Node.js child processes and pipes - OSX vs Ubuntu - node.js

Question

I am trying to get two long running node.js processes to communicate - a parent and a child - using pipes and Node's child-process module. I want the child to be able to send data back to the parent asynchronously, and I was hoping to use a pipe to do so.
Here's a simplified version of my code:
Parent:
cp = require('child_process')
es = require('event-stream')
child = cp.spawn('coffee', ['child.coffee'], {stdio: [null, null, null, 'pipe']})
so = child.stdout.pipe(es.split())
p3 = child.stdio[3].pipe(es.split())
so.on 'data', (data) ->
console.log('stdout: ' + data)
child.stderr.on 'data', (data) ->
console.log('stderr: ' + data);
p3.on 'data', (data) ->
console.log('stdio3: ' + data);
child.on 'close', (code) ->
console.log('child process exited with code ' + code)
child.stdin.write "a message from your parent", "utf8"
Child:
fs = require('fs')
p3 = fs.createWriteStream('/dev/fd/3', {encoding: 'utf8'})
process.stdin.on 'data', (data) ->
p3.write "hello #{process.pid} - #{data}\n", 'utf8'
process.stdout.write "world #{process.pid} - #{data}\n", 'utf8'
p3.end()
process.exit(0)
process.stdin.on 'end', (data) ->
console.log "end of stdin"
p3.end()
process.exit(0)
process.stdin.setEncoding('utf8')
process.stdin.resume()
The code works on OSX 10.9, but fails to run on a Ubuntu box. I have tried running it under both Ubuntu 12.04 and 14.04. I am running Node 10.2x.
/dev/fd/ under Ubuntu is symbolically linked to /proc/self/fd/ so I believe my child process is opening the right file.
The output from running the parent on Ubuntu is as follows:
$ coffee parent.coffee
stderr:
stderr: events.js:72
stderr: throw er; // Unhandled 'error' event
stderr:
stderr:
stderr:
stderr:
stderr: ^
stderr: Error: UNKNOWN, open '/dev/fd/3'
events.js:72
throw er; // Unhandled 'error' event
^
Error: read ECONNRESET
at errnoException (net.js:901:11)
at Pipe.onread (net.js:556:19)
I would expect to see (and do on a OSX box):
$ coffee parent.coffee
stdio3: hello 21101 - a message from your parent
stdout: world 21101 - a message from your parent
stdio3:
stdout:
child process exited with code 0
It is possible to communicate with the child using the command line also on Ubuntu, so the problem is likely in the parent when spawning the child process:
$ echo foo | coffee child.coffee 3>&1
hello 3077 - foo
world 3077 - foo
I have tried to investigate the kernel calls that node makes using strace, but couldn't make much sense of the output.

Answer 1

I figured it out myself. The error was in the child. Ubuntu linux is more strict when it comes to opening files that are already open, the line:
p3 = fs.createWriteStream('/dev/fd/3', {encoding: 'utf8'})
was throwing an error. The file descriptor 3 is already open when the child runs, so the code should look as follows:
Child:
fs = require('fs')
# parent opens the file descriptor 3 when spawning the child (and closes it when the child returns)
fd3write = (s) ->
b = new Buffer(s)
fs.writeSync(3,b,0,b.length)
process.stdin.on 'data', (data) ->
fd3write "p3 #{process.pid} - #{data}\n"
process.stdout.write "so #{process.pid} - #{data}\n", 'utf8'
process.exit(0)
process.stdin.on 'end', (data) ->
console.log "end of stdin"
process.exit(0)
process.stdin.setEncoding('utf8')
process.stdin.resume()
I hope this will be of help to someone else.
To use a pipe instead of stdin to send messages from the parent to the child this link might be of use: child-process-multiple-file-descriptors.