groue / semaphore Goto Github PK

I stumbled across https://github.com/adam-fowler/async-collections/blob/main/Sources/AsyncCollections/AsyncSemaphore.swift and I should ask questions to the author.

Could you add this function, thanks

Thank you for open sourcing this approach. I don't have a real issue, but wanted to share some thoughts. There's no discussions tab hence the issue.

I was curious to better understand how this works, because initially I assumed this might not be safe.

I thought locking across awaits is not safe, because locks have to be unlocked on the same thread.

"NSLock can be used safely but require caution to only use in synchronous code and not across an await."

Source: https://forums.swift.org/t/incremental-migration-to-structured-concurrency/54939/21

 public func wait() async {
        lock()
        // ...
        await withUnsafeContinuation { continuation in
            // ...
            unlock()
        }
    }

I wasn't sure if this could deadlock on small machine with a pool size of one, if at the suspension point marked by the await another async wait is run first and then trying to get the lock. Or if the closure would be executed on another thread as with Swift Concurrency there's not much guarantees about the thread on which an async function runs.

I now think this works, because of special behavior of withUnsafeContinuation:

"The closure is then immediately executed in the same execution context (in other words, the current thread) [...]"
Source: https://forums.swift.org/t/clarification-needed-on-unsafecontinuation-documentation/57803/2

If withTaskCancellationHandler and withUnsafeThrowingContinuation also have behave like that it starts to make more sense for me.

Or is there another reason why the locking code works in this case?