OS X: Removing a webcam source deadlocks

[SamCarlberg]

Reported by @AustinShalit

I don't have an apple computer so I can't tell if it's a general OS X thing or if it's specific to Austin's setup.

[AustinShalit]

@nightpool

[nightpool]

happens to me too.

1. installed GRIP from brew cask
2. Opened GRIP
3. added a new webcam
4. enabled the "preview", watched it for a bit
5. clicked on the "remove" button
6. GRIP deadlocked.

expected result: GRIP not to deadlock ;)

[SamCarlberg]

Just tested on a Windows 10 VM, can't reproduce. So it's definitely an OS X issue.

Here's what the webcam and JavaFX threads look like on OS X when removing the webcam:

"Webcam 0 Service" #31 prio=5 os_prio=31 tid=0x00007fabb4abc800 nid=0x12257 runnable [0x0000700005577000]
   java.lang.Thread.State: RUNNABLE
    at org.bytedeco.javacpp.opencv_videoio.cvReleaseCapture(Native Method)
    at org.bytedeco.javacv.OpenCVFrameGrabber.stop(OpenCVFrameGrabber.java:221)
    at edu.wpi.grip.core.sources.GrabberService.shutDown(GrabberService.java:107)
    at com.google.common.util.concurrent.AbstractExecutionThreadService$1$2.run(AbstractExecutionThreadService.java:76)
    at com.google.common.util.concurrent.Callables$3.run(Callables.java:100)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - None

"JavaFX Application Thread" #13 prio=5 os_prio=31 tid=0x00007fabaf0df000 nid=0x307 waiting on condition [0x00007fff5b8b2000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x000000076c24a1d0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitUninterruptibly(AbstractQueuedSynchronizer.java:1976)
    at com.google.common.util.concurrent.Monitor.awaitUninterruptibly(Monitor.java:1102)
    at com.google.common.util.concurrent.Monitor.enterWhenUninterruptibly(Monitor.java:473)
    at com.google.common.util.concurrent.AbstractService.awaitTerminated(AbstractService.java:278)
    at com.google.common.util.concurrent.AbstractExecutionThreadService.awaitTerminated(AbstractExecutionThreadService.java:217)
    at edu.wpi.grip.core.util.service.AutoRestartingService.stopAndAwait(AutoRestartingService.java:161)
    - locked <0x000000076c23acf8> (a edu.wpi.grip.core.util.service.AutoRestartingService)
    at edu.wpi.grip.core.sources.CameraSource.stopAndAwait(CameraSource.java:289)
    at edu.wpi.grip.core.sources.CameraSource.onSourceRemovedEvent(CameraSource.java:353)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:95)
    at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:154)
    - locked <0x000000076c24c958> (a com.google.common.eventbus.Subscriber$SynchronizedSubscriber)
    at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:80)
    at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:456)
    at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:76)
    at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:119)
    at com.google.common.eventbus.EventBus.post(EventBus.java:215)
    at edu.wpi.grip.ui.pipeline.source.SourceController.delete(SourceController.java:100)
    // Nothing relevant further up the call stack

So the UI thread is waiting for the camera to get released, and it never gets released because cvReleaseCapture isn't returning.

[JLLeitschuh]

Can you try FFMpegFrameGrabber? Or someone like @AustinShalit

[SamCarlberg]

FFmpegFrameGrabber only works for video files, you can't use it for webcams. @PeterJohnson, do you think CameraServer could be an alternative?

[PeterJohnson]

Yes, CameraServer will be a good alternative, but I haven't implemented IP camera support yet (ECD next week).

[SamCarlberg]

I think we'd only need it for webcams, we have a custom frame grabber for IP cameras

[ghost]

Is this issue related to #696?

[SamCarlberg]

No, #686.