routing: wait for topology clients to fully exit before closing ntfn chan

This commit fixes a send on closed channel panic by adding additional
synchronization when cancelling the notifications for a particular
topology client. We now ensure that all goroutines belonging to a
particular topology client exit fully before we close the notification
channel in order to avoid a panic.

routing/notifications.go

@@ -3,6 +3,7 @@ package routing
 import (
 	"fmt"
 	"net"
+	"sync"
 	"sync/atomic"
 
 	"github.com/davecgh/go-spew/spew"
@@ -101,6 +102,8 @@ type topologyClient struct {
 	// exit is a channel that is used internally by the channel router to
 	// cancel any active un-consumed goroutine notifications.
 	exit chan struct{}
+
+	wg sync.WaitGroup
 }
 
 // notifyTopologyChange notifies all registered clients of a new change in
@@ -116,7 +119,11 @@ func (r *ChannelRouter) notifyTopologyChange(topologyDiff *TopologyChange) {
 	}
 
 	for _, client := range r.topologyClients {
+		client.wg.Add(1)
+
 		go func(c topologyClient) {
+			defer c.wg.Done()
+
 			select {
 
 			// In this case we'll try to send the notification

routing/router.go

@@ -489,7 +489,10 @@ func (r *ChannelRouter) networkHandler() {
 			if ntfnUpdate.cancel {
 				if client, ok := r.topologyClients[ntfnUpdate.clientID]; ok {
 					delete(r.topologyClients, clientID)
+
 					close(client.exit)
+					client.wg.Wait()
+
 					close(client.ntfnChan)
 				}