It seems like `CoroutineExceptionHandler`s are ignored for exceptions thrown from a coroutine `launc...

# coroutines

[Zach Klippenstein (he/him) [MOD]]

It seems like `CoroutineExceptionHandler`s are ignored for exceptions thrown from a coroutine `launch`ed with a parent Job from a runBlocking scope. Changing the dispatcher doesn’t seem to matter, and coroutines `launch`ed from GlobalScope execute the handler as I’d expect. I can’t see any documentation about this or figure out how this makes sense.

E.g.

fun main(args: Array<String>) {
  val handler = CoroutineExceptionHandler { _, exception ->
    println("Caught $exception")
  }

  runBlocking(handler) {
    val job = launch {
      // This exception is thrown from runBlocking, and not seen by handler.
      throw AssertionError()
    }
    job.join()
  }
}

If you change that to the following, handler gets run:

runBlocking {
  val job = GlobalScope.launch(handler) {

Is this behavior by design?

[Zach Klippenstein (he/him) [MOD]]

Ah, I think I get it – the existence of a runBlocking parent job implies there is a well-defined place in the code where the exception can be caught, unlike a GlobalScope.launch.

[Dico]

Yeah, I think it's because there are 2 context elements with the same key. I don't know how they are prioritized. I guess this example would suggest the parent context is prioritized, but in the case of the intercepter it should not.

[Dico]

The continuation intercepter does get special treatment with regards to internal ordering for performance, which might have something to do with it.

[elizarov]

it is explained here: https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md#exception-handling

[elizarov]

In short: CoroutineExceptionHandler is a handler for uncaught exceptions, while child coroutines effectively have their exceptions “caught” by their parent

[elizarov]

Note that GlobalScope.launch completely decouples coroutine from its parent. If you want to inherit parent’s dispatcher, but make it separate in terms of cancellation and exception you can do launch(NonCancellable)

[Dico]

Good read!

[Zach Klippenstein (he/him) [MOD]]

Thanks!

[spand]

Any particular reason it is not named CoroutineUncaughtExceptionHandler then ?

[elizarov]

It could have been, but that is quite a long name