Can't catch exception throwing after Kotlin coroutine is cancelled

Question

Using kotlinx.coroutines lib I can't catch an exception if it was thrown after coroutine is canceled. This leads to app crash.

fun foo() {
  val job = launch(UI) {
     try {
        Log.d("TAG", "Start coroutine")
        run(CommonPool) {
           Log.d("TAG", "Start bg task")
           // Intentionally make bg task running for a long time
           SystemClock.sleep(2000)
           Log.d("TAG", "Throw bg task exception")
           throw RuntimeException("Bg task exception")
        }
     } catch (e: Exception) {
        Log.e("TAG", "Handle coroutine exception", e)
     }
  }

  launch(UI) {
     delay(1000)
     Log.d("TAG", "Cancel job = ${job.cancel()}")
  }

}

Running this functions on Android produces the following log output

07-26 15:09:10.038 31518-31518/co.foo.bar D/MainActivity: Start coroutine
07-26 15:09:10.044 31518-31547/co.foo.bar D/MainActivity: Start bg task
07-26 15:09:11.046 31518-31518/co.foo.bar D/MainActivity: Cancel job = true
07-26 15:09:11.047 31518-31518/co.foo.bar E/MainActivity: Handled coroutine exception
                           java.util.concurrent.CancellationException: Job was cancelled
                           at kotlinx.coroutines.experimental.JobSupport$CompletedExceptionally.getException(Job.kt:921)
                           at kotlinx.coroutines.experimental.RunCompletion.afterCompletion(Builders.kt:198)
                           ...
                           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
07-26 15:09:12.046 31518-31547/co.foo.bar D/MainActivity: Throwing bg task exception

--------- beginning of crash
07-26 15:09:12.046 31518-31547/co.foo.bar E/AndroidRuntime: FATAL EXCEPTION: ForkJoinPool.commonPool-worker-1
Process: co.foo.bar, PID: 31518
                           java.lang.RuntimeException: Bg task exception
                           at co.foo.barsample.MainActivity$onCreate$1$job$1$1.doResume(MainActivity.kt:36)
                           at kotlin.coroutines.experimental.jvm.internal.CoroutineImpl.resume(CoroutineImpl.kt:54)
                           at kotlinx.coroutines.experimental.DispatchTask.run(CoroutineDispatcher.kt:120)
                           at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1383)
                           at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:256)
                           at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1123)
                           at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1961)
                           at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1909)
                           at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:128)
07-26 15:09:12.050 1705-2190/system_process W/ActivityManager:   Force finishing activity co.foo.bar/co.foo.barsample.MainActivity

Seems to be calling cancel() throws CancellationException which is caught successfully. But the subsequent RuntimeException is not caught. I suppose the following exceptions after the job is canceled should be ignored by the lib? Or how I can cancel job silently without throwing a CancellationException exception?

Answer 1

Use CoroutineExceptionHandler as additinoal coroutine context to handle exceptions, for launch or run like

run(CommonPool + CoroutineExceptionHandler({ _, e ->
   Log.e("TAG", "CoroutineExceptionHandler", e)
})) {
    ...
}

or

launch(UI + CoroutineExceptionHandler({ _, e ->
   Log.e("TAG", "CoroutineExceptionHandler", e)
})) {
   ...
}