Inconsistent output depending on if it's piped somewhere or not

[fabiospampinato]

Version

22.2.0

Platform

Darwin MBP.local 21.6.0 Darwin Kernel Version 21.6.0: Wed Aug 10 14:28:23 PDT 2022; root:xnu-8020.141.5~2/RELEASE_ARM64_T6000 arm64

Subsystem

No response

What steps will reproduce the bug?

Put this into repro.js

for ( let i = 0, l = 1_000_000; i < l; i++ ) {
 console.log ( i );
}
process.exit ();

Then execute this, and notice how there's nothing to notice, it works like anybody would expect it to work:

node repro.js

Now execute this, and notice how the output is truncated for some reason:

node repro.js | cat

How often does it reproduce? Is there a required condition?

Always.

What is the expected behavior? Why is that the expected behavior?

The expected behavior is for that script to produce a consistent output. If the output is truncated just because we are piping it somewhere that doesn't really make sense to me.

According to the docs:

Calling process.exit() will force the process to exit as quickly as possible even if there are still asynchronous operations pending that have not yet completed fully, including I/O operations to process.stdout and process.stderr.

But I don't think anybody on the planet would want "output" to be truncated and not actually be logged when writing code like this:

console.log(output);
process.exit();

Regardless of that, the output begin inconsistent depending on whether we are piping it to cat or not, considering that the script wants to log the same stuff no matter what, seems like a bug.

What do you see instead?

Inconsistent output depending on whether we are piping to cat or not.

Additional information

No response

[avivkeller]

Calling process.exit() will force the process to exit as quickly as possible even if there are still asynchronous operations pending that have not yet completed fully, including I/O operations to process.stdout and process.stderr.

console.log is an asynchronous operation, so calling process.exit() will terminate it, even if it's in the middle of running.

[fabiospampinato]

console.log is an asynchronous operation, so calling process.exit() will terminate it, even if it's in the middle of running.

I don't see that happening unless I'm piping stdout somewhere else, so the behavior seems inconsistent.

[avivkeller]

IIRC it isn't supposed to be consistent. process.exit() means "Stop immediately", so depending on the process tick, the console will be able to log, or it won't. If you look a module hooks, logging won't occur at all because the process will exit before it has a chance.

I'm not an expert on the specifics of ticking, so take this with a hint of salt

CC @nodejs/process

[benjamingr]

s explained console.log is asynchronous and piped.output is buffered differently (tty'ness) but in any case even if it worked don't rely on it. If you want to write synchronously process.stdout.write and then wait for it to flush (by checking the return value and waiting for drain if it's false before exiting).