[rpc] Return fee and vsize from testmempoolaccept

Return fee and vsize if tx would pass ATMP.

Author: rajarshimaitra

doc/release-notes.md

@@ -102,6 +102,9 @@ will trigger BIP 125 (replace-by-fee) opt-in. (#11413)
   option `-deprecatedrpc=banscore` is used. The `banscore` field will be fully
   removed in the next major release. (#19469)
 
+- The `testmempoolaccept` RPC returns `vsize` and a `fee` object with the `base` fee
+  if the transaction passes validation. (#19940)
+
 - The `walletcreatefundedpsbt` RPC call will now fail with
   `Insufficient funds` when inputs are manually selected but are not enough to cover
   the outputs and fee. Additional inputs can automatically be added through the

src/rpc/rawtransaction.cpp

@@ -878,6 +878,11 @@ static UniValue testmempoolaccept(const JSONRPCRequest& request)
                         {
                             {RPCResult::Type::STR_HEX, "txid", "The transaction hash in hex"},
                             {RPCResult::Type::BOOL, "allowed", "If the mempool allows this tx to be inserted"},
+                            {RPCResult::Type::NUM, "vsize", "Virtual transaction size as defined in BIP 141. This is different from actual serialized size for witness transactions as witness data is discounted (only present when 'allowed' is true)"},
+                            {RPCResult::Type::OBJ, "fees", "Transaction fees (only present if 'allowed' is true)",
+                            {
+                                {RPCResult::Type::STR_AMOUNT, "base", "transaction fee in " + CURRENCY_UNIT},
+                            }},
                             {RPCResult::Type::STR, "reject-reason", "Rejection string (only present when 'allowed' is false)"},
                         }},
                     }
@@ -924,13 +929,22 @@ static UniValue testmempoolaccept(const JSONRPCRequest& request)
 
     TxValidationState state;
     bool test_accept_res;
+    CAmount fee;
     {
         LOCK(cs_main);
         test_accept_res = AcceptToMemoryPool(mempool, state, std::move(tx),
-            nullptr /* plTxnReplaced */, false /* bypass_limits */, max_raw_tx_fee, /* test_accept */ true);
+            nullptr /* plTxnReplaced */, false /* bypass_limits */, max_raw_tx_fee, /* test_accept */ true, &fee);
     }
     result_0.pushKV("allowed", test_accept_res);
-    if (!test_accept_res) {
+
+    // Only return the fee and vsize if the transaction would pass ATMP.
+    // These can be used to calculate the feerate.
+    if (test_accept_res) {
+        result_0.pushKV("vsize", virtual_size);
+        UniValue fees(UniValue::VOBJ);
+        fees.pushKV("base", ValueFromAmount(fee));
+        result_0.pushKV("fees", fees);
+    } else {
         if (state.IsInvalid()) {
             if (state.GetResult() == TxValidationResult::TX_MISSING_INPUTS) {
                 result_0.pushKV("reject-reason", "missing-inputs");

src/validation.cpp

@@ -475,6 +475,7 @@ class MemPoolAccept
          */
         std::vector<COutPoint>& m_coins_to_uncache;
         const bool m_test_accept;
+        CAmount* m_fee_out;
     };
 
     // Single transaction acceptance
@@ -687,6 +688,11 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         return false; // state filled in by CheckTxInputs
     }
 
+    // If fee_out is passed, return the fee to the caller
+    if (args.m_fee_out) {
+        *args.m_fee_out = nFees;
+    }
+
     // Check for non-standard pay-to-script-hash in inputs
     if (fRequireStandard && !AreInputsStandard(tx, m_view)) {
         return state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, "bad-txns-nonstandard-inputs");
@@ -1061,10 +1067,10 @@ bool MemPoolAccept::AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs
 /** (try to) add transaction to memory pool with a specified acceptance time **/
 static bool AcceptToMemoryPoolWithTime(const CChainParams& chainparams, CTxMemPool& pool, TxValidationState &state, const CTransactionRef &tx,
                         int64_t nAcceptTime, std::list<CTransactionRef>* plTxnReplaced,
-                        bool bypass_limits, const CAmount nAbsurdFee, bool test_accept) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+                        bool bypass_limits, const CAmount nAbsurdFee, bool test_accept, CAmount* fee_out=nullptr) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
     std::vector<COutPoint> coins_to_uncache;
-    MemPoolAccept::ATMPArgs args { chainparams, state, nAcceptTime, plTxnReplaced, bypass_limits, nAbsurdFee, coins_to_uncache, test_accept };
+    MemPoolAccept::ATMPArgs args { chainparams, state, nAcceptTime, plTxnReplaced, bypass_limits, nAbsurdFee, coins_to_uncache, test_accept, fee_out };
     bool res = MemPoolAccept(pool).AcceptSingleTransaction(tx, args);
     if (!res) {
         // Remove coins that were not present in the coins cache before calling ATMPW;
@@ -1083,10 +1089,10 @@ static bool AcceptToMemoryPoolWithTime(const CChainParams& chainparams, CTxMemPo
 
 bool AcceptToMemoryPool(CTxMemPool& pool, TxValidationState &state, const CTransactionRef &tx,
                         std::list<CTransactionRef>* plTxnReplaced,
-                        bool bypass_limits, const CAmount nAbsurdFee, bool test_accept)
+                        bool bypass_limits, const CAmount nAbsurdFee, bool test_accept, CAmount* fee_out)
 {
     const CChainParams& chainparams = Params();
-    return AcceptToMemoryPoolWithTime(chainparams, pool, state, tx, GetTime(), plTxnReplaced, bypass_limits, nAbsurdFee, test_accept);
+    return AcceptToMemoryPoolWithTime(chainparams, pool, state, tx, GetTime(), plTxnReplaced, bypass_limits, nAbsurdFee, test_accept, fee_out);
 }
 
