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, 9 insertions, 0 deletions

diff --git a/lib/open.c b/lib/open.c
index 2e2cc74dc6..e60b619949 100644
--- a/lib/open.c
+++ b/lib/open.c
@@ -63,6 +63,15 @@ open (const char *filename, int flags, ...)
       va_end (arg);
     }
 
+#if GNULIB_defined_O_NONBLOCK
+  /* The only known platform that lacks O_NONBLOCK is mingw, but it
+     also lacks named pipes and Unix sockets, which are the only two
+     file types that require non-blocking handling in open().
+     Therefore, it is safe to ignore O_NONBLOCK here.  It is handy
+     that mingw also lacks openat(), so that is also covered here.  */
+  flags &= ~O_NONBLOCK;
+#endif
+
 #if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
   if (strcmp (filename, "/dev/null") == 0)
     filename = "NUL";