Minimal subprocess API

[marzipankaiser]

Low-level-ish process api. Allows to:

• spawn a process
• with stdin potentially provided in a callback that writes to it
• stdout and stderr potentially redirected to a callback
• returning the exit code after completion

The API uses a (mutable) options object which allows to set those callbacks.

Fixes #1080.

[jiribenes]

I cannot attest to the LLVM version, but I think I sort-of understand the JS version and it looks alright.
I am fine with this being merged as-is and figuring out a more user-friendly API later (once I actually try to use it, that is). In general, I think it's a good idea to merge stdlib stuff earlier, so that we (and the students) can use it and see what's good and what's not good. :)

[jiribenes]

Here's just my general questions / comments for the JS version

[jiribenes]

This looks mutable to me.
Couldn't there be an issue when we try to reuse existing SpawnOptions?

Should we instead copy opts here?

[marzipankaiser]

It is mutable. I'm still unsure how an API that is both safe and does not extensively copy things on the C/JS side.
Having SpawnOptions be a extern interface (and then explicitly mutated) would be more ideal but I'm not sure how to pass extern interfaces over the FFI boundary.
I.e. something like spawn(...){ options: {SpawnOptions} => Unit }: ...

[jiribenes]

Is this runTopLevel or run? 🤔

[marzipankaiser]

It is a callback called from the JS side, so from somewhere in the event loop.
AFAICT, this means runToplevel

[jiribenes]

Can the process fail to spawn at all? 🤔

[marzipankaiser]

It might. This is not currently handeled (but could, and probably should, be)

[jiribenes]

This likely works now, but NodeJS can (and will) sometimes return false, and then we should be more async and wait for the 'drain' event.
https://nodejs.org/api/stream.html#writablewritechunk-encoding-callback

[jiribenes]

This ignores situations in which the child process is terminated.

I thought we had a string->number map for this somewhere in io/error, but I couldn't find it...
Anyways, here's something like what could be there (but I don't like the implementation)

  p.on('close', (exitcode, sig) => { 
    if (sig) {
      const sigMap = {
        'SIGHUP':1, 'SIGINT':2, 'SIGQUIT':3, 'SIGILL':4, 'SIGTRAP':5,
        'SIGABRT':6, 'SIGBUS':7, 'SIGFPE':8, 'SIGKILL':9, 'SIGUSR1':10,
        'SIGSEGV':11, 'SIGUSR2':12, 'SIGPIPE':13, 'SIGALRM':14, 'SIGTERM':15
      };
      k(128 + (sigMap[sig] || 15)); // '15' is default, this is obviously weird
    } else {
      k(exitcode || 0); // the `|| 0` is just defensive here
    }
  });

[marzipankaiser]

If we can reasonably from FFI, it would also be nice to just do signal('SIGHUP') or sim. instead of encoding it into an int 🤔