cloexec: add dup_cloexec

This is needed to enforce correct semantics of mkostemp_safer.
Meanwhile, it is one step closer to providing O_CLOEXEC support
to open, as well as implementing portions of fcntl for mingw.

* lib/cloexec.h (dup_cloexec): New prototype.  Add copyright
header and comments.
* lib/cloexec.c (set_cloexec_flag): Add comments.
(dup_cloexec): New function, with mingw implementation borrowed
from...
* lib/w32spawn.h (dup_noinherit): ...here.
* modules/execute (Depends-on): Add cloexec.
* modules/pipe (Depends-on): Likewise.
* modules/cloexec (Depends-on): Add dup2.
* modules/cloexec-tests (Files): New file.
* tests/test-cloexec.c: Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>

1 files changed, 36 insertions, 0 deletions

diff --git a/lib/cloexec.h b/lib/cloexec.h
index c25921d6b3..73334524d1 100644
--- a/lib/cloexec.h
+++ b/lib/cloexec.h
@@ -1,2 +1,38 @@
+/* closexec.c - set or clear the close-on-exec descriptor flag
+
+   Copyright (C) 2004, 2009 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+
 #include <stdbool.h>
+
+/* Set the `FD_CLOEXEC' flag of DESC if VALUE is true,
+   or clear the flag if VALUE is false.
+   Return 0 on success, or -1 on error with `errno' set.
+
+   Note that on MingW, this function does NOT protect DESC from being
+   inherited into spawned children.  Instead, either use dup_cloexec
+   followed by closing the original DESC, or use interfaces such as
+   open or pipe2 that accept flags like O_CLOEXEC to create DESC
+   non-inheritable in the first place.  */
+
 int set_cloexec_flag (int desc, bool value);
+
+/* Duplicates a file handle FD, while marking the copy to be closed
+   prior to exec or spawn.  Returns -1 and sets errno if FD could not
+   be duplicated.  */
+
+int dup_cloexec (int fd);