Multiplexer / Demultiplexer for standard error and standard output

The muxeo command combines a program's stderr and stdout streams into a single frame stream, followed by a frame containing the exit status code, all written to stdout. Frames are coded as follows:

+--------------+----Err---+---------------+
| kind: u8 = 0 | len: u32 | payload: [u8] |
+--------------+----------+---------------+
+-----Exit Status Code-----+
| kind: u8 = 1 | code: i32 |
+--------------+-----------+
+--------------+----Out---+---------------+
| kind: u8 = 2 | len: u32 | payload: [u8] |
+--------------+----------+---------------+

The demuxeo command knows how to decode the stream of frames received as stdin and then writes to both stderr and stdout depending on the frame kind (err/out) and exits with the decoded status code.

For example, the following command:

muxeo -- my-program --opt-1 --opt-2 -- arg-1 arg-2 | demuxeo

will produce a result similar to:

my-program --opt-1 --opt-2 -- arg-1 arg-2

One possible use case would be passing a program's stderr and stdout over HTTP. Here is an example Bun HTTP server:

import { $ } from "bun";

Bun.serve({
  async fetch() {
    return new Response(
      await $`muxeo -- my-program --opt-1 --opt-2 -- arg-1 arg-2`.arrayBuffer(),
    );
  },
  port: 3000,
});

We could then decode the HTTP stream response as follows, redirecting stderr/stdout if necessary.

curl --silent -- localhost:3000 | demuxeo