Create a non-blocking wait() function for child processes.

[ZoeyR]

Currently the only way to wait on a child is to call wait() which blocks until the child has finished. For some applications, it is more useful to just check if the child has exited without blocking if it hasn't. Usage would be similar to this:

let child = Command::new("sleep").arg("10").spawn().unwrap();
child.wait_no_hang();
// Continue with execution even if child hasn't exited yet.

[BurntSushi]

One possible low-tech solution to this problem is to just spawn your command in a separate thread and use a channel to communicate completion (which can be checked in a non-blocking fashion). This may not work for every use case, but I bet it's just fine for a majority of them. Otherwise, something like WNOHANG might be appropriate on unix. I don't know what the analogous option in Windows is.

[alexcrichton]

Some time ago I actually wrote a crate for this, although it's generalized a bit more to a timeout as opposed to just a wait_no_hang. Thankfully this certainly can be implemented robustly across all platforms, so no Windows concerns!

I do think that our process management story needs to be beefed up a little for external extensions (where I think something like this should live first), but I certainly like the idea!

[LukasKalbertodt]

cc me

[AkshatM]

What is the current status of this RFC, please?

[LukasKalbertodt]

I think this can be closed, now that Child::try_wait() is stable?

[Centril]

Closing per #1294 (comment).