node: avoid recomputing block hash in ReadBlock

Eliminate one SHA‑256 double‑hash computation of the header per block read by reusing the hash for:
* proof‑of‑work verification;
* (optional) integrity check against the supplied hash.

Author: l0rinc

src/node/blockstorage.cpp

@@ -38,6 +38,7 @@
 
 #include <cstddef>
 #include <map>
+#include <optional>
 #include <unordered_map>
 
 namespace kernel {
@@ -989,7 +990,7 @@ bool BlockManager::WriteBlockUndo(const CBlockUndo& blockundo, BlockValidationSt
     return true;
 }
 
-bool BlockManager::ReadBlock(CBlock& block, const FlatFilePos& pos) const
+bool BlockManager::ReadBlock(CBlock& block, const FlatFilePos& pos, const std::optional<uint256>& expected_hash) const
 {
     block.SetNull();
 
@@ -1007,8 +1008,10 @@ bool BlockManager::ReadBlock(CBlock& block, const FlatFilePos& pos) const
         return false;
     }
 
+    const auto block_hash{block.GetHash()};
+
     // Check the header
-    if (!CheckProofOfWork(block.GetHash(), block.nBits, GetConsensus())) {
+    if (!CheckProofOfWork(block_hash, block.nBits, GetConsensus())) {
         LogError("Errors in block header at %s while reading block", pos.ToString());
         return false;
     }
@@ -1019,21 +1022,19 @@ bool BlockManager::ReadBlock(CBlock& block, const FlatFilePos& pos) const
         return false;
     }
 
+    if (expected_hash && block_hash != *expected_hash) {
+        LogError("GetHash() doesn't match index at %s while reading block (%s != %s)",
+                 pos.ToString(), block_hash.ToString(), expected_hash->ToString());
+        return false;
+    }
+
     return true;
 }
 
 bool BlockManager::ReadBlock(CBlock& block, const CBlockIndex& index) const
 {
     const FlatFilePos block_pos{WITH_LOCK(cs_main, return index.GetBlockPos())};
-
-    if (!ReadBlock(block, block_pos)) {
-        return false;
-    }
-    if (block.GetHash() != index.GetBlockHash()) {
-        LogError("GetHash() doesn't match index for %s at %s while reading block", index.ToString(), block_pos.ToString());
-        return false;
-    }
-    return true;
+    return ReadBlock(block, block_pos, index.GetBlockHash());
 }
 
 bool BlockManager::ReadRawBlock(std::vector<uint8_t>& block, const FlatFilePos& pos) const

src/node/blockstorage.h

@@ -411,7 +411,7 @@ class BlockManager
     void UnlinkPrunedFiles(const std::set<int>& setFilesToPrune) const;
 
     /** Functions for disk access for blocks */
-    bool ReadBlock(CBlock& block, const FlatFilePos& pos) const;
+    bool ReadBlock(CBlock& block, const FlatFilePos& pos, const std::optional<uint256>& expected_hash = {}) const;
     bool ReadBlock(CBlock& block, const CBlockIndex& index) const;
     bool ReadRawBlock(std::vector<uint8_t>& block, const FlatFilePos& pos) const;