try to fix the issue that socket always returns "i/o timeout" or "Closed explicitly" error

Author: bachue

session.go

@@ -4800,13 +4800,13 @@ func (s *Session) acquireSocket(slaveOk bool) (*mongoSocket, error) {
 	s.m.RLock()
 	// If there is a slave socket reserved and its use is acceptable, take it as long
 	// as there isn't a master socket which would be preferred by the read preference mode.
-	if s.slaveSocket != nil && s.slaveOk && slaveOk && (s.masterSocket == nil || s.consistency != PrimaryPreferred && s.consistency != Monotonic) {
+	if s.slaveSocket != nil && s.slaveSocket.dead == nil && s.slaveOk && slaveOk && (s.masterSocket == nil || s.consistency != PrimaryPreferred && s.consistency != Monotonic) {
 		socket := s.slaveSocket
 		socket.Acquire()
 		s.m.RUnlock()
 		return socket, nil
 	}
-	if s.masterSocket != nil {
+	if s.masterSocket != nil && s.masterSocket.dead == nil {
 		socket := s.masterSocket
 		socket.Acquire()
 		s.m.RUnlock()
@@ -4820,12 +4820,20 @@ func (s *Session) acquireSocket(slaveOk bool) (*mongoSocket, error) {
 	defer s.m.Unlock()
 
 	if s.slaveSocket != nil && s.slaveOk && slaveOk && (s.masterSocket == nil || s.consistency != PrimaryPreferred && s.consistency != Monotonic) {
-		s.slaveSocket.Acquire()
-		return s.slaveSocket, nil
+		if s.slaveSocket.dead == nil {
+			s.slaveSocket.Acquire()
+			return s.slaveSocket, nil
+		} else {
+			s.unsetSocket()
+		}
 	}
 	if s.masterSocket != nil {
-		s.masterSocket.Acquire()
-		return s.masterSocket, nil
+		if s.masterSocket.dead == nil {
+			s.masterSocket.Acquire()
+			return s.masterSocket, nil
+		} else {
+			s.unsetSocket()
+		}
 	}
 
 	// Still not good.  We need a new socket.
@@ -4876,9 +4884,11 @@ func (s *Session) setSocket(socket *mongoSocket) {
 // unsetSocket releases any slave and/or master sockets reserved.
 func (s *Session) unsetSocket() {
 	if s.masterSocket != nil {
+		debugf("unset master socket from session %p", s)
 		s.masterSocket.Release()
 	}
 	if s.slaveSocket != nil {
+		debugf("unset slave socket from session %p", s)
 		s.slaveSocket.Release()
 	}
 	s.masterSocket = nil