 CTransactionRef GetTransaction(const CBlockIndex* const block_index, const CTxMemPool* const mempool, const uint256& hash, const Consensus::Params& consensusParams, uint256& hashBlock)

src/validation.h

@@ -199,10 +199,11 @@ void UnlinkPrunedFiles(const std::set<int>& setFilesToPrune);
 void PruneBlockFilesManual(int nManualPruneHeight);
 
 /** (try to) add transaction to memory pool
- * plTxnReplaced will be appended to with all transactions replaced from mempool **/
+ * plTxnReplaced will be appended to with all transactions replaced from mempool
+ * @param[out] fee_out optional argument to return tx fee to the caller **/
 bool AcceptToMemoryPool(CTxMemPool& pool, TxValidationState &state, const CTransactionRef &tx,
                         std::list<CTransactionRef>* plTxnReplaced,
-                        bool bypass_limits, const CAmount nAbsurdFee, bool test_accept=false) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+                        bool bypass_limits, const CAmount nAbsurdFee, bool test_accept=false, CAmount* fee_out=nullptr) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
 /** Get the BIP9 state for a given deployment at the current tip. */
 ThresholdState VersionBitsTipState(const Consensus::Params& params, Consensus::DeploymentPos pos);

test/functional/mempool_accept.py

@@ -4,6 +4,7 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test mempool acceptance of raw transactions."""
 
+from decimal import Decimal
 from io import BytesIO
 import math
 
@@ -91,20 +92,22 @@ def run_test(self):
         tx.deserialize(BytesIO(hex_str_to_bytes(raw_tx_0)))
         txid_0 = tx.rehash()
         self.check_mempool_result(
-            result_expected=[{'txid': txid_0, 'allowed': True}],
+            result_expected=[{'txid': txid_0, 'allowed': True, 'vsize': 110, 'fees': {'base': Decimal(str(fee))}}],
             rawtxs=[raw_tx_0],
         )
 
         self.log.info('A final transaction not in the mempool')
         coin = coins.pop()  # Pick a random coin(base) to spend
+        output_amount = 0.025
         raw_tx_final = node.signrawtransactionwithwallet(node.createrawtransaction(
             inputs=[{'txid': coin['txid'], 'vout': coin['vout'], "sequence": 0xffffffff}],  # SEQUENCE_FINAL
-            outputs=[{node.getnewaddress(): 0.025}],
+            outputs=[{node.getnewaddress(): output_amount}],
             locktime=node.getblockcount() + 2000,  # Can be anything
         ))['hex']
         tx.deserialize(BytesIO(hex_str_to_bytes(raw_tx_final)))
+        fee_expected = int(coin['amount']) - output_amount
         self.check_mempool_result(
-            result_expected=[{'txid': tx.rehash(), 'allowed': True}],
+            result_expected=[{'txid': tx.rehash(), 'allowed': True, 'vsize': 188, 'fees': {'base': Decimal(str(fee_expected))}}],
             rawtxs=[tx.serialize().hex()],
             maxfeerate=0,
         )
@@ -127,7 +130,7 @@ def run_test(self):
         tx.deserialize(BytesIO(hex_str_to_bytes(raw_tx_0)))
         txid_0 = tx.rehash()
         self.check_mempool_result(
-            result_expected=[{'txid': txid_0, 'allowed': True}],
+            result_expected=[{'txid': txid_0, 'allowed': True, 'vsize': 110, 'fees': {'base': Decimal(str(2 * fee))}}],
             rawtxs=[raw_tx_0],
         )
 
@@ -187,7 +190,7 @@ def run_test(self):
         tx.deserialize(BytesIO(hex_str_to_bytes(raw_tx_reference)))
         # Reference tx should be valid on itself
         self.check_mempool_result(
-            result_expected=[{'txid': tx.rehash(), 'allowed': True}],
+            result_expected=[{'txid': tx.rehash(), 'allowed': True, 'vsize': 110, 'fees': { 'base': Decimal(str(0.1 - 0.05))}}],
             rawtxs=[tx.serialize().hex()],
             maxfeerate=0,
         )

test/functional/p2p_segwit.py

@@ -3,6 +3,7 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test segwit transactions and blocks on P2P network."""
+from decimal import Decimal
 import math
 import random
 import struct
@@ -695,13 +696,13 @@ def test_standardness_v0(self):
         if not self.segwit_active:
             # Just check mempool acceptance, but don't add the transaction to the mempool, since witness is disallowed
             # in blocks and the tx is impossible to mine right now.
-            assert_equal(self.nodes[0].testmempoolaccept([tx3.serialize_with_witness().hex()]), [{'txid': tx3.hash, 'allowed': True}])
+            assert_equal(self.nodes[0].testmempoolaccept([tx3.serialize_with_witness().hex()]), [{'txid': tx3.hash, 'allowed': True, 'vsize': 93, 'fees': { 'base': Decimal('0.00001000')}}])
             # Create the same output as tx3, but by replacing tx
             tx3_out = tx3.vout[0]
             tx3 = tx
             tx3.vout = [tx3_out]
             tx3.rehash()
-            assert_equal(self.nodes[0].testmempoolaccept([tx3.serialize_with_witness().hex()]), [{'txid': tx3.hash, 'allowed': True}])
+            assert_equal(self.nodes[0].testmempoolaccept([tx3.serialize_with_witness().hex()]), [{'txid': tx3.hash, 'allowed': True, 'vsize': 93, 'fees': { 'base': Decimal('0.00011000')}}])
         test_transaction_acceptance(self.nodes[0], self.test_node, tx3, with_witness=True, accepted=True)
 
         self.nodes[0].generate(1)