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doc: describe child.kill() pitfalls on linux #6052

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doc: describe child.kill() pitfalls on linux #6052

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26 changes: 24 additions & 2 deletions doc/api/child_process.markdown
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Expand Up @@ -732,7 +732,29 @@ delivered to that process instead which can have unexpected results.
Note that while the function is called `kill`, the signal delivered to the
child process may not actually terminate the process.

See `kill(2)`
See `kill(2)` for reference.

Also note: on Linux, child processes of child processes will not be terminated
when attempting to kill their parent. This is likely to happen when running a
new process in a shell or with use of the `shell` option of `ChildProcess`, such
as in this example:

```js
'use strict';
const spawn = require('child_process').spawn;

let child = spawn('sh', ['-c',
`node -e "setInterval(() => {
console.log(process.pid + 'is alive')
}, 500);"`
], {
stdio: ['inherit', 'inherit', 'inherit']
});

setTimeout(() => {
child.kill(); // does not terminate the node process in the shell
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"So what do I do"?

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Unfortunately there currently isn't much you can do about this:

You need to keep references to the grand children's pid and kill it directly. Unfortunately that isn't very easy in such a scenario. If you are lucky you can assume that the pid is the child's pid +1.

I am currently writing (or rather researching) a libuv addition to query all children's pids of a process. This is not too straightforward, since e.g. on OS X there is no C function for that.

The alternative is to detach and just orphan the grandchild

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On Unixen, use 'detached' flag, and process.kill(-child.pid) to kill the entire process group <--- think this will work, have not tested.

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That would work. But for example not on OS X. The question then would be whether we want to document this.
I am making progress with the the get_child_processes thing I was mentioning. If libuv accepts it I would rather point to such a method then, suggesting a rationale: query all children, kill all children. This negative pid number thing on some unices seems very unexpressive to me.

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There is a userland package that solves this: tree-kill.

But I don't think that it's good to recommend a specific userland package.

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@eljefedelrodeodeljefe

But for example not on OS X.

Why not? Its a BSD derived system, it supports process groups.

}, 2000);
```

### child.pid

Expand Down Expand Up @@ -998,4 +1020,4 @@ to the same value.
[`options.stdio`]: #child_process_options_stdio
[`stdio`]: #child_process_options_stdio
[synchronous counterparts]: #child_process_synchronous_process_creation
[`JSON.stringify()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify
[`JSON.stringify()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify