discovery: limit NumBlocks to best known height for outgoing QueryChannelRange

This is done to ensure we don't receive replies for channels in blocks
not currently known to us, which we wouldn't be able to process.

discovery/syncer.go

@@ -237,6 +237,9 @@ type gossipSyncerCfg struct {
 	// This prevents ranges with old start times from causing us to dump the
 	// graph on connect.
 	ignoreHistoricalFilters bool
+
+	// bestHeight returns the latest height known of the chain.
+	bestHeight func() uint32
 }
 
 // GossipSyncer is a struct that handles synchronizing the channel graph state
@@ -834,9 +837,17 @@ func (g *GossipSyncer) genChanRangeQuery(
 		startHeight = uint32(newestChan.BlockHeight - chanRangeQueryBuffer)
 	}
 
+	// Determine the number of blocks to request based on our best height.
+	// We'll take into account any potential underflows and explicitly set
+	// numBlocks to its minimum value of 1 if so.
+	bestHeight := g.cfg.bestHeight()
+	numBlocks := bestHeight - startHeight
+	if int64(numBlocks) < 1 {
+		numBlocks = 1
+	}
+
 	log.Infof("GossipSyncer(%x): requesting new chans from height=%v "+
-		"and %v blocks after", g.cfg.peerPub[:], startHeight,
-		math.MaxUint32-startHeight)
+		"and %v blocks after", g.cfg.peerPub[:], startHeight, numBlocks)
 
 	// Finally, we'll craft the channel range query, using our starting
 	// height, then asking for all known channels to the foreseeable end of
@@ -844,7 +855,7 @@ func (g *GossipSyncer) genChanRangeQuery(
 	query := &lnwire.QueryChannelRange{
 		ChainHash:        g.cfg.chainHash,
 		FirstBlockHeight: startHeight,
-		NumBlocks:        math.MaxUint32 - startHeight,
+		NumBlocks:        numBlocks,
 	}
 	g.curQueryRangeMsg = query