Making an array or dictionary a value type by definition, but then actually copying it only when one reference to it tries to modify it is a lovely idea, but it makes me wary in a multi-queued/threaded context. I need to know:
Is Swift's copy-on-write capability thread-safe? eg: If I create an array on one queue and pass it to another queue, is it safe for either queue to modify it while the other might be reading or modifying it? Since by definition the copy was made when the array reference was passed into the second queue, can we assume that the Swift engineers did the right thing and implemented copy-on-write in a queue-safe way?
I found this old discussion of this, which seems authoritative, but in both directions! https://developer.apple.com/forums/thread/53488
Some credible voices say it's thread-safe, others say it isn't. I imagine that this may be because in some early version of Swift it was not, while perhaps in Swift 5 it is. Does anyone here know for sure for Swift 5?
Here's some sample code to illustrate the issue:
func func1()
{
var strings1: [String] = ["A", "B", "C"]
var strings2: [String] = strings1 // array not actually copied
queue.async()
{
strings2.append("D")
}
print(strings1[0]) // is this reference thread-safe?
strings1.append("E") // is this modification thread-safe?
}