I'd like there to be an API that can spawn processes similarly to popen(3) / subprocess.run.

The high-level API would be something like:

import process

process::spawn("cp", ["from.txt", "to.txt"]) // : ???

There are two main questions:

1. How to actually implement this / what properties do we want?

Should spawn only return the process ID? What would be the callback when the command gets run? Is it (a-)synchronous?

NodeJS has child_process[.spawn] for this, libuv (for the LLVM backend) has spawn. It's necessary to have such an API that works on both the [Node.]JS and the LLVM backends.

2. How do we communicate with the process?

In simpler terms, how do we get the stdin/stdout/stderr of the new process?
Can/should it be streaming? How do we process failures (exit = 1) and return codes in general? Can we do something with signals? IPC?...