core, eth, les: more efficient hash-based header chain retrieval (#16946)

core/headerchain.go

@@ -307,6 +307,43 @@ func (hc *HeaderChain) GetBlockHashesFromHash(hash common.Hash, max uint64) []co
 	return chain
 }
 
+// GetAncestor retrieves the Nth ancestor of a given block. It assumes that either the given block or
+// a close ancestor of it is canonical. maxNonCanonical points to a downwards counter limiting the
+// number of blocks to be individually checked before we reach the canonical chain.
+//
+// Note: ancestor == 0 returns the same block, 1 returns its parent and so on.
+func (hc *HeaderChain) GetAncestor(hash common.Hash, number, ancestor uint64, maxNonCanonical *uint64) (common.Hash, uint64) {
+	if ancestor > number {
+		return common.Hash{}, 0
+	}
+	if ancestor == 1 {
+		// in this case it is cheaper to just read the header
+		if header := hc.GetHeader(hash, number); header != nil {
+			return header.ParentHash, number - 1
+		} else {
+			return common.Hash{}, 0
+		}
+	}
+	for ancestor != 0 {
+		if rawdb.ReadCanonicalHash(hc.chainDb, number) == hash {
+			number -= ancestor
+			return rawdb.ReadCanonicalHash(hc.chainDb, number), number
+		}
+		if *maxNonCanonical == 0 {
+			return common.Hash{}, 0
+		}
+		*maxNonCanonical--
+		ancestor--
+		header := hc.GetHeader(hash, number)
+		if header == nil {
+			return common.Hash{}, 0
+		}
+		hash = header.ParentHash
+		number--
+	}
+	return hash, number
+}
+
 // GetTd retrieves a block's total difficulty in the canonical chain from the
 // database by hash and number, caching it if found.
 func (hc *HeaderChain) GetTd(hash common.Hash, number uint64) *big.Int {