Merge #664: Do non-depth check for peg-in witness where possible

8838e76 Do non-depth check for peg-in witness where possible (Gregory Sanders)

Pull request description:

  The non-depth check is very cheap, and could be made even cheaper in the future given the proper assumptions. Depth checks are only required for `CheckTxInputs`.

Tree-SHA512: 46b9214146c49a0a82147927b897e51af98852ecb86a7a6dd220b7daf4a4a443d262dc576f958642e70476f283733e1c7022f3c0c7214d57581a8cbf3a8f0f2c

src/consensus/tx_verify.cpp

@@ -162,7 +162,8 @@ int64_t GetTransactionSigOpCost(const CTransaction& tx, const CCoinsViewCache& i
         CTxOut prevout;
         if (tx.vin[i].m_is_pegin) {
             std::string err;
-            if (tx.witness.vtxinwit.size() <= i || !IsValidPeginWitness(tx.witness.vtxinwit[i].m_pegin_witness, tx.vin[i].prevout, err, true)) {
+            // Make sure witness exists and is properly formatted
+            if (tx.witness.vtxinwit.size() != tx.vin.size() || !IsValidPeginWitness(tx.witness.vtxinwit[i].m_pegin_witness, tx.vin[i].prevout, err, false)) {
                 continue;
             }
             prevout = GetPeginOutputFromWitness(tx.witness.vtxinwit[i].m_pegin_witness);

src/validation.cpp

@@ -700,12 +700,11 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
             // This only checks the UTXO set for already claimed pegins. For mempool conflicts,
             // we rely on the GetConflictTx check done above.
             if (txin.m_is_pegin) {
-                // Quick sanity check on witness first.
-                if (tx.witness.vtxinwit.size() <= i ||
-                        tx.witness.vtxinwit[i].m_pegin_witness.stack.size() < 6 ||
-                        uint256(tx.witness.vtxinwit[i].m_pegin_witness.stack[2]).IsNull() ||
-                        tx.vin[i].prevout.hash.IsNull()) {
-                    return state.Invalid(false, REJECT_INVALID, "pegin-no-witness");
+                // Peg-in witness is required, check here without validating existence in parent chain
+                std::string err_msg = "no peg-in witness attached";
+                if (tx.witness.vtxinwit.size() != tx.vin.size() ||
+                        !IsValidPeginWitness(tx.witness.vtxinwit[i].m_pegin_witness, tx.vin[i].prevout, err_msg, false)) {
+                    return state.Invalid(false, REJECT_INVALID, "pegin-no-witness", err_msg);
                 }
 
                 std::pair<uint256, COutPoint> pegin = std::make_pair(uint256(tx.witness.vtxinwit[i].m_pegin_witness.stack[2]), tx.vin[i].prevout);