eth/downloader: fix ancestor searching for light syncing (#19136)

eth/downloader/downloader.go

@@ -665,30 +665,32 @@ func (d *Downloader) findAncestor(p *peerConnection, remoteHeader *types.Header)
 		localHeight = d.lightchain.CurrentHeader().Number.Uint64()
 	}
 	p.log.Debug("Looking for common ancestor", "local", localHeight, "remote", remoteHeight)
+
+	// Recap floor value for binary search
 	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)
+	}
+	// 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
 				}
-			}
-			// We already know the "genesis" block number, cap floor to that
-			if floor < int64(d.genesis)-1 {
-				floor = int64(d.genesis) - 1
+				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)
 
 	p.log.Trace("Span searching for common ancestor", "count", count, "from", from, "skip", skip)