process.on beforeExit broken on Windows

[joaocgreis]

• Version: all >=4.0.0
• Platform: Windows
• Subsystem: v8, lib (?)

Investigating #8821 (comment) , I found that process.on('beforeExit', ...) is broken on Windows if node is not invoked from the Windows Command Prompt. Here is a simple repro:

const child = require('child_process');
//const script = "console.log('start');"; // OK
//const script = "process.on('exit', () => console.log('exit')); console.log('start');"; // OK
const script = "process.on('beforeExit', () => console.log('beforeExit')); console.log('start');";
const proc = child.spawn(process.execPath, ['-e', script], { encoding: 'utf8' });
proc.stdout.on('data', (data) => console.log(`stdout: ${data}`));
proc.stderr.on('data', (data) => console.log(`stderr: ${data}`));

This triggers an infinite loop printing beforeExit to stdout. This happens when node spawns itself (as in the repro above) or when node is started from gitbash. This does not happen when node is started from the Windows Command Prompt with

node -e "process.on('beforeExit', () => console.log('beforeExit')); console.log('start')"

but happens with

node -e "process.on('beforeExit', () => console.log('beforeExit')); console.log('start')" | cat

so it might be related to output redirection or output not being a TTY.

Bisected, first bad commit is c431725 (V8 4.5 update before node v4.0.0).

cc @nodejs/platform-windows @bnoordhuis

[Fishrock123]

@joaocgreis what happens if you follow it up by adding a setImmediate() to the end?

[joaocgreis]

node -e "process.on('beforeExit', () => console.log('beforeExit')); console.log('start'); setImmediate(function(){});"

does not make any difference, still gets stuck in a loop printing beforeExit.

[Fishrock123]

@joaocgreis I meant, does that make it loop with a pipe, or still not?

[bnoordhuis]

node -e "process.on('beforeExit', () => console.log('beforeExit')); console.log('start')" | cat

That isn't too surprising. The same could happen on Unices with a slow reader and a full pipe.

Bisected, first bad commit is c431725 (V8 4.5 update before node v4.0.0).

That seems an unlikely culprit, seeing how it only touches files in deps/v8.

[seishun]

Seems to be expected behavior to me. console.log is non-blocking when writing to a pipe, so calling it schedules additional work for the event loop, effectively cancelling the exit.

[sam-github]

Expected behaviour as described by @seishun, and documented: https://nodejs.org/api/process.html#process_event_beforeexit

[joaocgreis]

This was surprising for me. Thanks for your help!