[swift-users] Data races with copy-on-write

[Jens Alfke]

> On Dec 5, 2017, at 6:24 AM, Michel Fortin via swift-users <swift-users at swift.org> wrote:
> 
> The array *storage* is copy on write. The array variable (which essentially contains a pointer to the storage) is not copy on write. If you refer to the same array variable from multiple threads, you have a race. Rather, use a different copy of the variable to each thread. Copied variables will share the same storage but will make a copy of the storage when writing to it.

I think you’ve misunderstood. The race condition Romain is referring to is when the two threads both access the shared storage, through separate array variables.

Romain:
From the thread sanitizer warnings, it sounds like the implementation of Array is not using synchronization around its call(s) to isKnownUniquelyReferenced … which would mean the class is not thread-safe.

It’s pretty common for the regular (mutable) collection classes supplied by a framework to be non-thread-safe; for example consider Foundation and Java. The reason is that the overhead of taking a lock every time you access an array element is pretty high. Generally it’s preferable to use larger-granularity locks, i.e. grab an external lock before performing a number of array operations.

—Jens
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