"withContext" and "prompt cancellation guarantee"

[yvolk]

KDoc for the withContext function (

kotlinx.coroutines/kotlinx-coroutines-core/common/src/Builders.common.kt

Line 131 in 865c229

* when it completes. Note that the result of `withContext` invocation is

) says:

Note that the result of withContext invocation is dispatched into the original context in a cancellable way with a prompt cancellation guarantee,
which means that if the original [coroutineContext], in which withContext was invoked,
is cancelled by the time its dispatcher starts to execute the code,
it discards the result of withContext and throws [CancellationException].

But this example https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-core/jvm/test/guide/example-cancel-05.kt
shows that even if the job is cancelled, "withContext" (in the finally block) starts normally.

I even checked this adding two lines inside finally block:

...
    } finally {
        println("Context in finally:  coroutineContext isActive:${coroutineContext.isActive}")
        println("Context in finally:  job isCancelled:${coroutineContext[Job]?.isCancelled}")
        withContext(NonCancellable) {
            println("job: I'm running finally")
...

and get this:

Context in finally:  coroutineContext isActive:false
Context in finally:  job isCancelled:true
job: I'm running finally

As I understand, either documentation or behavior should be changed.

[yvolk]

Looks like copy-paste bug?!

[elizarov]

That's a subtle effect of a combination of withContext and NonCancellable and it works so by design. I'll see how it could be properly documented.