:wave: Need an advice about finding causes of this...

# coroutines

[Eugene]

👋 Need an advice about finding causes of this error crash in Android app:

kotlinx.coroutines.JobCancellationException: StandaloneCoroutine was cancelled

It has no stacktrace (due to https://github.com/Kotlin/kotlinx.coroutines/issues/1866).
The only case that I was able to reproduce is when you throw this exception outside of coroutines.
Any ideas?

[Sam]

You should expect to see this exception when using coroutines. It indicates normal cancellation of a job and isn’t an error. Generally, don’t catch it and don’t log it.

[Eugene]

This is a crash or process, it’s obviously not catched or logged or throwed manually

[Sam]

There are places where cancellation exceptions can propagate into places where they shouldn’t… examples I can think of include:
• Calling await on a cancelled deferred
• Calling cancel() on the current job, especially inside a coroutineScope or withContext block

[Sam]

If a cancellation exception is causing the app to crash, it suggests it’s propagating outside of the coroutine that was supposed to be cancelled 😞

[Eugene]

especially inside a coroutineScope or withContext block

I couldn’t reproduce it manually inside any scope/coroutine. Any cancellation exception was swallowed silently as expected.

[Sam]

The simplest reproduction I know of is this:

suspend fun main() = coroutineScope {
   cancel()
}

https://pl.kotl.in/69W7CAite
I’m not familiar with Android so I’m not sure how I would repro it there

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

You could set an exception breakpoint for that exception type in the debugger to see the raw stack trace at least. Might be noisy tho

[Eugene]

It works only if you can reproduce it think smart
The thing is, we didn’t encounter this crash in debug builds yet.