Mutex.withLock() added EXACTLY_ONCE contract

[ZhouGongZaiShi]

Mutex.withLock(owner: Any? = null, action: () -> T) should be like synchronized (lock: Any, block:()-> R), add EXACTLY_ONCE contract.
This is very helpful when the compiler infers variable initialization.

val str : String
mMutex.withLock {
   str = "If no Exception is thrown, it will be assigned."
}
println("I should be able to use str field. str = $str")

[fvasco]

Same issue on Semaphore.withPermit

[zach-klippenstein]

I think this needs to be AT_MOST_ONCE since the block won't be ran at all if lock throws (eg cancelled while suspended).

[ZhouGongZaiShi]

I think this needs to be AT_MOST_ONCE since the block won't be ran at all if lock throws (eg cancelled while suspended).

I think that if mutex.withlock{} throws a CancellationException(or other exception) while suspended, then the coroutine will not run the code after mutex.withlock{}.

[zach-klippenstein]

Yep, I forgot that the compiler will assume anything run inside a try wasn't executed if it throws.

[elizarov]

Looks good to me, but let's also add a similar contract on Semaphore.withPermit in the same PR for consistency.

[elizarov]

Good to go now. Waiting for tests to pass.