Use lighter atomic counters instead of mutexes

Where appropriate, it makes sense to use lighter weight atomic counters
instead of mutexes.

rpcserver.go

@@ -210,8 +210,7 @@ type rpcServer struct {
 	server      *server
 	authsha     [fastsha256.Size]byte
 	ntfnMgr     *wsNotificationManager
-	numClients      int
-	numClientsMutex sync.Mutex
+	numClients  int32
 	statusLines map[int]string
 	statusLock  sync.RWMutex
 	wg          sync.WaitGroup
@@ -353,10 +352,7 @@ func (s *rpcServer) writeHTTPResponseHeaders(req *http.Request, headers http.Hea
 //
 // This function is safe for concurrent access.
 func (s *rpcServer) limitConnections(w http.ResponseWriter, remoteAddr string) bool {
-	s.numClientsMutex.Lock()
-	defer s.numClientsMutex.Unlock()
-
-	if s.numClients+1 > cfg.RPCMaxClients {
+	if int(atomic.LoadInt32(&s.numClients)+1) > cfg.RPCMaxClients {
 		rpcsLog.Infof("Max RPC clients exceeded [%d] - "+
 			"disconnecting client %s", cfg.RPCMaxClients,
 			remoteAddr)
@@ -373,10 +369,7 @@ func (s *rpcServer) limitConnections(w http.ResponseWriter, remoteAddr string) b
 //
 // This function is safe for concurrent access.
 func (s *rpcServer) incrementClients() {
-	s.numClientsMutex.Lock()
-	defer s.numClientsMutex.Unlock()
-
-	s.numClients++
+	atomic.AddInt32(&s.numClients, 1)
 }
 
 // decrementClients subtracts one from the number of connected RPC clients.
@@ -385,10 +378,7 @@ func (s *rpcServer) incrementClients() {
 //
 // This function is safe for concurrent access.
 func (s *rpcServer) decrementClients() {
-	s.numClientsMutex.Lock()
-	defer s.numClientsMutex.Unlock()
-
-	s.numClients--
+	atomic.AddInt32(&s.numClients, -1)
 }
 
 // checkAuth checks the HTTP Basic authentication supplied by a wallet