[swift-evolution] Introducing synthesized locks

[David Smith]

It's relatively unlikely that Swift's concurrency model, once it gains one, will look all that similar to Objective-C's. For example*, if Swift were to adopt a shared-nothing model where heap storage was per-thread and could only be accessed by another thread by copying or moving the value to its heap, then a 'synchronized' construct wouldn't be applicable. Until Swift does have a concurrency model, it's probably best to focus on library-level solutions like libdispatch. 

	David

*and this is just an example, there haven't been any concrete proposals for a direction on concurrency beyond a few old speculative drafts you can find in the repository

> On Jun 12, 2017, at 2:10 AM, Erik Aigner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> In my day to day tasks, synchronization primitives are used quite often. ObjC had the @synchronized attribute for methods. I didn’t find anything about this in swift evolution, so I thought i bring it up here. I think it would quite easily be possible to introduce a synchronized qualifier for struct/class objects that automatically synthesize a semaphore variable on the object and use it to lock said method. Here is an example of how that would work
> 
> Without the synchronized attribute (code written in mail, not compiled):
> 
> class Obj {
> 
> 	 private let sema = DispatchSemaphore(value: 1)
> 
> 	func synchronizedMethod() {
> 		sema.wait()
> 		defer {
> 			sema.signal()
> 		}
> 		// do something...
> 	}
> }
> 
> With synchronized attribute (the semaphore/wait/deferred-signal is synthesized by Swift automatically)
> 
> class Obj {
> 
> 	synchronized func method() {
> 		// semaphore is synthesized automatically, do something…
> 	}
> }
> 
> 
> Cheers,
> Erik
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