Node 0.8 breaks use of /dev/stdin in child processes

[David Glasser]

See https://gist.github.com/3751746 for details.

Basically, in Node 0.6, if you start a child_process with
child_process.spawn, the pipe2 system call was used to create the
pipes between parent and child.

This was changed (I believe) in this commit by indutny:
https://github.com/joyent/libuv/commit/c0081f0e6675131721dbb5138fd398792a8c2163

Now the socketpair call is used to create those fds.

I'm not sure why that choice was made, but it has at least one bad
effect: it breaks the ability to use "/dev/stdin" or "/proc/self/fd/0"
on the child, at least on Linux.

The gist shows a Node program that basically just spawns "cat
/proc/self/fd/0", closes the pipe to the cat, and shows what the cat
outputs. In 0.6.17 cat happily cats the empty stdin and exits 1. In
0.8.8 cat fails to open the /proc/self/fd/0.

The C program in the gist shows that the difference really is just
socketpair vs pipe2. If you run the version with pipe2 the open call
succeeds. Comment out pipe2 and uncomment socketpair, and the open
call fails.

Was this behavior change intentional?
What benefits are created by switching from pipe2 to socketpair?

--dave

[Ben Noordhuis]

Yes.

> What benefits are created by switching from pipe2 to socketpair?

The ability to send file descriptors to the child process (which was
something you could do in v0.4 but not v0.6).

It's been brought up on the issue tracker a few times. A
straightforward workaround is to insert a pipe. E.g.:

spawn('foo')

Becomes:

spawn('/bin/sh', ['-c', 'cat | foo'])

[David Glasser]

Ah, yes, that'll do it. And yes, that's the workaround we're using.

[dhruvbird]

Hello,

So, I'm still trying to understand the implications of this change. Suppose I spawn "cat" and write to the stdin of the spawned process, then will the "cat" process not be able to read it?

var cat = spawn('cat');
cat.stdin.write('Foo Bar');
cat.stdout.on('data', function(d) { console.log("Child Says:", String(d); });

Will this do what I expect it to?

[Ben Noordhuis]

On Thu, Sep 20, 2012 at 4:07 PM, dhruvbird <dhru...@gmail.com> wrote:
> Hello,
>
> So, I'm still trying to understand the implications of this change. Suppose
> I spawn "cat" and write to the stdin of the spawned process, then will the
> "cat" process not be able to read it?
>
> var cat = spawn('cat');
> cat.stdin.write('Foo Bar');
> cat.stdout.on('data', function(d) { console.log("Child Says:", String(d);
> });
>
> Will this do what I expect it to?

Yes. It's only when the child process explicitly opens /dev/stdin (or
/dev/stdout) that it doesn't work.

[Shawn Parrish]

Ben,

I noticed this line in the announcement for 0.8.10

* unix: put child process stdio fds in blocking mode (Ben Noordhuis)

Does that by any chance address this issue? I continue to struggle with it.

Thanks,
Shawn

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[Ben Noordhuis]

On Wed, Sep 26, 2012 at 12:43 AM, Shawn Parrish <spar...@nodeping.com> wrote:
> Ben,
>
> I noticed this line in the announcement for 0.8.10
>
> * unix: put child process stdio fds in blocking mode (Ben Noordhuis)
>
> Does that by any chance address this issue? I continue to struggle with it.

No, that fixes another issue: child processes going wild because they
don't expect stdio to be non-blocking. Only happens in some cases.