POSIX: Add code for shuffling file descriptors.

POSIX: Add code for shuffling file descriptors.

When forking a child process, one often wants to move existing file
descriptors to well known locations (stdout, stderr etc). However,
this is a process bedeviled with corner cases. Consider the case where
you open two file descriptors, get assigned fds 1 and 0 for them and
wish to shuffle them so that they end up in slots 0 and 1. Our current
code fails in this case.

We also have a problem where we're currently trying to mark file
descriptors as close-on-exec rather than closing them in the child
process. This is inherently broken in a multithreaded process where
another thread can open a file descriptor and race the loop which is
trying to mark them.

Thus, on Linux we switch to close-after-fork where we known that no
other threads exist in the process. On Mac, the code is sufficiently
different that this simple fix isn't applicable and one of the Mac
folks will need to take a look.

[Evan Martin, 2009-04-28 21:37:03]

I think Markus, being the other Unixy person around here, might be a good
reviewer for this.

[jeremy, 2009-04-28 22:22:42]

+mark, tvl

agl: Thanks for fixing this, in future could you cc some Mac folk if there are
questions about the OS X implementation?

The discussion of the OS X implementation should probalby live in a bug, but I
wanted to start it here in case there' something simple we can do in the scope
of this CL?

On OS X we use the posix_spawn() call which provides some atomicity around this
type of operation, but still doesn't solve the issues you raise in the CL
description.

tvl: I know you had some thoughts about reworking this code.  Could we solve the
race condition by forking before calling posix_spawn?  Then augment agl's code
to build up the posix_spawn_file_actions_t correctly?

[Markus (顧孟勤), 2009-04-28 23:15:17]

LGTM