How to send stdout/stderr to grandparent process? - node.js

Question

I have this node.js code:
const k = cp.spawn('bash');
k.stdin.end(`
set -e;
echo; echo; echo 'du results:';
( cd "${extractDir}" && tar -xzvf "${createTarball.pack.value}" ) > /dev/null;
find "${extractDir}/package" -type f | xargs du --threshold=500KB;
`);
k.stdout.pipe(pt(chalk.redBright('this is a big file (>500KB) according to du: '))).pipe(process.stdout);
k.stderr.pipe(pt(chalk.magenta('du stderr: '))).pipe(process.stderr);
the problem is that multiple grandchildren will write all it's stdout/stderr to the parent.
I want to do something like this instead:
const pid = process.pid;
const k = cp.spawn('bash');
k.stdin.end(`
set -e;
exec 3<>/proc/${pid}/fd/1
echo > 3 ; echo > 3; echo 'du results:' > 3;
( cd "${extractDir}" && tar -xzvf "${createTarball.pack.value}" ) > /dev/null;
find "${extractDir}/package" -type f | xargs du --threshold=500KB ;
`);
k.stdout.pipe(pt(chalk.redBright('this is a big file (>500KB) according to du: '))).pipe(process.stdout);
k.stderr.pipe(pt(chalk.magenta('du stderr: '))).pipe(process.stderr);
but that technique won't work on MacOS because Mac doesn't have the /proc/pid fs feature. Does anyone know what I am trying to do and maybe a good workaround?

Answer 1

Install libsys or some other means of making raw syscalls from node. Then, do the syscall dup(2) with 1 as its argument, and store the result to variable fd. Once you do that, fd will be a number of a file descriptor that's a duplicate of node's stdout, and it will be inherited by child processes. At that point, just remove exec 3<>/proc/${pid}/fd/1 from bash, and replace all of your >&3s with >&${fd}s.

Answer 2

So I solved this problem a different way, the OP was a bit of an XY question - so I basically use mkfifo since I am on MacOS and don't have a /proc/<pid> file:
first we call this:
mkfifo "$fifoDir/fifo"
and then we have
const fifo = cp.spawn('bash');
fifo.stdout.setEncoding('utf8');
fifo.stdout
.pipe(pt(chalk.yellow.bold('warning: this is a big file (>5KB) according to du: ')))
.pipe(process.stdout);
fifo.stdin.end(`
while true; do
cat "${fifoDir}/fifo"
done
`);
const k = cp.spawn('bash');
k.stdin.end(`
set -e;
echo; echo 'du results:';
tar -xzvf "${createTarball.pack.value}" -C "${extractDir}" > /dev/null;
${cmd} > "${fifoDir}/fifo";
kill -INT ${fifo.pid};
`);
k.stdout.pipe(process.stdout);
k.stderr.pipe(pt(chalk.magenta('du stderr: '))).pipe(process.stderr);
k.once('exit', code => {
if (code > 0) {
log.error('Could not run cmd:', cmd);
}
cb(code);
});
},
the "fifo" child process is a sibling process to process k, which is obvious when you look at the above code block, and the "fifo" process ends up handling the stdout of one of the subprocesses of the k process - they intercommunicate by way of the named pipe (mkfifo).