How do I merge stdout and stderr of a child process?

[Darksonn]

If I open a child process with tokio::process::Command, the ChildStderr and ChildStdout types are two separate streams of data. I would like to merge the two streams so I can read the output line-by-line from both streams in an interleaved manner.

[Darksonn]

The general approach for merging the two streams is the following:

1. Put stdout and stderr into tokio::io::BufReader.
2. Put the buffered streams into tokio::io::Lines.
3. Wrap it in tokio_stream::wrappers::LinesStream.
4. Merge them with tokio_stream::StreamExt::merge.

This gives the following code:

use tokio::io::{AsyncBufReadExt, BufReader};
use tokio_stream::StreamExt;
use tokio_stream::wrappers::LinesStream;

// Variable of type tokio::process::Child.
let child = ...;

// Take ownership of stdout and stderr from child.
let stdout = child.stdout.take().unwrap();
let stderr = child.stderr.take().unwrap();

// Wrap them up and merge them.
let stdout = LinesStream::new(BufReader::new(stdout).lines());
let stderr = LinesStream::new(BufReader::new(stderr).lines());
let mut merged = StreamExt::merge(stdout, stderr);

// Iterate through the stream line-by-line.
while let Some(line) = merged.next().await {
    // Since reading a line may fail, we use the question-mark to unwrap the line.
    let line = line?;
    println!("{}", line);
}

[nayeemrmn]

Thanks for looking into this, Deno has a need for this feature.

I would like to merge the two streams so I can read the output line-by-line from both streams in an interleaved manner.

Systems which support combining pipes can do better than buffering line-by-line. Running the following script should show that shells multiplex the two streams more granularly:

#!/bin/bash
printf "abc"
printf "def" 1>&2
printf "ghi"
printf "jkl" 1>&2

abcdefghijkl

The following shows that Python 3's pipe combination can do the same:

#!/usr/bin/env python3
import subprocess
print(subprocess.run(["./temp.sh"], stdout=subprocess.PIPE, stderr=subprocess.STDOUT))

CompletedProcess(args=['./temp.sh'], returncode=0, stdout=b'abcdefghijkl')

Ultimately, I think we need a mirror of https://docs.rs/os_pipe/0.9.2/os_pipe/ where https://docs.rs/os_pipe/0.9.2/os_pipe/struct.PipeReader.html can be asynchronously read.