000000 dB, Final SW Volume = -14.500000 dB, External Volume Handler: No
default 11:36:57.109008 -0700 mediaserverd OutputVolumeControl_HAL_Common.cpp:663:CommitVolume: PhysicalDevice UID = "Speaker"
default 11:36:57.109721 -0700 mediaserverd OutputVolumeControl_HAL_Common.cpp:664:CommitVolume: Scope = 1869968496 ("outp")
default 11:36:57.110704 -0700 mediaserverd OutputVolumeControl_HAL_Common.cpp:665:CommitVolume: Element = 0 ("0")
default 11:36:57.112062 -0700 mediaserverd OutputVolumeControl_HAL_Common.cpp:698:CommitVolume: SW Volume: 0.750000 scalar
default 11:36:57.114393 -0700 mediaserverd VirtualAudio_Device.cpp:2066:SetPropertyData: Client request to set nominal sample rate to 44100.000000 on VAD 354.
default 11:36:57.116038 -0700 assertiond [Runner:14591] Attempting to acquire assertion for Runner:14591: <BKProcessAssertion: 0x101349110; "CMSession.14591."com.monkela.audioRecordSample"."MediaPlayback".isPlayingProcessAssertion" (audio:inf); id:…C74721DE1F95>
default 11:36:57.116572 -0700 assertiond [Runner:14591] Add assertion: <BKProcessAssertion: 0x101349110; id: 13077-914DDDE2-35CE-4420-85ED-C74721DE1F95; name: CMSession.14591."com.monkela.audioRecordSample"."MediaPlayback".isPlayingProcessAssertion; state: active; reason: audio; duration: infs> {
owner = <BSProcessHandle: 0x101009be0; mediaserverd:13077; valid: YES>;
flags = preventSuspend, preventThrottleDownCPU, preventIdleSleep, preventSuspendOnSleep;
}
When recording is working:
default 11:39:19.865022 -0700 mediaserverd 2863: { "action":"activate", "session":{"ID":"0x3315826","PID":14609,"name":"Runner"}, "details":"entry" }
default 11:39:19.880637 -0700 mediaserverd CAReportingClient.mm:328:-[CAReportingClient sendMessage:category:type:reporters:]: message {
"active devices" = "[ { vdef, [ ], [ pspk ( 0 ) ] }, { vspd, [ pmbi ( 0 ) ], [ ] }, { vcal, [ pdln ( 0 ) ], [ puln ( 0 ) ] } ]";
category = csav;
"going_active" = 1;
mode = imdf;
"staying_active" = 0;
}: (
857014546
)
The output when recording is broken suggests that my attempt to start recording actually starts playing something (i.e. invoking the wrong library).

Answer 1

I tracked this down and figured out how to make the libraries work together by modifying the stereo package. It turns out the stereo package was activating a playback audio session in application didFinishLaunchingWithOptions. Essentially these lines:
AVAudioSession *session = [AVAudioSession sharedInstance];
[session setCategory:AVAudioSessionCategoryPlayback error:&error];
[session setActive:YES error:&error];
I forked the stereo package and changed the code to activate the playback session only when it is actually needed. Magically my record function started working.
I'm not sure why it ever worked in the simulator. Somehow the simulator must be able to have both a playback and recording session going simultaneously.
UPDATE: The author of the stereo package accepted my pull request so as long as you have version 1.0.1 of the package, you should be able to avoid this interference issue.