Hei. I am seeing few issues with `evalOn`, and `co...

# arrow

[Satyam Agarwal]

Hei.
I am seeing few issues with evalOn, and coroutineContexts. Made 2 test files to show what is happening and what I expect.
https://github.com/satyamagarwal/arrow-issues/tree/master/src/test/kotlin/com/arrow/issues

Can any one help me please ? feel free to take a pull and test it out.

[raulraja]

Hi Satyam, Do you get the same behavior if insted of the KotlinX runners you use the Arrow Fx Coroutines ones https://github.com/arrow-kt/arrow-fx/blob/master/arrow-fx-coroutines/src/main/kotlin/arrow/fx/coroutines/Enviroment.kt#L39 ?

[Satyam Agarwal]

@raulraja

Replaced this line https://github.com/satyamagarwal/arrow-issues/blob/master/src/test/kotlin/com/arrow/issues/CoroutinesTest.kt#L27

with Environment(MDCContext()).unsafeRunSync {

Same result.

in test, key: myValue
I am in main
in runBlocking, key: myValue
I am in parTupledN, ForkJoinPool-1-worker-3
in parTupledN block, key: null
I am in parTupledN, ForkJoinPool-1-worker-5
in parTupledN block, key: null
I am in ForkJoinPool-1-worker-3
in either block, key: null
I am in ForkJoinPool-1-worker-5
in evalOn block, key: null
Evaluated thread result : ForkJoinPool-1-worker-5 in ForkJoinPool-1-worker-5
in either block after evalOn, key: null
I am in ForkJoinPool-1-worker-5
in parTupledN block after evalOn, key: null
I am in ForkJoinPool-1-worker-3
in parTupledN block after evalOn, key: null
Evaluated either block result : ForkJoinPool-1-worker-5 in ForkJoinPool-1-worker-3
in mdcContext after either block, key: null
in test, key: myValue

with original program :

in test, key: myValue
I am in main @coroutine#3
in runBlocking, key: myValue
I am in parTupledN, ForkJoinPool-1-worker-3
in parTupledN block, key: null
I am in parTupledN, ForkJoinPool-1-worker-5
in parTupledN block, key: null
I am in main @coroutine#3
in either block, key: myValue
I am in ForkJoinPool-1-worker-5
in evalOn block, key: null
Evaluated thread result : ForkJoinPool-1-worker-5 in ForkJoinPool-1-worker-5
in either block after evalOn, key: null
I am in ForkJoinPool-1-worker-3
in parTupledN block after evalOn, key: null
I am in ForkJoinPool-1-worker-3
in parTupledN block after evalOn, key: null
Evaluated either block result : ForkJoinPool-1-worker-5 in ForkJoinPool-1-worker-3
in mdcContext after either block, key: null
in test, key: myValue

[raulraja]

It seems like there is a bug somewhere then. Can you file an issue and include the code snippets please? thanks!

[Satyam Agarwal]

ok.
Did you check out the other test as well KtorClientTest ? where evalOn hangs when ConnectionRefused is thrown ?

[Satyam Agarwal]

Should I make a new issue for that, or include all in one issue ?

[simon.vergauwen]

Hey @Satyam Agarwal, this is expected but probably not expected..

MDCContext is not a simple CoroutineContext, it's a ThreadContextElement which is a type that is hard-wired in the KotlinX library so we cananot add support for it.

[simon.vergauwen]

https://github.com/satyamagarwal/arrow-issues/blob/26a2adfe6dfc5e920872cef52adafca3585f542e/src/test/kotlin/com/arrow/issues/CoroutinesTest.kt#L95

This can be considered a bug, since Enviroment actually requires a ContinuationInterceptor as an argument, which MDCContext isn't. So we can default back to adding ComputationPool by default if the provided CoroutineContext isn't a ContinuationInterceptor. That would fix the incorrect thread reading.

The reason for this is that to go back to an original thread you need a dispatching ContinuationInterceptor, otherwise that's not possible.

Although that still wouldn't solve the issue you're having with MDCContext. If known this issue for quite a while, but am unsure how we can solve it best or what we can do about it. Ideally the interface would exist in the Standard libray and so all libraries that want to support it can do so but that's probably not happening soon.

[Satyam Agarwal]

@simon.vergauwen

https://github.com/arrow-kt/arrow-fx/issues/304

I have added 2 more test which do not use MDC context at all, and yet do wrong thread reading. Please check them out and see if what I am reporting is is correct 🙂

https://github.com/arrow-kt/arrow-fx/issues/305

Is another issue which is the same repo I created, can you look at this as well please ?

[simon.vergauwen]

I'll comment on the issues, and we can discuss it further there!