Read stdout/stderr from subprocess asynchronously

General Usage
1

I want to call a process that might write to its stdout or stderr however it chooses. The process could take a while, and it might die (exit with nonzero status).

Conceptually, I want something like this:

cmd = `sh -c 'echo "My stdout"; echo "My stderr" 1>&2; sleep 5; exit 1'`
open(cmd) do io  # WRONG - doesn't capture stderr
    for line in eachline(io)
        @info "sub> $line"
    end
end

I want:

  1. The sub> ... outputs should appear immediately, not wait for the process to finish
  2. I don’t need to distinguish between stdout and stderr on incoming lines
  3. I do need to distinguish between a zero & nonzero exit code - but if the process had spewed some lines before dying, I need to make sure I’ve captured all of them in my eachline() loop

I toyed around with creating a custom IO subtype that just does @info "sub> $line" whenever it receives any input, and using it as both stdout and stderr for a pipeline, but I couldn’t get my IO to be full-featured enough to work in this way (it would be great if there were an interface for IO objects).

Any advice for getting this to work?