Can someone please explain the result of this small snippet?

# coroutines

[Sebouh Aguehian]

Can someone please explain the result of this small snippet?

[Sebouh Aguehian]

fun main() {
  val job = GlobalScope.launch(Dispatchers.Main) {
    println("This is executed before toBeCancelled")
    toBeCancelled()
    println("This is executed after toBeCancelled")
  }
}

suspend fun toBeCancelled() {
  withContext(EmptyCoroutineContext) {
    coroutineContext[Job]!!.cancel()
    println("This is executed before the delay")
    delay(2000)
    println("This is executed after the delay")
  }
}

This is executed before toBeCancelled
This is executed before the delay

How come “This is executed after toBeCancelled” is not printed? My understanding is that withContext would create a child job, and calling coroutineContext[Job]!!.cancel() should only cancel that child job. How come it is impacting the parent?

[streetsofboston]

Because calling withContext { ... } throws a CancellationException. This bubbles up to the caller.

[streetsofboston]

Try this and you can see the exception being thrown:

suspend fun toBeCancelled() {
    try {
      withContext(EmptyCoroutineContext) {
          coroutineContext[Job]!!.cancel()
          println("This is executed before the delay")
          delay(2000)
          println("This is executed after the delay")
      }
    } catch (e: Throwable) { println("withContext threw exception $e"); throw e }
}

[streetsofboston]

Removing the throw e from the catch clause will eat that cancellation exception and “This is executed after toBeCancelled” will be printed out (note: Don’t eat/forget-to-rethrow CancellationExceptions. It can break the Structured Concurrency!)

[Sebouh Aguehian]

Hmm. Is it a special case with withContext? The following doesn’t cancel the parent.

fun main() {
  val job = GlobalScope.launch(Dispatchers.Main) {
    println("This is executed before toBeCancelled")
    launch {
      coroutineContext[Job]!!.cancel()
      println("This is executed before the delay")
      delay(2000)
      println("This is executed after the delay")
    }.join()
    println("This is executed after toBeCancelled")
  }
}

[Sebouh Aguehian]

Is the parent/child relationship different here?

[streetsofboston]

Because launch { ... }.join() is not like withContex

[streetsofboston]

withContext is more like async { .... }.await(). It returns the value of its lambda.

[Sebouh Aguehian]

I see

[Sebouh Aguehian]

So when a result is expected, a cancellation bubbles up then.

[streetsofboston]

A call to launch will never throw an exception. A call to join won’t either. A launch will cause the CoroutineScope’s CoroutineExceptionHandler to be called.

a call to async will never throw an exception either, but a call to await will throw an exception (if there was one thrown by the lambda of async)

[streetsofboston]

withContext, like async { ...}.await(), re-throws any exception thrown in its lambda. And that exception can be a CancellationException (caused by a call to cancel somewhere)

[streetsofboston]

You may be interested in the article I wrote here 🙂 :
https://medium.com/the-kotlin-chronicle/coroutine-exceptions-3378f51a7d33

[Sebouh Aguehian]

But in case of a non-cancellation exception within launch, that bubble up right?

[Sebouh Aguehian]

Just tested it, indeed seems to.

[Sebouh Aguehian]

Ok, that’s clearer in my head now. Thanks a lot. I’ll check your article.

[streetsofboston]

I won’t bubble up outside of the launch.

An exception from a launch will be handled by the CoroutineScope’s CoroutineExceptionHandler (CEH).
If that exception is a CancellationException, nothing really will happen, except for the CoroutineScope getting cancelled (unless a SupervisorScope is used). If it is not a CancelationException, depending whether you have a CEH installed or not and the implementation of your CEH and the implementation of your app’s ‘uncaught-exception-handler’, your app may crash.

[Sebouh Aguehian]

In the following case:

fun main() {
  val job = GlobalScope.launch(Dispatchers.Main) {
    println("This is executed before toBeCancelled")
    try {
      launch {
        throw Exception("shiiiii")
      }.join()
    } catch (e: Exception) {
      println (e)
    }
    println("This is executed after toBeCancelled")
  }
}

I’m getting:

This is executed before toBeCancelled
JobCancellationException: StandaloneCoroutine is cancelling; caused by Exception: shiiiii; job=StandaloneCoroutine{Cancelling}@11
This is executed after toBeCancelled
Exception: shiiiii

Seems like I’m able to catch it outside the launch?

[Pablichjenkov]

I think in your initial code:
coroutineContext[Job]!!.cancel()
inside withContext will return the parents job. withContext switches Context but I don't think it creates a new child Job().

[Sebouh Aguehian]

I printed the jobs and they were different.

[Pablichjenkov]

humm, that defeats my statement. Never mind.