eth/downloader: fix light client cht binary search issue

eth/downloader/downloader.go

@@ -99,6 +99,7 @@ type Downloader struct {
 	mode SyncMode       // Synchronisation mode defining the strategy used (per sync cycle)
 	mux  *event.TypeMux // Event multiplexer to announce sync operation events
 
+	genesis uint64   // Genesis block number to limit sync to (e.g. light client CHT)
 	queue   *queue   // Scheduler for selecting the hashes to download
 	peers   *peerSet // Set of active peers from which download can proceed
 	stateDB ethdb.Database
@@ -664,7 +665,28 @@ func (d *Downloader) findAncestor(p *peerConnection, remoteHeader *types.Header)
 	}
 	p.log.Debug("Looking for common ancestor", "local", localHeight, "remote", remoteHeight)
 	if localHeight >= MaxForkAncestry {
+		// We're above the max reorg threshold, find the earliest fork point
 		floor = int64(localHeight - MaxForkAncestry)
+
+		// If we're doing a light sync, ensure the floor doesn't go below the CHT, as
+		// all headers before that point will be missing.
+		if d.mode == LightSync {
+			// If we dont know the current CHT position, find it
+			if d.genesis == 0 {
+				header := d.lightchain.CurrentHeader()
+				for header != nil {
+					d.genesis = header.Number.Uint64()
+					if floor >= int64(d.genesis)-1 {
+						break
+					}
+					header = d.lightchain.GetHeaderByHash(header.ParentHash)
+				}
+			}
+			// We already know the "genesis" block number, cap floor to that
+			if floor < int64(d.genesis)-1 {
+				floor = int64(d.genesis) - 1
+			}
+		}
 	}
 	from, count, skip, max := calculateRequestSpan(remoteHeight, localHeight)