[refactor] return MempoolAcceptResult

This creates a cleaner interface with ATMP, allows us to make results const,
and makes accessing values that don't make sense (e.g. fee when tx is
invalid) an error.

9 files changed, 106 insertions, 92 deletions

diff --git a/src/bench/block_assemble.cpp b/src/bench/block_assemble.cpp
index af5a82f69f..9f7d582e8a 100644
--- a/src/bench/block_assemble.cpp
+++ b/src/bench/block_assemble.cpp
@@ -48,9 +48,8 @@ static void AssembleBlock(benchmark::Bench& bench)
         LOCK(::cs_main); // Required for ::AcceptToMemoryPool.
 
         for (const auto& txr : txs) {
-            TxValidationState state;
-            bool ret{::AcceptToMemoryPool(*test_setup.m_node.mempool, state, txr, nullptr /* plTxnReplaced */, false /* bypass_limits */)};
-            assert(ret);
+            const MempoolAcceptResult res = ::AcceptToMemoryPool(*test_setup.m_node.mempool, txr, false /* bypass_limits */);
+            assert(res.m_result_type == MempoolAcceptResult::ResultType::VALID);
         }
     }
 
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index b68453759a..ba80da287a 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -2178,10 +2178,10 @@ void PeerManagerImpl::ProcessOrphanTx(std::set<uint256>& orphan_work_set)
         if (orphan_it == mapOrphanTransactions.end()) continue;
 
         const CTransactionRef porphanTx = orphan_it->second.tx;
-        TxValidationState state;
-        std::list<CTransactionRef> removed_txn;
+        const MempoolAcceptResult result = AcceptToMemoryPool(m_mempool, porphanTx, false /* bypass_limits */);
+        const TxValidationState& state = result.m_state;
 
-        if (AcceptToMemoryPool(m_mempool, state, porphanTx, &removed_txn, false /* bypass_limits */)) {
+        if (result.m_result_type == MempoolAcceptResult::ResultType::VALID) {
             LogPrint(BCLog::MEMPOOL, "   accepted orphan tx %s\n", orphanHash.ToString());
             RelayTransaction(orphanHash, porphanTx->GetWitnessHash(), m_connman);
             for (unsigned int i = 0; i < porphanTx->vout.size(); i++) {
@@ -2193,7 +2193,7 @@ void PeerManagerImpl::ProcessOrphanTx(std::set<uint256>& orphan_work_set)
                 }
             }
             EraseOrphanTx(orphanHash);
-            for (const CTransactionRef& removedTx : removed_txn) {
+            for (const CTransactionRef& removedTx : result.m_replaced_transactions.value()) {
                 AddToCompactExtraTransactions(removedTx);
             }
             break;
@@ -3197,10 +3197,10 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
             return;
         }
 
-        TxValidationState state;
-        std::list<CTransactionRef> lRemovedTxn;
+        const MempoolAcceptResult result = AcceptToMemoryPool(m_mempool, ptx, false /* bypass_limits */);
+        const TxValidationState& state = result.m_state;
 
-        if (AcceptToMemoryPool(m_mempool, state, ptx, &lRemovedTxn, false /* bypass_limits */)) {
+        if (result.m_result_type == MempoolAcceptResult::ResultType::VALID) {
             m_mempool.check(&::ChainstateActive().CoinsTip());
             // As this version of the transaction was acceptable, we can forget about any
             // requests for it.
@@ -3223,7 +3223,7 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
                 tx.GetHash().ToString(),
                 m_mempool.size(), m_mempool.DynamicMemoryUsage() / 1000);
 
-            for (const CTransactionRef& removedTx : lRemovedTxn) {
+            for (const CTransactionRef& removedTx : result.m_replaced_transactions.value()) {
                 AddToCompactExtraTransactions(removedTx);
             }
 
