eth/downloader: resolve local header by hash for beacon sync (#24691)

* eth/downlaoder: resolve local header by hash for beacon sync

* eth/downloader: fix error message

* eth/downloader: cap the reverse header resolving

* eth/downloader: re-enable tests

* eth/downloader: add warning logs

eth/downloader/downloader.go

@@ -159,9 +159,6 @@ type LightChain interface {
 	// GetHeaderByHash retrieves a header from the local chain.
 	GetHeaderByHash(common.Hash) *types.Header
 
-	// GetHeaderByNumber retrieves a block header from the local chain by number.
-	GetHeaderByNumber(number uint64) *types.Header
-
 	// CurrentHeader retrieves the head header from the local chain.
 	CurrentHeader() *types.Header
 
@@ -486,7 +483,15 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td, ttd *
 			// Retrieve the pivot header from the skeleton chain segment but
 			// fallback to local chain if it's not found in skeleton space.
 			if pivot = d.skeleton.Header(number); pivot == nil {
-				pivot = d.lightchain.GetHeaderByNumber(number)
+				_, oldest, _ := d.skeleton.Bounds() // error is already checked
+				if number < oldest.Number.Uint64() {
+					count := int(oldest.Number.Uint64() - number) // it's capped by fsMinFullBlocks
+					headers := d.readHeaderRange(oldest, count)
+					if len(headers) == count {
+						pivot = headers[len(headers)-1]
+						log.Warn("Retrieved pivot header from local", "number", pivot.Number, "hash", pivot.Hash(), "latest", latest.Number, "oldest", oldest.Number)
+					}
+				}
 			}
 			// Print an error log and return directly in case the pivot header
 			// is still not found. It means the skeleton chain is not linked
@@ -1772,3 +1777,25 @@ func (d *Downloader) DeliverSnapPacket(peer *snap.Peer, packet snap.Packet) erro
 		return fmt.Errorf("unexpected snap packet type: %T", packet)
 	}
 }
+
+// readHeaderRange returns a list of headers, using the given last header as the base,
+// and going backwards towards genesis. This method assumes that the caller already has
+// placed a reasonable cap on count.
+func (d *Downloader) readHeaderRange(last *types.Header, count int) []*types.Header {
+	var (
+		current = last
+		headers []*types.Header
+	)
+	for {
+		parent := d.lightchain.GetHeaderByHash(current.ParentHash)
+		if parent == nil {
+			break // The chain is not continuous, or the chain is exhausted
+		}
+		headers = append(headers, parent)
+		if len(headers) >= count {
+			break
+		}
+		current = parent
+	}
+	return headers
+}