morph: Fix subscription deadlock

Scenario:
0. at least one subscription has been performed
1. another subscription is being done
2. a notification from one of the `0.` point's subs is received

If `2.` happens b/w `0.` and `1.` a deadlock appears since the notification
routing process is locked on the subscription lock while the subscription lock
cannot be unlocked since the subscription RPC cannot be done before the
just-arrived notification is handled (read from the neo-go subscription
channel).

`switchLock` does the same thing to the `routeNotifications`: it ensures that no
routine is doing/will be doing changes with the subscription channels, even
though `subs`'s lock was created for this purpose initially.

Relates #2559.

Signed-off-by: Pavel Karpy <[email protected]>

pkg/morph/client/notifications.go

@@ -206,8 +206,6 @@ func (c *Client) Notifications() (<-chan *state.ContainedNotificationEvent, <-ch
 }
 
 type subscriptions struct {
-	sync.RWMutex
-
 	// notification consumers (Client sends
 	// notifications to these channels)
 	notifyChan chan *state.ContainedNotificationEvent
@@ -220,6 +218,8 @@ type subscriptions struct {
 	curBlockChan  chan *block.Block
 	curNotaryChan chan *result.NotaryRequestEvent
 
+	sync.RWMutex // for subscription fields only
+
 	// cached subscription information
 	subscribedEvents            map[util.Uint160]struct{}
 	subscribedToAllNotaryEvents bool
@@ -237,11 +237,11 @@ func (c *Client) routeNotifications() {
 routeloop:
 	for {
 		var connLost bool
-		c.subs.RLock()
+		c.switchLock.RLock()
 		notifCh := c.subs.curNotifyChan
 		blCh := c.subs.curBlockChan
 		notaryCh := c.subs.curNotaryChan
-		c.subs.RUnlock()
+		c.switchLock.RUnlock()
 		select {
 		case <-c.closeChan:
 			break routeloop