[refactor] Cleanup merkle root checks

1 files changed, 41 insertions, 14 deletions

diff --git a/src/validation.cpp b/src/validation.cpp
index 98383cd133..e02ce2ad1d 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -3642,35 +3642,59 @@ static bool CheckBlockHeader(const CBlockHeader& block, BlockValidationState& st
 static bool CheckMerkleRoot(const CBlock& block, BlockValidationState& state)
 {
     bool mutated;
-    uint256 hashMerkleRoot2 = BlockMerkleRoot(block, &mutated);
-    if (block.hashMerkleRoot != hashMerkleRoot2)
-        return state.Invalid(BlockValidationResult::BLOCK_MUTATED, "bad-txnmrklroot", "hashMerkleRoot mismatch");
+    uint256 merkle_root = BlockMerkleRoot(block, &mutated);
+    if (block.hashMerkleRoot != merkle_root) {
+        return state.Invalid(
+            /*result=*/BlockValidationResult::BLOCK_MUTATED,
+            /*reject_reason=*/"bad-txnmrklroot",
+            /*debug_message=*/"hashMerkleRoot mismatch");
+    }
 
     // Check for merkle tree malleability (CVE-2012-2459): repeating sequences
     // of transactions in a block without affecting the merkle root of a block,
     // while still invalidating it.
-    if (mutated)
-        return state.Invalid(BlockValidationResult::BLOCK_MUTATED, "bad-txns-duplicate", "duplicate transaction");
+    if (mutated) {
+        return state.Invalid(
+            /*result=*/BlockValidationResult::BLOCK_MUTATED,
+            /*reject_reason=*/"bad-txns-duplicate",
+            /*debug_message=*/"duplicate transaction");
+    }
 
     return true;
 }
 
+/** CheckWitnessMalleation performs checks for block malleation with regard to
+ * its witnesses.
+ *
+ * Note: If the witness commitment is expected (i.e. `expect_witness_commitment
+ * = true`), then the block is required to have at least one transaction and the
+ * first transaction needs to have at least one input. */
 static bool CheckWitnessMalleation(const CBlock& block, bool expect_witness_commitment, BlockValidationState& state)
 {
     if (expect_witness_commitment) {
         int commitpos = GetWitnessCommitmentIndex(block);
         if (commitpos != NO_WITNESS_COMMITMENT) {
-            bool malleated = false;
-            uint256 hashWitness = BlockWitnessMerkleRoot(block, &malleated);
+            assert(!block.vtx.empty() && !block.vtx[0]->vin.empty());
+            const auto& witness_stack{block.vtx[0]->vin[0].scriptWitness.stack};
+
+            if (witness_stack.size() != 1 || witness_stack[0].size() != 32) {
+                return state.Invalid(
+                    /*result=*/BlockValidationResult::BLOCK_MUTATED,
+                    /*reject_reason=*/"bad-witness-nonce-size",
+                    /*debug_message=*/strprintf("%s : invalid witness reserved value size", __func__));
+            }
+
             // The malleation check is ignored; as the transaction tree itself
             // already does not permit it, it is impossible to trigger in the
             // witness tree.
-            if (block.vtx[0]->vin[0].scriptWitness.stack.size() != 1 || block.vtx[0]->vin[0].scriptWitness.stack[0].size() != 32) {
-                return state.Invalid(BlockValidationResult::BLOCK_MUTATED, "bad-witness-nonce-size", strprintf("%s : invalid witness reserved value size", __func__));
-            }
-            CHash256().Write(hashWitness).Write(block.vtx[0]->vin[0].scriptWitness.stack[0]).Finalize(hashWitness);
-            if (memcmp(hashWitness.begin(), &block.vtx[0]->vout[commitpos].scriptPubKey[6], 32)) {
-                return state.Invalid(BlockValidationResult::BLOCK_MUTATED, "bad-witness-merkle-match", strprintf("%s : witness merkle commitment mismatch", __func__));
+            uint256 hash_witness = BlockWitnessMerkleRoot(block, /*mutated=*/nullptr);
+
+            CHash256().Write(hash_witness).Write(witness_stack[0]).Finalize(hash_witness);
+            if (memcmp(hash_witness.begin(), &block.vtx[0]->vout[commitpos].scriptPubKey[6], 32)) {
+                return state.Invalid(
+                    /*result=*/BlockValidationResult::BLOCK_MUTATED,
+                    /*reject_reason=*/"bad-witness-merkle-match",
+                    /*debug_message=*/strprintf("%s : witness merkle commitment mismatch", __func__));
             }
 
             return true;
@@ -3680,7 +3704,10 @@ static bool CheckWitnessMalleation(const CBlock& block, bool expect_witness_comm
     // No witness data is allowed in blocks that don't commit to witness data, as this would otherwise leave room for spam
     for (const auto& tx : block.vtx) {
         if (tx->HasWitness()) {
-            return state.Invalid(BlockValidationResult::BLOCK_MUTATED, "unexpected-witness", strprintf("%s : unexpected witness data found", __func__));
+            return state.Invalid(
+                /*result=*/BlockValidationResult::BLOCK_MUTATED,
+                /*reject_reason=*/"unexpected-witness",
+                /*debug_message=*/strprintf("%s : unexpected witness data found", __func__));
         }
     }