services: fix chain locked by WS subscriptions handlers

Blockchain's subscriptions, unsubscriptions and notifications are
handled by a single notificationDispatcher routine. Thus, on attempt
to send the subsequent event to Blockchain's subscribers, dispatcher
can't handle subscriptions\unsubscriptions. Make subscription and
unsubscription to be a non-blocking operation for blockchain on the
server side, otherwise it may cause the dispatcher locks.

To achieve this, use a separate lock for those code that make calls
to blockchain's subscription API and for subscription counters on
the server side.

pkg/services/rpcsrv/server.go

@@ -136,18 +136,21 @@ type (
 		sessionsLock sync.Mutex
 		sessions     map[string]*session
 
-		subsLock          sync.RWMutex
-		subscribers       map[*subscriber]bool
+		subsLock    sync.RWMutex
+		subscribers map[*subscriber]bool
+
+		subsCounterLock   sync.RWMutex
 		blockSubs         int
 		executionSubs     int
 		notificationSubs  int
 		transactionSubs   int
 		notaryRequestSubs int
-		blockCh           chan *block.Block
-		executionCh       chan *state.AppExecResult
-		notificationCh    chan *state.ContainedNotificationEvent
-		transactionCh     chan *transaction.Transaction
-		notaryRequestCh   chan mempoolevent.Event
+
+		blockCh         chan *block.Block
+		executionCh     chan *state.AppExecResult
+		notificationCh  chan *state.ContainedNotificationEvent
+		transactionCh   chan *transaction.Transaction
+		notaryRequestCh chan mempoolevent.Event
 	}
 
 	// session holds a set of iterators got after invoke* call with corresponding
@@ -591,14 +594,17 @@ requestloop:
 		case resChan <- res:
 		}
 	}
+
 	s.subsLock.Lock()
 	delete(s.subscribers, subscr)
+	s.subsLock.Unlock()
+	s.subsCounterLock.Lock()
 	for _, e := range subscr.feeds {
 		if e.event != neorpc.InvalidEventID {
 			s.unsubscribeFromChannel(e.event)
 		}
 	}
-	s.subsLock.Unlock()
+	s.subsCounterLock.Unlock()
 	close(resChan)
 	ws.Close()
 }
@@ -2441,9 +2447,9 @@ func (s *Server) subscribe(reqParams params.Params, sub *subscriber) (interface{
 	}
 
 	s.subsLock.Lock()
-	defer s.subsLock.Unlock()
 	select {
 	case <-s.shutdown:
+		s.subsLock.Unlock()
 		return nil, neorpc.NewInternalServerError("server is shutting down")
 	default:
 	}
@@ -2454,16 +2460,21 @@ func (s *Server) subscribe(reqParams params.Params, sub *subscriber) (interface{
 		}
 	}
 	if id == len(sub.feeds) {
+		s.subsLock.Unlock()
 		return nil, neorpc.NewInternalServerError("maximum number of subscriptions is reached")
 	}
 	sub.feeds[id].event = event
 	sub.feeds[id].filter = filter
+	s.subsLock.Unlock()
+
+	s.subsCounterLock.Lock()
 	s.subscribeToChannel(event)
+	s.subsCounterLock.Unlock()
 	return strconv.FormatInt(int64(id), 10), nil
 }
 
 // subscribeToChannel subscribes RPC server to appropriate chain events if
-// it's not yet subscribed for them. It's supposed to be called with s.subsLock
+// it's not yet subscribed for them. It's supposed to be called with s.subsCounterLock
 // taken by the caller.
 func (s *Server) subscribeToChannel(event neorpc.EventID) {
 	switch event {
@@ -2502,20 +2513,24 @@ func (s *Server) unsubscribe(reqParams params.Params, sub *subscriber) (interfac
 		return nil, neorpc.ErrInvalidParams
 	}
 	s.subsLock.Lock()
-	defer s.subsLock.Unlock()
 	if len(sub.feeds) <= id || sub.feeds[id].event == neorpc.InvalidEventID {
+		s.subsLock.Unlock()
 		return nil, neorpc.ErrInvalidParams
 	}
 	event := sub.feeds[id].event
 	sub.feeds[id].event = neorpc.InvalidEventID
 	sub.feeds[id].filter = nil
+	s.subsLock.Unlock()
+
+	s.subsCounterLock.Lock()
 	s.unsubscribeFromChannel(event)
+	s.subsCounterLock.Unlock()
 	return true, nil
 }
 
 // unsubscribeFromChannel unsubscribes RPC server from appropriate chain events
-// if there are no other subscribers for it. It's supposed to be called with
-// s.subsLock taken by the caller.
+// if there are no other subscribers for it. It must be called with s.subsConutersLock
+// holding by the caller.
 func (s *Server) unsubscribeFromChannel(event neorpc.EventID) {
 	switch event {
 	case neorpc.BlockEventID:
@@ -2635,6 +2650,7 @@ chloop:
 	// should be running concurrently to this one. And even if one is to run
 	// after unlock, it'll see closed s.shutdown and won't subscribe.
 	s.subsLock.Lock()
+	s.subsCounterLock.Lock()
 	// There might be no subscription in reality, but it's not a problem as
 	// core.Blockchain allows unsubscribing non-subscribed channels.
 	s.chain.UnsubscribeFromBlocks(s.blockCh)
@@ -2644,6 +2660,7 @@ chloop:
 	if s.chain.P2PSigExtensionsEnabled() {
 		s.coreServer.UnsubscribeFromNotaryRequests(s.notaryRequestCh)
 	}
+	s.subsCounterLock.Unlock()
 	s.subsLock.Unlock()
 drainloop:
 	for {