Move flags to recvmsg function in netlink

netlink_multicast used 3 calls to fcntl in order to set O_NONBLOCK on
socket. It is possible to pass MSG_DONTWAIT flag just to recvmsg function,
without setting it permanently on socket. Save few kernel calls and use
recvmsg flags.

It is supported since kernel 2.2, should be fine for any device still
receiving updates.

Author: pemensik

src/netlink.c

@@ -106,7 +106,7 @@ char *netlink_init(void)
   return NULL;
 }
 
-static ssize_t netlink_recv(void)
+static ssize_t netlink_recv(int flags)
 {
   struct msghdr msg;
   struct sockaddr_nl nladdr;
@@ -122,7 +122,8 @@ static ssize_t netlink_recv(void)
       msg.msg_iovlen = 1;
       msg.msg_flags = 0;
 
-      while ((rc = recvmsg(daemon->netlinkfd, &msg, MSG_PEEK | MSG_TRUNC)) == -1 && errno == EINTR);
+      while ((rc = recvmsg(daemon->netlinkfd, &msg, flags | MSG_PEEK | MSG_TRUNC)) == -1 &&
+	     errno == EINTR);
 
       /* make buffer big enough */
       if (rc != -1 && (msg.msg_flags & MSG_TRUNC))
@@ -139,7 +140,7 @@ static ssize_t netlink_recv(void)
 
       /* read it for real */
       msg.msg_flags = 0;
-      while ((rc = recvmsg(daemon->netlinkfd, &msg, 0)) == -1 && errno == EINTR);
+      while ((rc = recvmsg(daemon->netlinkfd, &msg, flags)) == -1 && errno == EINTR);
 
       /* Make sure this is from the kernel */
       if (rc == -1 || nladdr.nl_pid == 0)
@@ -201,7 +202,7 @@ int iface_enumerate(int family, void *parm, int (*callback)())
 
   while (1)
     {
-      if ((len = netlink_recv()) == -1)
+      if ((len = netlink_recv(0)) == -1)
 	{
 	  if (errno == ENOBUFS)
 	    {
@@ -353,20 +354,12 @@ void netlink_multicast(void)
 {
   ssize_t len;
   struct nlmsghdr *h;
-  int flags;
   unsigned state = 0;
 
   /* don't risk blocking reading netlink messages here. */
-  if ((flags = fcntl(daemon->netlinkfd, F_GETFL)) == -1 ||
-      fcntl(daemon->netlinkfd, F_SETFL, flags | O_NONBLOCK) == -1) 
-    return;
-  
-  while ((len = netlink_recv()) != -1)
+  while ((len = netlink_recv(MSG_DONTWAIT)) != -1)
     for (h = (struct nlmsghdr *)iov.iov_base; NLMSG_OK(h, (size_t)len); h = NLMSG_NEXT(h, len))
       state = nl_async(h, state);
-  
-  /* restore non-blocking status */
-  fcntl(daemon->netlinkfd, F_SETFL, flags);
 }
 
 static unsigned nl_async(struct nlmsghdr *h, unsigned state)