child_process: add public API to unref ipc channel

[mscdex]

• Version: all
• Platform: n/a
• Subsystem: child_process

It would be nice to have a public API to unref() a child process's ipc channel. My use case for this is that I spawn a child process, send some messages back and forth via ipc, then at some point I want to detach the child process. child.unref() is not enough because that only unrefs the C++ ProcessWrap handle. Currently I have to resort to also doing child._channel.unref(). I am not sure if this should be done automatically inside child.unref() or if there should be a separate function or similar.

[sam-github]

I've unrefed _channel, too, for similar reasons, and other reasons.

I'm not sure ._channel should be quietly unrefed when you unref the ChildProcess. It seems inconsistent to me, because child#unref() doesn't unref the other pipes, like stdout, stderr, or even pipes on descriptors higher than 2.

I'd prefer for .channel to just be documented as a property that comes into existence when 'ipc' is in the stdio: options, as .stdout and the others are. This would mean changing the name of _channel, aliasing its name so both names exist, or just documenting _channel. I'd prefer aliasing in v7, deprecating in v8, and removing in v9.

[cjihrig]

I like that idea. I've opened #9322 as a potential starting point.