withContext doesn't check for cancellation before running the block

[LouisCAD]

The current implementation of withContext doesn't seem to check for cancellation (like yield() would for example) before running the passed block.
I didn't double check, but if I remember correctly, it doesn't check for cancellation after running the block either.

I think withContext checked for cancellation before and after running the block in the past (at some point before 1.0). I don't recall seeing any release notes about this though, so maybe I made false assumptions.

Anyway, I think it would be great to throw a CancellationException if the coroutine has been cancelled before running the block, and after it has been run before resuming the parent coroutine.

Cancellation is an important part of the efficiency coroutines brings to our programs. The earlier you stop doing something no longer needed, the earlier you start saving resources.

[qwwdfsad]

I've partially addressed this issue in #791
All builders now unconditionally check cancellation status on exit.

I don't think it's really necessary to check it on the beginning of the block because it will be thrown on the first suspension point anyway, and if you are using some in-place builder you will suspend soon. While a desire to insert cancellation checks everywhere is understandable, we should remember that they are not free.

[LouisCAD]

Nothing is free, but I'm wondering about the cost of checking the boolean property isActive and proceed on cancellation only when it's false. Is accessing this property costly?

About checking cancellation only on exit, I'm now wondering if checking cancellation before enter is useful in practical use cases, as you usually launch the coroutine already on the first dispatcher you want to use, and if you switch afterwards, it was likely because of previous suspension points (that may check for cancellation before you reach withContext). Do you think there are practical use cases for this? If there is none, #791 probably resolves this issue on its own.

[Tolriq]

Can I ask a small confirmation as my tests did gave a different result about withContext checking for cancellation at the end.

@qwwdfsad does your PR changes also withContext and it is currently not checking?

I've made a lot of tests and finally left the code secure with a manual check for isActive after withContext

        launch {
            val result = withContext(IO) { repository.getAll() }
            if (isActive) {
                liveData.value = result
            }
        }

But from all my tests when cancelling the coroutine it's stopped before reaching the if.