detached child_process doesn't inherit stdio

[yepitschunked]

The following snippet of code prints the child process' output if detached is true, and doesn't if it's false:

  cmd = '...'
  child_process.spawn(cmd, [], {
    stdio: 'inherit',
    detached: true
  });

This part of the docs seems relevant:

If the parent's stdio is inherited, the child will remain attached to the controlling terminal.

Which seems to imply that inheriting these streams is supported. Not sure if this is just me misreading the docs or...

[bnoordhuis]

{ detached: true } is sugar for setsid(2), meaning it disconnects from the controlling tty. Does that answer you question? If not, can you post a complete example with expected and actual output?

[cjihrig]

@bnoordhuis is this expected behavior? With Node 5 on OS X, and the following code:

process.stdout;
require('child_process').spawn('ls', [], {
  stdio: 'inherit',
  detached: true
});

Nothing is printed. However, if I comment out the process.stdout on the first line, then ls output is displayed.

[bnoordhuis]

/cc @indutny - I'm guessing it's related to OS X's broken kqueue support for special files and libuv's workaround. I can reproduce on OS X but it works fine on Linux, FWIW.

[indutny]

The only explanation that I have is that reopening stdout as /dev/tty somehow interferes with it.

[indutny]

Gosh, it is twice as odd, considering that piping the output like this: node test.js | less works just fine in both cases. I suspect some OS X oddness atm...

[indutny]

And of course it is related to the reopening of /dev/tty in libuv: https://github.com/libuv/libuv/blob/25506bb33156f15e27a837979773264dfe8d06be/src/unix/tty.c#L64-L87 . Removing that branch fixes the problem.

[indutny]

I think there is some fishy stuff with the current session and the way /dev/tty attaches to it on OS X, but I am not yet quite sure what exactly is happening there.

[indutny]

It looks like for a child process write returns EIO:

write(0x1, "\0", 0x1)        = -1 Err#5

[indutny]

@wonnage may I ask you to give a try to this patch?

diff --git a/deps/uv/src/unix/process.c b/deps/uv/src/unix/process.c
index 571f8cd..c02c750 100644
--- a/deps/uv/src/unix/process.c
+++ b/deps/uv/src/unix/process.c
@@ -283,8 +283,10 @@ static void uv__process_child_init(const uv_process_options_t* options,
   int use_fd;
   int fd;

-  if (options->flags & UV_PROCESS_DETACHED)
+  if (options->flags & UV_PROCESS_DETACHED) {
+    setpgid(getpid(), getpid());
     setsid();
+  }

   /* First duplicate low numbered fds, since it's not safe to duplicate them,
    * they could get replaced. Example: swapping stdout and stderr; without

I suspect that this is an OS X kernel bug and that for some reason pg->pg_jobc is 0 if setsid() is called alone.

[indutny]

Gosh, I have figured it out. The actual device behind /dev/tty lives in bsd/kern/tty_tty.c. The thing about this device is that it always looks up the tty using current proc's session. So when you create a new session, it doesn't have any tty assigned to it, and all syscalls on it will result in EIO error.

I'll need some time to think if there is any workaround for this.

[indutny]

I failed at it miserably :( Submitted Apple bugreport # 23994737

[yepitschunked]

Hey! sorry i've been inactive on this thread - switched jobs and no longer working on the code that produced this error in the first place :)