doc: buffers are not sent over IPC with a socket

[timkuijsten]

Checklist

• documentation is changed or added
• the commit message follows commit guidelines

Affected core subsystem(s)

doc/child_process

Description of change

If a socket is sent to a child, any data that is buffered in the
socket will not be sent to the child. The child will only receive
data from the socket that is sent after the child has the socket.

PoC:

From #6951:

$ node --version
v4.4.7

$ cat m.js
const fork = require('child_process').fork
const net = require('net')

const cp = fork('./child.js')

net.createServer(c => {
  setTimeout(() => {
    cp.send({}, c)
  }, 500) // send the connection after a delay
}).listen(1234, () => {
  net.createConnection(1234, function() {
    var i = 0
    setInterval(() => {
      this.write(`${i++} `)
    }, 100) // start sending data before the connection is sent to the child
  })
})

$ cat child.js 
process.on('message', (m, c) => {
  console.log('child: got connection')
  c.pipe(process.stdout)
})

$ node m.js
child: got connection
4 5 6 7 8 ^C

[mscdex]

To be clear, this is a backport from master.

[cjihrig]

LGTM

[mscdex]

Also, I think we're still using v4.x-staging for backporting commits instead of v4.x directly.

[MylesBorins]

we indeed use v4.x-staging.

Generally we also maintain the original meta data... so you should include

PR-URL: #6951
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

at the end of the commit message

regarding staging, I'll make sure to land it to that branch, no need to open another PR

[MylesBorins]

Manually landed with appropriate meta data in cf43254