child_process ipc send should communicate back pressure

[tjfontaine]

In v0.10 child_process target.send sometimes returns if the writeQueueSize is smaller than some "arbitrary" value https://github.com/joyent/node/blob/v0.10/lib/child_process.js#L481 but not all code paths do this.

In v0.12 we don't even bother trying to report the value https://github.com/joyent/node/blob/v0.12/lib/child_process.js#L415

We should make the code paths consistent for v0.10, merge the branch into v0.12, and then fix the v0.12 issue.

[cjihrig]

@tjfontaine if you want to implement this on 0.10, this commit should probably be included. Are you OK with that? Also, what do you mean by "then fix the v0.12 issue"?

You also mentioned that 65536 * 2 is an arbitrary value. Did you want this to be a parameter?

[orangemocha]

Was the backpressure mechanism (the return from send()) ever documented? I cannot find this in the docs http://nodejs.org/api/all.html#all_child_send_message_sendhandle

If not, this would make this a feature rather than a bug fix.

[tjfontaine]

@orangemocha it wasn't documented, but works sometimes in v0.10, so ideally we would resolve that inconsistency, but I am fine with delaying that all as a feature for v0.12

@cjihrig I'm not sure I want to bring that commit down to v0.10 would need to talk more about it.

[cjihrig]

As far as I can tell, this is resolved.