process: process.kill() and signals on Windows

[vsemozhetbyt]

• Version: many
• Platform: Windows 7 x64
• Subsystem: process

The doc states:

Note: Windows does not support sending signals, but Node.js offers some emulation with process.kill(), and ChildProcess.kill().

WRT supporting signals on Windows, the doc makes only this remark:

SIGTERM is not supported on Windows, it can be listened on.

As for the rest, process.kill() behavior on Windows is somehow confusing and hazardous. A test file with just this line:

process.kill(process.pid, SIGNAL_NAME_MENTIONED_IN_THE_DOC);

gives three types of results.

1. SIGTERM, SIGINT, SIGKILL (despite the doc note wrt SIGTERM) — silently exit.
2. SIGUSR1, SIGPIPE, SIGSTOP, SIGBUS — throw this error:

internal/process.js:184
        throw new Error(`Unknown signal: ${sig}`);
        ^

Error: Unknown signal: SIGUSR1
    at process.kill (internal/process.js:184:15)
    at Object.<anonymous> (...\test.js:5:9)
    at Module._compile (module.js:607:30)
    at Object.Module._extensions..js (module.js:618:10)
    at Module.load (module.js:516:32)
    at tryModuleLoad (module.js:466:12)
    at Function.Module._load (module.js:458:3)
    at Module.runMain (module.js:643:10)
    at run (bootstrap_node.js:441:7)
    at startup (bootstrap_node.js:144:9)

3. SIGHUP, SIGBREAK, SIGWINCH, SIGFPE, SIGSEGV, SIGILL — throw this error:

internal/process.js:190
      throw errnoException(err, 'kill');
      ^

Error: kill ENOSYS
    at exports._errnoException (util.js:1057:11)
    at process.kill (internal/process.js:190:13)
    at Object.<anonymous> (...\test.js:6:9)
    at Module._compile (module.js:607:30)
    at Object.Module._extensions..js (module.js:618:10)
    at Module.load (module.js:516:32)
    at tryModuleLoad (module.js:466:12)
    at Function.Module._load (module.js:458:3)
    at Module.runMain (module.js:643:10)
    at run (bootstrap_node.js:441:7)

As with this issue, I am not sure, if this is a Node.js or libuv domain. Also I am not sure if this should be addressed in code or in docs. So just reporting for extra precaution.

[vsemozhetbyt]

In addition, I can't attach an event listener for any signal fired by the process.kill(), so the example here does not work on Windows with any signal (it either throws or silently exits). Maybe this is also worth to be documented.

[sam-github]

The docs could doubtless be improved, but they say:

Note: Windows does not support sending signals, but Node.js offers some emulation with process.kill(), and ChildProcess.kill(). Sending signal 0 can be used to test for the existence of a process. Sending SIGINT, SIGTERM, and SIGKILL cause the unconditional termination of the target process.

Then you say:

SIGTERM, SIGINT, SIGKILL (despite the doc note) — silently exit.

I don't get it, "silent exit" === "unconditional termination", doesn't it? What doc note are you talking about?

And like the docs say, all other signals are unsupported.

They should perhaps be un-supported more uniformly, perhaps with ENOSYS instead of Unknown signal.

This is probably happening because node does some checking to convert signal strings to numbers, and it fails on Windows because it doesn't know about some signal strings.

But even if it suceeds in converting a signal string to a number, Windows doens't support kill, thus its ENOSYS.

[sam-github]

per "example doesn't work", it links to the signal docs, which say under what conditions it works (3 signals have some emulation on windows). Perhaps its better to rework the docs, moving the signal list into two places, one in process.kill which describes behaviour when signal is sent, and one in process.on() which describes when signal is received. The current structure is I think contributing to the confusion.

[vsemozhetbyt]

@sam-github 'despite the doc note': I mean, the doc states 'SIGTERM is not supported on Windows', but exactly SIGTERM does not throw any ENOSYS or Unknown signal error messages, while other many signals throw errors and doc states nothing about support for them.

[sam-github]

Maybe you can improve it now that you know how it works? To be clear, SIGTERM is not supported on Windows as a signal. Windows as an O/S has absolutely no signal support. Like it says later (should probably be moved closer to the note you mention), sending SIGTERM/KILL/INT cause windows processes to be terminated on Windows, as a way of UV trying to emulate signals where they do not exist.

If SIGTERM was supported on Windows, you could listen for it and receive it... but you cannot. You can listen..., but you'll never receive it.

[vsemozhetbyt]

I try to abstain from writing English texts of a significant length, as I am clumsy at that. And, to be honest, signals on Windows is a murky topic for me. I do feel it is somehow wrong for me to just dump such issues that I can't fix myself. Sorry. Maybe somebody from @nodejs/platform-windows will find some time to refactor the doc on this topic in future.

[refack]

[offtopic] anyone knows a TODO app for windows that can track github issues? I would like to address this issue, but after I finish some stuff I'm doing. By them most probably I'd forget about this one.

[sam-github]

been trying octobox.io lately, I usually just keep a markdown file, http://todotxt.com/ can be nice, too.

[gibfahn]

@refack alternately you could just assign it to yourself, then it'll show up in the GitHub list of assigned to me issues.

[alohaninja]

The windows subsystem lacks signaling support using process.kill(). Managing complex jobs/processes on windows without proper signaling is still doable though - it just takes some creative workarounds.
process.send will allow you to message child processes to notify them of pending termination.
For hung child processes (such as long-running tests), it's best to fire up exec/execFile to kill any processes that have been running longer than a set threshold.
This helper will kill orphaned processes via powershell.exe running longer than 30 min: Get-Process -name myprocess | ? { $_.StartTime -lt (Get-Date).AddMinutes(-30) } | Stop-Process -Force

[refack]

“Fear is the path to the dark side. Fear leads to anger. Anger leads to hate. Hate leads to suffering.”
– Yoda

@alohaninja no need to be so negative...