Does String copy storage between actors?

[AlexanderM (Alexander Momchilov)]

@Max_Desiatov

I'd expect isKnownUniquelyReferenced in CoW containers under the hood is used to check an atomic reference count on modification universally, including concurrent access, which is why it is atomic in the first place.

Correct! It's thread-safe by virtue of the reference counting being synchronized already. It's just doing a single check, and doesn't also modify the ref count, so there's no TOC/TOU concerns.

By the time you're able to call isKnownUniquelyReferenced(x), you've got a strong reference to x. At that point, it can check if the reference count is >1, indicating that some other reference must exist.

Here's an enlightening thread on this matter: isKnownUniquelyReferenced thread safety