Hi,
I'm trying to better understand how copy-on-write works, especially in a
multithreaded environment, but there are a few things that confuse me.
From the documentation, it is said that:
"If the instance passed as object is being accessed by multiple threads
simultaneously, isKnownUniquelyReferenced(_:) may still return true. Therefore,
you must only call this function from mutating methods with appropriate thread
synchronization. That will ensure that isKnownUniquelyReferenced(_:) only
returns true when there is really one accessor, or when there is a race
condition, which is already undefined behavior."
Let's consider this sample code:
func mutateArray(_ array: [Int]) {
var elements = array
elements.append(1)
}
let q1 = DispatchQueue(label: "testQ1")
let q2 = DispatchQueue(label: "testQ2")
let q3 = DispatchQueue(label: "testQ3")
let iterations = 1000
var array: [Int] = [1, 2, 3]
q1.async {
for _ in 0..<iterations {
mutateArray(array)
}
}
q2.async {
for _ in 0..<iterations {
mutateArray(array)
}
}
q3.async {
for _ in 0..<iterations {
mutateArray(array)
}
}
// ...
From what I understand, since Array<T> implements copy-on-write, the array
should be copied only when the mutating append(_:) method is called in
mutateArray(_:). Therefore, isKnownUniquelyReferenced(_:) should be called to
determine whether a copy is required or not.
However, it is being accessed by multiple threads simultaneously, which may
cause a race condition, right? So why does the thread sanitizer never detect a
race condition here? Is there some compiler optimization going on here?
On the other hand, the thread sanitizer always detects a race condition, for
instance, if I add the following code to mutate directly the array:
for _ in 0..<iterations {
array.append(1)
}
In this case, is it because I mutate the array buffer that is being copied from
other threads?
Even strangier, let's consider the following sample code:
class SynchronizedArray<Element> {
// [...]
private var lock = NSLock()
private var _elements: Array<Element>
var elements: Array<Element> {
lock.lock()
defer { lock.unlock() }
return _elements
}
@discardableResult
public final func access<R>(_ closure: (inout T) throws -> R) rethrows
-> R {
lock.lock()
defer { lock.unlock() }
return try closure(&_value)
}
}
let syncArray = SynchronizedArray<Int>()
func mutateArray() {
syncArray.access { array in
array.append(1)
}
var elements = syncArray.elements
var copy = elements // [X] no race condition detected by TSan when I
add this line
elements.append(1) // race condition detected by TSan (if previous line
is missing)
}
// Call mutateArray() from multiple threads like in the first sample code.
The line marked with [X] does nothing useful, yet adding this line prevents the
race condition at the next line to be detected by the thread sanitizer. Is this
again because of some compiler optimization?
However, when the array buffer is being copied, we can mutate the same buffer
with the append(_:) method, right? So, shouldn't the thread sanitizer detect a
race condition here?
Please let me know if I ever misunderstood how copy-on-write works in Swift.
Also, I'd like to know:
- besides capture lists, what are the correct ways to pass a copy-on-write
value between threads?
- for thread-safe classes that expose an array as a property, should I always
copy the private array variable before returning it from the public getter? If
so, is there any recommended way to force-copy a value type in Swift ?
Any help would be greatly appreciated.
Thanks.
Note: I'm using Swift 4 with the latest Xcode version (9.2 (9C40b)) and the
thread sanitizer enabled.
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