Add SOCK_NONBLOCK option to UDP sockets.

src/core/iomgr/udp_server.c

@@ -370,7 +370,8 @@ int grpc_udp_server_add_port(grpc_udp_server *s, const void *addr, int addr_len,
     /* Try listening on IPv6 first. */
     addr = (struct sockaddr *)&wild6;
     addr_len = sizeof(wild6);
-    fd = grpc_create_dualstack_socket(addr, SOCK_DGRAM, IPPROTO_UDP, &dsmode);
+    fd = grpc_create_dualstack_socket(addr, SOCK_DGRAM | SOCK_NONBLOCK,
+                                      IPPROTO_UDP, &dsmode);
     allocated_port1 = add_socket_to_server(s, fd, addr, addr_len, read_cb);
     if (fd >= 0 && dsmode == GRPC_DSMODE_DUALSTACK) {
       goto done;
@@ -384,7 +385,8 @@ int grpc_udp_server_add_port(grpc_udp_server *s, const void *addr, int addr_len,
     addr_len = sizeof(wild4);
   }
 
-  fd = grpc_create_dualstack_socket(addr, SOCK_DGRAM, IPPROTO_UDP, &dsmode);
+  fd = grpc_create_dualstack_socket(addr, SOCK_DGRAM | SOCK_NONBLOCK,
+                                    IPPROTO_UDP, &dsmode);
   if (fd < 0) {
     gpr_log(GPR_ERROR, "Unable to create socket: %s", strerror(errno));
   }