Process does not die when child process to which stdin is piped dies

[masaeedu]

• Version: v10.3.0
• Platform: Linux plutobox 4.18.16-arch1-1-ARCH deps: update openssl to 1.0.1j #1 SMP PREEMPT Sat Oct 20 22:06:45 UTC 2018 x86_64 GNU/Linux
• Subsystem: child_process/stream/process

Basically a duplicate of: #2276, which was either closed without having been resolved or has regressed.

Stick the following in a file:

#!/usr/bin/env node
const cp = require("child_process");

const { stdin, stdout, stderr } = process;
const child = cp.execFile("ps", [], { stdio: ["pipe", "pipe", "pipe"] });

process.stdin.pipe(child.stdin); // this is the problematic one
child.stdout.pipe(process.stdout);
child.stderr.pipe(process.stderr);

child.on("exit", (code, signal) =>
  console.log({ msg: "Exited", code, signal }),
);

chmod +x and run it in some shell, and you'll see it'll output the stuff from ps, then just hang. Assuming your shell's stdin is being forwarded to the node process, hitting enter will unstick it. If you comment out the process.stdin... line (since ps doesn't actually need any stdin), you'll see it end as expected.

The workaround I'm using for this is to add the event handler child.on("exit", (code, signal) => code !== null ? process.exit(code) : signal !== null ? process.kill(process.pid, signal) : fail("Impossible situation, process exited but neither exited nor was killed")). I can't really use this in the general case, because I may need to do more stuff while the child process is running and exiting, and can't always assume the parent needs to die precisely when this one specific child process dies. At the same time the parent should be free to exit when all children are dead, and shouldn't hang like this.

[masaeedu]

I tried manually unpipe-ing stdin in the on("exit", ...) handler, in the hopes that this would fix the problem, but neither process.stdin.unpipe(child.stdin) nor process.stdin.unpipe(undefined) seem to make any difference.

[Trott]

@nodejs/child_process

[santigimeno]

Why not just call: process.stdin.unref() ?

[bnoordhuis]

^ This. It's not a bug, it's a ps quirk. cat | ps works the same way.

[masaeedu]

@bnoordhuis If ps is quirky so is scp, because that also hangs the parent process (in fact my actual use case was uploading several files to a remote ssh server):

#!/usr/bin/env node
const util = require("util");
const cp = require("child_process");

const exe = "scp";
const args = [
  "-o",
  "StrictHostKeyChecking=no",
  "-o",
  "LogLevel=ERROR",
  "-i",
  "~/.ssh/test_machine_rsa",
  "-r",
  "test.js",
  "Administrator@192.168.33.164:C:/test.txt",
];

const { stdin, stdout, stderr } = process;
const child = cp.spawn(exe, args, {
  stdio: ["pipe", "pipe", "pipe"],
});

process.stdin.pipe(child.stdin); // this is the problematic one
child.stdout.pipe(process.stdout);
child.stderr.pipe(process.stderr);

child.on("exit", (code, signal) => {
  console.log({ msg: "Exited", code, signal });
});

Maybe the quirk common to both is that neither actually consumes anything from stdin.

[bnoordhuis]

Maybe the quirk common to both is that neither actually consumes anything from stdin.

That's right; cat | true is another example. I mentioned ps because you did. :-)