Extract process logic from Utils.execute_in_background into Subprocess.spawn_detached

[jawshooah]

Move process logic from Utils.execute_in_background to Subprocess.spawn_in_background. Use ChildProcess rather than low-level Process module for future-proofing, since ChildProcess is Windows-compatible.

Edit: Per the conversation in the comments, this PR now simply extracts the Process.detach(Process.spawn(...)) logic to Subprocess.spawn_detached in the interest of separation of concerns.

[sds]

How does this deal with long-running background processes that continue running after the hook run has completed?

The reason we used Process.detach here was to ensure that we wouldn't create a zombie process. I'm not sure how ChildProcess deals with this situation, but so far it hasn't been necessary to worry about because we don't use it for these sorts of background tasks—it's always been used in a context that blocks the parent Overcommit process until it completes.

Furthermore, I'm not sure what happens to the child process when terminating the parent process (does ChildProcess auto-terminate the child for us?). We wouldn't want this to happen for hooks like IndexTags which can take many seconds to index large repositories.

[jawshooah]

We can detach the ChildProcess using process.detach = true before executing process.start. I only omitted that line because process.exited? throws an error when process is detached, which caused the spec to fail. Didn't consider the problem of zombie processes, and wasn't sure how to work around the error.

[jawshooah]

It doesn't seem like ChildProcess does anything special to terminate the created child process when the parent dies, but that should happen anyway unless the child process has been detached, right?

[jawshooah]

Regardless, we can just use the original Process.detach(Process.spawn(...)) logic here and save the ChildProcess stuff for later, when we figure out Windows compatibility.

[sds]

Merged in 3b67c5d.