> Explain how and why this is atomic in a doc comment here. 

Added.

> Is it important that this be safe for multiple Sass processes concurrently using the same cache?

Yes. People leaving multiple watchers running on accident is a real and common occurrence.

>  If not, it's probably cleaner to use a mutex

An in-memory Mutex only works for a single process. Using `File.flock` doesn't work on windows. Also, a mutex introduces delays and deadlock possibilities. Lock-free implementations like the one we are using here are the "cleaner" implementation to handle concurrency.

> rely on the presumptive atomicity of the filesystem

This is a posix guarantee and it is also true on windows' filesystems. It is a very commonly employed strategy. If there are filesystems that do not make this guarantee and need to be supported, we can address those when a user comes to us with that need. I don't know of such a filesystem that would ever by used by a Sass developer.