ntp: skip loop test if no server socket is open

If there is no socket that could receive a request from a client or
peer, we know that nothing can be synchronized to us and no loop is
possible.

ntp_core.c

@@ -1474,9 +1474,9 @@ check_sync_loop(NCR_Instance inst, NTP_Packet *message, NTP_Local_Address *local
   NTP_Leap leap_status;
   uint32_t our_ref_id;
 
-  /* Check if our server may be enabled, i.e. a client or peer can actually
+  /* Check if a server socket is open, i.e. a client or peer can actually
      be synchronised to us */
-  if (REF_GetMode() != REF_ModeNormal)
+  if (!NIO_IsServerSocketOpen())
     return 1;
 
   /* Check if the source indicates that it is synchronised to our address

ntp_io.c

@@ -329,6 +329,14 @@ NIO_IsServerSocket(int sock_fd)
 
 /* ================================================== */
 
+int
+NIO_IsServerSocketOpen(void)
+{
+  return server_sock_fd4 != INVALID_SOCK_FD || server_sock_fd6 != INVALID_SOCK_FD;
+}
+
+/* ================================================== */
+
 int
 NIO_IsServerConnectable(NTP_Remote_Address *remote_addr)
 {

ntp_io.h

@@ -53,6 +53,9 @@ extern void NIO_CloseServerSocket(int sock_fd);
 /* Function to check if socket is a server socket */
 extern int NIO_IsServerSocket(int sock_fd);
 
+/* Function to check if a server socket is currently open */
+extern int NIO_IsServerSocketOpen(void);
+
 /* Function to check if client packets can be sent to a server */
 extern int NIO_IsServerConnectable(NTP_Remote_Address *remote_addr);
 

test/unit/ntp_core.c

@@ -39,6 +39,7 @@ static int req_length, res_length;
 #define NIO_OpenClientSocket(addr) ((addr)->ip_addr.family != IPADDR_UNSPEC ? 101 : 0)
 #define NIO_CloseClientSocket(fd) assert(fd == 101)
 #define NIO_IsServerSocket(fd) (fd == 100)
+#define NIO_IsServerSocketOpen() 1
 #define NIO_SendPacket(msg, to, from, len, process_tx) (memcpy(&req_buffer, msg, len), req_length = len, 1)
 #define SCH_AddTimeoutByDelay(delay, handler, arg) (1 ? 102 : (handler(arg), 1))
 #define SCH_AddTimeoutInClass(delay, separation, randomness, class, handler, arg) \