discovery: allow gossip syncer to perform historical syncs

In this commit, we introduce the ability for gossip syncers to perform
historical syncs. This allows us to reconcile any channels we're missing
that the remote peer has starting from the genesis block of the chain.
This commit serves as a prerequisite to the SyncManager, introduced in a
later commit, where we'll be able to make spot checks by performing
historical syncs with peers to ensure we have as much of the graph as
possible.

discovery/syncer_test.go

@@ -730,7 +730,7 @@ func TestGossipSyncerGenChanRangeQuery(t *testing.T) {
 	// If we now ask the syncer to generate an initial range query, it
 	// should return a start height that's back chanRangeQueryBuffer
 	// blocks.
-	rangeQuery, err := syncer.genChanRangeQuery()
+	rangeQuery, err := syncer.genChanRangeQuery(false)
 	if err != nil {
 		t.Fatalf("unable to resp: %v", err)
 	}
@@ -743,7 +743,22 @@ func TestGossipSyncerGenChanRangeQuery(t *testing.T) {
 	}
 	if rangeQuery.NumBlocks != math.MaxUint32-firstHeight {
 		t.Fatalf("wrong num blocks: expected %v, got %v",
-			rangeQuery.NumBlocks, math.MaxUint32-firstHeight)
+			math.MaxUint32-firstHeight, rangeQuery.NumBlocks)
+	}
+
+	// Generating a historical range query should result in a start height
+	// of 0.
+	rangeQuery, err = syncer.genChanRangeQuery(true)
+	if err != nil {
+		t.Fatalf("unable to resp: %v", err)
+	}
+	if rangeQuery.FirstBlockHeight != 0 {
+		t.Fatalf("incorrect chan range query: expected %v, %v", 0,
+			rangeQuery.FirstBlockHeight)
+	}
+	if rangeQuery.NumBlocks != math.MaxUint32 {
+		t.Fatalf("wrong num blocks: expected %v, got %v",
+			math.MaxUint32, rangeQuery.NumBlocks)
 	}
 }
 
@@ -2082,3 +2097,46 @@ func TestGossipSyncerSyncTransitions(t *testing.T) {
 		})
 	}
 }
+
+// TestGossipSyncerHistoricalSync tests that a gossip syncer can perform a
+// historical sync with the remote peer.
+func TestGossipSyncerHistoricalSync(t *testing.T) {
+	t.Parallel()
+
+	// We'll create a new gossip syncer and manually override its state to
+	// chansSynced. This is necessary as the syncer can only process
+	// historical sync requests in this state.
+	msgChan, syncer, _ := newTestSyncer(
+		lnwire.ShortChannelID{BlockHeight: latestKnownHeight},
+		defaultEncoding, defaultChunkSize,
+	)
+	syncer.setSyncType(PassiveSync)
+	syncer.setSyncState(chansSynced)
+
+	syncer.Start()
+	defer syncer.Stop()
+
+	syncer.historicalSync()
+
+	// We should expect to see a single lnwire.QueryChannelRange message be
+	// sent to the remote peer with a FirstBlockHeight of 0.
+	expectedMsg := &lnwire.QueryChannelRange{
+		FirstBlockHeight: 0,
+		NumBlocks:        math.MaxUint32,
+	}
+
+	select {
+	case msgs := <-msgChan:
+		if len(msgs) != 1 {
+			t.Fatalf("expected to send a single "+
+				"lnwire.QueryChannelRange message, got %d",
+				len(msgs))
+		}
+		if !reflect.DeepEqual(msgs[0], expectedMsg) {
+			t.Fatalf("expected to send message: %v\ngot: %v",
+				spew.Sdump(expectedMsg), spew.Sdump(msgs[0]))
+		}
+	case <-time.After(time.Second):
+		t.Fatalf("expected to send a lnwire.QueryChannelRange message")
+	}
+}