discovery/syncer: make gossip sends synchronous

This commit makes all replies in the gossip syncer synchronous, meaning
that they will wait for each message to be successfully written to the
remote peer before attempting to send the next. This helps throttle
messages the remote peer has requested, preventing unintended
disconnects when the remote peer is slow to process messages. This
changes also helps out congestion in the peer by forcing the syncer to
buffer the messages instead of dumping them into the peer's queue.

discovery/syncer_test.go

@@ -151,6 +151,10 @@ func newTestSyncer(hID lnwire.ShortChannelID,
 			msgChan <- msgs
 			return nil
 		},
+		sendToPeerSync: func(msgs ...lnwire.Message) error {
+			msgChan <- msgs
+			return nil
+		},
 		delayedQueryReplyInterval: 2 * time.Second,
 	}
 	syncer := newGossipSyncer(cfg)
@@ -540,30 +544,37 @@ func TestGossipSyncerReplyShortChanIDs(t *testing.T) {
 		t.Fatalf("unable to query for chan IDs: %v", err)
 	}
 
-	select {
-	case <-time.After(time.Second * 15):
-		t.Fatalf("no msgs received")
+	for i := 0; i < len(queryReply)+1; i++ {
+		select {
+		case <-time.After(time.Second * 15):
+			t.Fatalf("no msgs received")
 
-		// We should get back exactly 4 messages. The first 3 are the same
-		// messages we sent above, and the query end message.
-	case msgs := <-msgChan:
-		if len(msgs) != 4 {
-			t.Fatalf("wrong messages: expected %v, got %v",
-				4, len(msgs))
-		}
+		// We should get back exactly 4 messages. The first 3 are the
+		// same messages we sent above, and the query end message.
+		case msgs := <-msgChan:
+			if len(msgs) != 1 {
+				t.Fatalf("wrong number of messages: "+
+					"expected %v, got %v", 1, len(msgs))
+			}
 
-		if !reflect.DeepEqual(queryReply, msgs[:3]) {
-			t.Fatalf("wrong set of messages: expected %v, got %v",
-				spew.Sdump(queryReply), spew.Sdump(msgs[:3]))
-		}
+			isQueryReply := i < len(queryReply)
+			finalMsg, ok := msgs[0].(*lnwire.ReplyShortChanIDsEnd)
 
-		finalMsg, ok := msgs[3].(*lnwire.ReplyShortChanIDsEnd)
-		if !ok {
-			t.Fatalf("expected lnwire.ReplyShortChanIDsEnd "+
-				"instead got %T", msgs[3])
-		}
-		if finalMsg.Complete != 1 {
-			t.Fatalf("complete wasn't set")
+			switch {
+			case isQueryReply &&
+				!reflect.DeepEqual(queryReply[i], msgs[0]):
+
+				t.Fatalf("wrong message: expected %v, got %v",
+					spew.Sdump(queryReply[i]),
+					spew.Sdump(msgs[0]))
+
+			case !isQueryReply && !ok:
+				t.Fatalf("expected lnwire.ReplyShortChanIDsEnd"+
+					" instead got %T", msgs[3])
+
+			case !isQueryReply && finalMsg.Complete != 1:
+				t.Fatalf("complete wasn't set")
+			}
 		}
 	}
 }