lib/connections: Trust the model to tell us if we are connected

This should address issue as described in https://forum.syncthing.net/t/stun-nig-party-with-paused-devices/10942/13
Essentially the model and the connection service goes out of sync in terms of thinking if we are connected or not.
Resort to model as being the ultimate source of truth.

I can't immediately pin down how this happens, yet some ideas.

ConfigSaved happens in separate routine, so it's possbile that we have some sort of device removed yet connection comes in parallel kind of thing.
However, in this case the connection exists in the model, and does not exist in the connection service and the only way for the connection to be removed
in the connection service is device removal from the config.

Given the subject, this might also be related to the device being paused.

Also, adds more info to the logs

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4533

lib/connections/service.go

@@ -89,9 +89,6 @@ type Service struct {
 	listeners          map[string]genericListener
 	listenerTokens     map[string]suture.ServiceToken
 	listenerSupervisor *suture.Supervisor
-
-	curConMut         sync.Mutex
-	currentConnection map[protocol.DeviceID]completeConn
 }
 
 func NewService(cfg *config.Wrapper, myID protocol.DeviceID, mdl Model, tlsCfg *tls.Config, discoverer discover.Finder,
@@ -129,9 +126,6 @@ func NewService(cfg *config.Wrapper, myID protocol.DeviceID, mdl Model, tlsCfg *
 			FailureThreshold: 2,
 			FailureBackoff:   600 * time.Second,
 		}),
-
-		curConMut:         sync.NewMutex(),
-		currentConnection: make(map[protocol.DeviceID]completeConn),
 	}
 	cfg.Subscribe(service)
 
@@ -228,15 +222,11 @@ next:
 
 		// If we have a relay connection, and the new incoming connection is
 		// not a relay connection, we should drop that, and prefer this one.
-		connected := s.model.ConnectedTo(remoteID)
-		s.curConMut.Lock()
-		ct, ok := s.currentConnection[remoteID]
-		s.curConMut.Unlock()
-		priorityKnown := ok && connected
+		ct, connected := s.model.Connection(remoteID)
 
 		// Lower priority is better, just like nice etc.
-		if priorityKnown && ct.internalConn.priority > c.priority {
-			l.Debugln("Switching connections", remoteID)
+		if connected && ct.Priority() > c.priority {
+			l.Debugf("Switching connections %s (existing: %s new: %s)", remoteID, ct, c)
 		} else if connected {
 			// We should not already be connected to the other party. TODO: This
 			// could use some better handling. If the old connection is dead but
@@ -244,7 +234,7 @@ next:
 			// this one. But in case we are two devices connecting to each other
 			// in parallel we don't want to do that or we end up with no
 			// connections still established...
-			l.Infof("Connected to already connected device (%s)", remoteID)
+			l.Infof("Connected to already connected device %s (existing: %s new: %s)", remoteID, ct, c)
 			c.Close()
 			continue
 		}
@@ -284,9 +274,6 @@ next:
 		l.Infof("Established secure connection to %s at %s (%s)", remoteID, name, tlsCipherSuiteNames[c.ConnectionState().CipherSuite])
 
 		s.model.AddConnection(modelConn, hello)
-		s.curConMut.Lock()
-		s.currentConnection[remoteID] = modelConn
-		s.curConMut.Unlock()
 		continue next
 	}
 }
@@ -329,19 +316,13 @@ func (s *Service) connect() {
 				continue
 			}
 
-			connected := s.model.ConnectedTo(deviceID)
-			s.curConMut.Lock()
-			ct, ok := s.currentConnection[deviceID]
-			s.curConMut.Unlock()
-			priorityKnown := ok && connected
+			ct, connected := s.model.Connection(deviceID)
 
-			if priorityKnown && ct.internalConn.priority == bestDialerPrio {
+			if connected && ct.Priority() == bestDialerPrio {
 				// Things are already as good as they can get.
 				continue
 			}
 
-			l.Debugln("Reconnect loop for", deviceID)
-
 			var addrs []string
 			for _, addr := range deviceCfg.Addresses {
 				if addr == "dynamic" {
@@ -355,6 +336,8 @@ func (s *Service) connect() {
 				}
 			}
 
+			l.Debugln("Reconnect loop for", deviceID, addrs)
+
 			seen = append(seen, addrs...)
 
 			dialTargets := make([]dialTarget, 0)
@@ -394,8 +377,8 @@ func (s *Service) connect() {
 
 				prio := dialerFactory.Priority()
 
-				if priorityKnown && prio >= ct.internalConn.priority {
-					l.Debugf("Not dialing using %s as priority is less than current connection (%d >= %d)", dialerFactory, dialerFactory.Priority(), ct.internalConn.priority)
+				if connected && prio >= ct.Priority() {
+					l.Debugf("Not dialing using %s as priority is less than current connection (%d >= %d)", dialerFactory, dialerFactory.Priority(), ct.Priority())
 					continue
 				}
 
@@ -492,9 +475,6 @@ func (s *Service) CommitConfiguration(from, to config.Configuration) bool {
 
 	for _, dev := range from.Devices {
 		if !newDevices[dev.DeviceID] {
-			s.curConMut.Lock()
-			delete(s.currentConnection, dev.DeviceID)
-			s.curConMut.Unlock()
 			warningLimitersMut.Lock()
 			delete(warningLimiters, dev.DeviceID)
 			warningLimitersMut.Unlock()