WorkManager CoroutineWorker's stream cancellation

Question

I have the following setup:

override suspend fun doWork(): Result = coroutineScope {
    flowOfEvents
        .onEach(eventChannel::send)
        .launchIn(this)  

    if (isQueueEmpty()) return@coroutineScope Result.success()

    ... 
}

What I'm seeing is the following: when isQueueEmpty() is true, I return Result.success() and I'd expect the flowOfEvents...launchIn(this) stream to be disposed/cancelled as well, but I keep receiving events from that stream.

Am I doing something wrong?

Capturing the Job from launchIn(this) and explicitly calling job.cancel() before each return statement works, but feels unnecessary/wrong.

Answer 1

After a bit of research, it was made clear that this is a Kotlin coroutines behavior and isn't related to WorkManager at all.

• Listening to a Flow<Event> is a never-ending job
• By definition, parent coroutines wait for the child coroutines to finish

Given these 2 points, it is now apparent why I kept receiving Events even after I returned from the parent coroutine.

The solution: call coroutineContext[Job]?.cancelChildren() before returning from the parent coroutine.

override suspend fun doWork(): Result = coroutineScope {
    flowOfEvents
        .onEach(eventChannel::send)
        .launchIn(this)  

    if (isQueueEmpty()) {
        coroutineContext[Job]?.cancelChildren()
        return@coroutineScope Result.success()
    }

    ... 
}

Note: calling parent's cancel() throws a CancellationException, which in the context of a WorkManager meant that the Worker#doWork method would actually result in a failed job.