Exit Code 137 when a background subprocess is closed externally

[wellwelwel]

What version of Bun is running?

1.1.8+89d25807f

---

What platform is your computer?

Darwin 23.4.0 arm64 arm

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What steps can reproduce the bug?

When a subprocess is started, but is terminated externally (e.g., using kill -9), Bun executes the entire script normally until its end, but terminates the process with code 137 if this subprocess was previously called (using fetch, for example), even if it all worked properly.

I've created a minimal repository for a simple step-by-step repro using solely native Bun features:
https://github.com/wellwelwel/bun-unexpected-exit-code-137

Steps

• Start a subprocess (e.g., spawn) that will be used in background
• Fetch it
• Close the subprocess directly by PID
• Wait for the main process to be completed

If "fetch" step is removed, everything will work perfectly.

lib/server.ts

// Nothing wrong here (it could be any service or server, using Bun or not) 🙋🏻‍♂️

Bun.serve({
  port: 4000,
  fetch: () => new Response('✅ Server is Live'),
});

lib/pid.ts

// Nothing wrong here (just for context) 🙋🏻‍♂️

const findPID = (port: number): Promise<number[]> =>
  new Promise(async (resolve) => {
    const PIDs: Set<number> = new Set();
    const service = Bun.spawn(['lsof', '-t', '-i', `:${Number(port)}`]);

    const output = (await new Response(service.stdout).text())
      .trim()
      .split('\n');

    output.forEach((pid) => {
      if (pid) PIDs.add(Number(pid));
    });

    return resolve(Array.from(PIDs));
  });

const killPID = async (PID: number) => {
  const proc = Bun.spawn(['kill', '-9', PID.toString()]);

  return await proc.exited;
};

export const killPort = async (port: number) => {
  const PIDs = await findPID(port);

  PIDs.forEach(async (PID) => await killPID(PID));
};

repro.error.ts

import { killPort } from './lib/pid.js';

// Start the server in background
try {
  const proc = Bun.spawn(['bun', 'lib/server.ts'], {
    // It doesn't run if "fetch" is used
    onExit: (proc) => {
      console.log('✅ Sub process signal Code is', proc.signalCode);
      console.log('✅ Sub process exit code is', proc.exitCode);
    },
  });

  const error = await new Response(proc.stderr).text();
  if (error) console.log(error);
} catch (quiet) {
}

const repro = async () => {
  const res = await (await fetch('http://localhost:4000')).text();
  console.log(res);

  await killPort(4000);

  console.log('✅ Module continues to work normally after closing the PIDs');
};

setTimeout(repro, 1000);

// It doesn't run if "fetch" is used
process.on('exit', (code) => {
  console.log('✅ Exit Code is', code);
});

Output

bun repro.error.ts

✅ Server is Live
✅ Module continues to work normally after closing the PIDs
[1]    7901 killed     bun repro.error.ts

ℹ️ Note that the error is not exactly related to fetch itself. Even using node:http or an external package, the issue seems to be related to the simple fact of Bun triggers a service running in the background.

Testing manually, this behavior also occurred on Linux, including the official Docker Alpine images to check different Bun versions and the platform linux/amd64.

---

What is the expected behavior?

I tried the same approach for Deno and Node.js, and everything worked normally in all cases, as expected.

Also, in Bun, by just not calling the background subprocess, everything will work as usual:

repro.success.ts

import { killPort } from './lib/pid.js';

try {
  const proc = Bun.spawn(['bun', 'lib/server.ts'], {
    // Without "fetch", everything works fine
    onExit: (proc) => {
      console.log('✅ Sub process signal Code is', proc.signalCode);
      console.log('✅ Sub process exit code is', proc.exitCode);
    },
  });

  const error = await new Response(proc.stderr).text();
  if (error) console.log(error);
} catch (quiet) {
}

const repro = async () => {
  // const res = await (await fetch('http://localhost:4000')).text();
  // console.log(res);

  await killPort(4000);

  console.log('✅ Module continues to work normally after closing the PIDs');
};

setTimeout(repro, 1000);

// Without "fetch", everything works fine
process.on('exit', (code) => {
  console.log('✅ Exit Code is', code);
});

• Same code, just commenting fetch

Output

bun repro.success.ts

✅ Module continues to work normally after closing the PIDs
✅ Sub process signal Code is SIGKILL
✅ Sub process exit code is null
✅ Exit Code is 0

---

What do you see instead?

A visual output of both the error and success examples:

---

Additional information

I noticed that both Bun and Deno reproduce the issue #7441. To solve this in my package, I've allowed the user to pass a port when finishing a subprocess to ensure its end.

But even if the process terminates completely successfully, this happens (due to the 137 exit code — this reported issue):

The following test results use node:child_process and node:http, so this issue also occurs by using solely native Node.js features and using Bun just to run it (although it works normally for Node.js and Deno).

---

Thanks for your time 🙋🏻‍♂️

[wellwelwel]

Thanks, @paperdave 🤝

Same approach worked for Windows, using netstat instead.