Uncaught exception may not appear on console when main is waiting for a job

[elizarov]

Consider the following code:

fun main(args: Array<String>) = runBlocking {
    val job = launch {
        doSomething()
    }
    job.join()
}

fun doSomething() {
    TODO("Not implemented")
}

Because of the order events are handled internally, main can wake up and terminate application before exception handler had a chance to print onto the console from its background thread.

Consider handling uncaught exceptions in launch before everything else.

[Starcarr]

This is still happening on 0.22 with Kotlin 1.2.21.

Code (same as the body of the issue)

fun main(args: Array<String>) = runBlocking {
    val job = launch {
        doSomething()
    }
    job.join()
}

fun doSomething() {
    TODO("Not implemented")
}

Sample output:

/usr/lib/jvm/java-8-openjdk/bin/java...
Process finished with exit code 0

[elizarov]

Confirm. I've fixed the order in which invocations happen normally, but there is a problem in another place, too.

[elizarov]

Here is what I'm observing in this case:

1. The job launches, executed and crashes even before the main coroutine reaches job.join statement.
2. However, the OS scheduler resumes execution on the main thread before the crashed background thread has a change to print its error message on the console.
3. Note, that by that time the state of the job is already complete (we update state first and the print exception on the console), so when main thread reaches join it does not even suspend, but goes straight though to the exit, terminates the process and does not leave any chance to the background thread to print its exception.

[elizarov]

Fixed in version 0.22.1. Now, when launch Job starts to fail (completes with exception) it starts the process by transitioning into completing state, then it handles the exception (printing its stacktrace by default), then it becomes complete (allowing for a coroutine that is using join to go ahead).