[swift-users] Is this really a race condition?

Guillaume Lessard glessard at tffenterprises.com
Wed Mar 1 17:29:53 CST 2017

> On Mar 1, 2017, at 3:21 PM, Edward Connell via swift-users <swift-users at swift.org> wrote:
> The thread sanitizer on Linux is reporting that I have race conditions in libswiftcore. I eliminated enough code down to this trivial example. Is there really a race condition here or are these bogus errors?
> 	let count = 1000
> 	var items = [[UInt8]?](repeating: nil, count: count)
> 	DispatchQueue.concurrentPerform(iterations: count) {
> 		items[$0] = [UInt8](repeating: 7, count: 10)
> 	}
> My real scenario is retrieving data asynchronously, so I just threw in a buffer assignment.

The assignments to array elements are where the race lies.

I don’t know about the libswiftcore part, but: assigning to a shared Array concurrently from multiple threads won't work, because of Array's copy-on-write behaviour. You could do

let items = UnsafeMutablePointer<[UInt8]?>.allocate(capacity: 1)
items.initialize(to: nil, count: count)

DispatchQueue.concurrentPerform(iterations: count) {
  items[$0].initialize([UInt8](repeating: 7, count: 10))

// you’ll be able to see here that they’re all initialized

items.deallocate(capacity: count)

Guillaume Lessard

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