nonblocking: provide O_NONBLOCK for mingw

Mingw is the only known platform that lacks O_NONBLOCK (possibly
via the alternate spelling O_NDELAY).  But mingw also lacks any
files where open() needs to enforce non-blocking behavior, and
lacks openat(), so it is relatively simple to provide a non-zero
flag.  A future patches will make use of O_NONBLOCK for pipe2.

* modules/nonblocking (Depends-on): Add open.
(configure.ac): Set new witness macro.
* m4/fcntl_h.m4 (gl_FCNTL_H_DEFAULTS): Provide default for it.
* modules/fcntl-h (Makefile.am): Substitute it.
* lib/fcntl.in.h (O_NONBLOCK): Guarantee non-zero definition when
nonblocking module is in use.
* lib/nonblocking.c: Adjust portability test.
* lib/open.c (open): Don't let native open see gnulib flag.
* tests/test-fcntl-h.c (main): Enhance test.
* tests/test-open.h (test_open): Likewise.
* doc/posix-headers/fcntl.texi (fcntl.h): Document the replacement.

Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/tests/test-open.h b/tests/test-open.h
index 3e5c5e1726..2ba5d137bf 100644
--- a/tests/test-open.h
+++ b/tests/test-open.h
@@ -63,6 +63,12 @@ test_open (int (*func) (char const *, int, ...), bool print)
   ASSERT (write (fd, "c", 1) == 1);
   ASSERT (close (fd) == 0);
 
+  /* Although O_NONBLOCK on regular files can be ignored, it must not
+     cause a failure.  */
+  fd = func (BASE "file", O_NONBLOCK | O_RDONLY);
+  ASSERT (0 <= fd);
+  ASSERT (close (fd) == 0);
+
   /* Symlink handling, where supported.  */
   if (symlink (BASE "file", BASE "link") != 0)
     {