is it actually invalid to invoke a `Continuation` in the sam

Question

is it actually invalid to invoke a `Continuation` in the same stack frame or is that just a best practice thing to avoid blowing the stack

Dominaezzz · Answer

It s valid <https kotlinlang org api latest jvm stdlib kotlin coroutines experimental suspend coroutine html>

jw · Answer

I don t mean inside of `suspendCoroutine` I mean the `Continuation` parameter that is passed directly in bytecode

jw · Answer

This will cause the caller s state machine to resume beneath the current stack frame Seems like it would create an invalid state since the existing state has not yet received a return value of `CORUTINE SUSPENDED`

jw · Answer

unless it s already technically in that state the more i think about it the worse the implications become

Dominaezzz · Answer

I see what you mean now

Dominaezzz · Answer

For CPS to work Invoking the `Continuation` in any stackframe has to be valid

Dominaezzz · Answer

Out of curiosity what do you need this info for

jw · Answer

I m working around a bug that s caused by `suspendContinuation` throwing a checked exception synchronously that was supplied to its `Continuation` in a seemingly asynchronous way

jw · Answer

writing a blog post about it now

jw · Answer

to be clear it s not doing anything wrong it s just causing some unsavory behavior in a case that seems impossible

Dominaezzz · Answer

Check exception Are you using java

jw · Answer

It s passing through a library written in Java yes

Dominaezzz · Answer

Send me a link when you re done This sounds interesting

jw · Answer

Will do should be tomorrow

Marko Mitic · Answer

Post it to this thread

Ilmir Usmanov [JB] · Answer

gt Seems like it would create an invalid state Let the function be `foo` `BaseContinuationImpl resumeWith` calls `Continuation invokeSuspend` which calls `foo` Since you are using the same continuation object in recursive call after the call s completion `BaseContinuationImpl resumeWith` calls `completion resumeWith` effectively dropping the frames since recursion is unrolled At first glance I see no harm in resuming at the same frame

Ilmir Usmanov [JB] · Answer

But after completing the root continuation you will return to the very first frame The continuation object is now corrupted The label field will be recovered but spilled variables is a different story if they are primitives they will recover but references now have completely different values and point to completely different objects TL DR Do not do this unless you are completely sure about what you are going Even then a compiler update with change in spilling unspilling is likely to break the code

Ilmir Usmanov [JB] · Answer

Now I am thinking about exploiting the feature

Dico · Answer

It really seems bad

Dico · Answer

Not something the framework has been engineered towards either understandably

jw · Answer

elizarov · Answer

Great writeup Now I fully understand your problem Since the exception is always delivered from another thread you can also replace `suspendCoroutine` with intrinsic version without protection It is Ok to race it with resumption from another thread <https kotlinlang org api latest jvm stdlib kotlin coroutines intrinsics suspend coroutine unintercepted or return html>

elizarov · Answer

It is the lowest level function it directly gives you access to continuation as if you ve been implementing suspend function in Java code

jw · Answer

So the real Retrofit code is actually using `suspendCancellableCoroutine` which I don t think has an equivalent right I had to hand wave over some stuff or the post would have been twice as long

elizarov · Answer

Ah that is more complicated indeed Some food for thought