I'm trying to track down the source of a `JobCancellationException`. My crash reporting tool isn't o...

# coroutines

[Tim Malseed]

I'm trying to track down the source of a JobCancellationException. My crash reporting tool isn't offering any useful information, and I don't quite understand this exception..

[Tim Malseed]

kotlinx.coroutines.JobCancellationException · Job was cancelled
unknown fileunknown method

[Tim Malseed]

Looking at the breadcrumbs, it looks like this occurs after MainActivity.onDestroy() is called in my Android app. So presumably, someone is swiping the app away from recents (potentially killing the app process)

[Tim Malseed]

There's a coroutine that runs when the app is launched, which I suspect is the source. The scope used to launch this coroutine is:

CoroutineScope(Dispatchers.Main + job + coroutineExceptionHandler)

Where:
job is a SupervisorJob, and
coroutineExceptionHandler is an instance of CoroutineExceptionHandler that logs errors

[Tim Malseed]

I'm not sure if I'm looking in the wrong place, or the app is throwing an exception that is swallowed by coroutines, or the 'crash' is simply that the job is being cancelled. Any insight would be great, I'm still wrapping my head around JobCancellationExceptions and CoroutineExceptionHandlers

[elizarov]

Turning on debug mode should help, because it enables stacktrace in JCE

[Tim Malseed]

Thanks for the quick reply. The issue here is, I'm not able to reproduce this crash myself

[Tim Malseed]

Oh, I wonder if I can ship the app (it's only in alpha) with debug mode on.. Would that be crazy?

Just had a read of https://github.com/Kotlin/kotlinx.coroutines/blob/master/docs/debugging.md

Thanks

[louiscad]

@Tim Malseed Are you using Channel and the offer function?

[Tim Malseed]

I am in some places, yes

[louiscad]

@Tim Malseed
Then it's quite likely you have these exceptions thrown here either:
• because you're using consume, consumeEach or consumeAsFlow() when, respectively, you should not, when you should use a plain for loop or receive(), or when you should use receiveAsFlow()
• because you have a race condition, for example in `callbackFlow`/`channelFlow` or elsewhere, which is this issue: https://github.com/Kotlin/kotlinx.coroutines/issues/974
• because you close or cancel the channel in a way or another, where a JobCancellationException is passed as a cause.

[Tim Malseed]

Thanks for the info