[PATCH] accept4: Support SOCK_NONBLOCK, if defined

[Eric Blake <eblake@redhat.com>]

Ideally, we would improve our replacement <sys/socket.h> to define a
replacement SOCK_NONBLOCK on all platforms, and teach socket() to
honor it as well; but that's a bigger task.  In the meantime, if the
platform already has SOCK_NONBLOCK, we should honor it when doing a
fallback.

* lib/accept4.c (accept4): If SOCK_NONBLOCK is defined, honor it.
---
 ChangeLog     |  3 +++
 lib/accept4.c | 21 ++++++++++++++++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index 6253f9221..f5eda2fa3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2019-08-20  Eric Blake  <eblake@redhat.com>

+	accept4: Support SOCK_NONBLOCK, if defined
+	* lib/accept4.c (accept4): If SOCK_NONBLOCK is defined, honor it.
+
 	accept4: Fix compilation when native accept4() exists.
 	Reported by Richard W.M. Jones <rjones@redhat.com> in
 	https://lists.gnu.org/archive/html/bug-gnulib/2019-08/msg00029.html
diff --git a/lib/accept4.c b/lib/accept4.c
index c6e59c955..9ce78fa96 100644
--- a/lib/accept4.c
+++ b/lib/accept4.c
@@ -31,6 +31,9 @@
 #ifndef SOCK_CLOEXEC
 # define SOCK_CLOEXEC 0
 #endif
+#ifndef SOCK_NONBLOCK
+# define SOCK_NONBLOCK 0
+#endif

 int
 accept4 (int sockfd, struct sockaddr *addr, socklen_t *addrlen, int flags)
@@ -58,7 +61,7 @@ accept4 (int sockfd, struct sockaddr *addr, socklen_t *addrlen, int flags)
 #endif

   /* Check the supported flags.  */
-  if ((flags & ~(SOCK_CLOEXEC | O_TEXT | O_BINARY)) != 0)
+  if ((flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK | O_TEXT | O_BINARY)) != 0)
     {
       errno = EINVAL;
       return -1;
@@ -121,6 +124,22 @@ accept4 (int sockfd, struct sockaddr *addr, socklen_t *addrlen, int flags)
 # endif
 #endif

+#if SOCK_CLOEXEC
+  if (flags & SOCK_NONBLOCK)
+    {
+      int fcntl_flags;
+
+      if ((fcntl_flags = fcntl (fd, F_GETFL, 0)) < 0
+          || fcntl (fd, F_SETFL, fcntl_flags | O_NONBLOCK) == -1)
+        {
+          int saved_errno = errno;
+          close (fd);
+          errno = saved_errno;
+          return -1;
+        }
+    }
+#endif
+
 #if O_BINARY
   if (flags & O_BINARY)
     set_binary_mode (fd, O_BINARY);
-- 
2.21.0