diff --git a/src/node/transaction.cpp b/src/node/transaction.cpp
index d3bb9687a8..6f5acf41e3 100644
--- a/src/node/transaction.cpp
+++ b/src/node/transaction.cpp
@@ -50,22 +50,22 @@ TransactionError BroadcastTransaction(NodeContext& node, const CTransactionRef t
     }
     if (!node.mempool->exists(hashTx)) {
         // Transaction is not already in the mempool.
-        TxValidationState state;
         if (max_tx_fee > 0) {
             // First, call ATMP with test_accept and check the fee. If ATMP
             // fails here, return error immediately.
-            CAmount fee{0};
-            if (!AcceptToMemoryPool(*node.mempool, state, tx,
-                nullptr /* plTxnReplaced */, false /* bypass_limits */, /* test_accept */ true, &fee)) {
-                return HandleATMPError(state, err_string);
-            } else if (fee > max_tx_fee) {
+            const MempoolAcceptResult result = AcceptToMemoryPool(*node.mempool, tx, false /* bypass_limits */,
+                                                                  true /* test_accept */);
+            if (result.m_result_type != MempoolAcceptResult::ResultType::VALID) {
+                return HandleATMPError(result.m_state, err_string);
+            } else if (result.m_base_fees.value() > max_tx_fee) {
                 return TransactionError::MAX_FEE_EXCEEDED;
             }
         }
         // Try to submit the transaction to the mempool.
-        if (!AcceptToMemoryPool(*node.mempool, state, tx,
-                nullptr /* plTxnReplaced */, false /* bypass_limits */)) {
-            return HandleATMPError(state, err_string);
+        const MempoolAcceptResult result = AcceptToMemoryPool(*node.mempool, tx, false /* bypass_limits */,
+                                                              false /* test_accept */);
+        if (result.m_result_type != MempoolAcceptResult::ResultType::VALID) {
+            return HandleATMPError(result.m_state, err_string);
         }
 
         // Transaction was accepted to the mempool.
diff --git a/src/rpc/rawtransaction.cpp b/src/rpc/rawtransaction.cpp
index 31190ef9fc..784a53e060 100644
--- a/src/rpc/rawtransaction.cpp
+++ b/src/rpc/rawtransaction.cpp
@@ -946,18 +946,13 @@ static RPCHelpMan testmempoolaccept()
     result_0.pushKV("txid", tx->GetHash().GetHex());
     result_0.pushKV("wtxid", tx->GetWitnessHash().GetHex());
 
-    TxValidationState state;
-    bool test_accept_res;
-    CAmount fee{0};
-    {
-        LOCK(cs_main);
-        test_accept_res = AcceptToMemoryPool(mempool, state, std::move(tx),
-            nullptr /* plTxnReplaced */, false /* bypass_limits */, /* test_accept */ true, &fee);
-    }
+    const MempoolAcceptResult accept_result = WITH_LOCK(cs_main, return AcceptToMemoryPool(mempool, std::move(tx),
+                                                  false /* bypass_limits */, /* test_accept */ true));
 
     // Only return the fee and vsize if the transaction would pass ATMP.
     // These can be used to calculate the feerate.
-    if (test_accept_res) {
+    if (accept_result.m_result_type == MempoolAcceptResult::ResultType::VALID) {
+        const CAmount fee = accept_result.m_base_fees.value();
         // Check that fee does not exceed maximum fee
         if (max_raw_tx_fee && fee > max_raw_tx_fee) {
             result_0.pushKV("allowed", false);
@@ -972,6 +967,7 @@ static RPCHelpMan testmempoolaccept()
         result.push_back(std::move(result_0));
     } else {
         result_0.pushKV("allowed", false);
+        const TxValidationState state = accept_result.m_state;
         if (state.GetResult() == TxValidationResult::TX_MISSING_INPUTS) {
             result_0.pushKV("reject-reason", "missing-inputs");
         } else {
diff --git a/src/test/txvalidation_tests.cpp b/src/test/txvalidation_tests.cpp
index 7e6246d68f..9b0d2e3135 100644
--- a/src/test/txvalidation_tests.cpp
+++ b/src/test/txvalidation_tests.cpp
@@ -30,25 +30,21 @@ BOOST_FIXTURE_TEST_CASE(tx_mempool_reject_coinbase, TestChain100Setup)
 
     BOOST_CHECK(CTransaction(coinbaseTx).IsCoinBase());
 
-    TxValidationState state;
-
     LOCK(cs_main);
 
     unsigned int initialPoolSize = m_node.mempool->size();
+    const MempoolAcceptResult result = AcceptToMemoryPool(*m_node.mempool, MakeTransactionRef(coinbaseTx),
+                true /* bypass_limits */);
 
-    BOOST_CHECK_EQUAL(
-            false,
-            AcceptToMemoryPool(*m_node.mempool, state, MakeTransactionRef(coinbaseTx),
-                nullptr /* plTxnReplaced */,
-                true /* bypass_limits */));
+    BOOST_CHECK(result.m_result_type == MempoolAcceptResult::ResultType::INVALID);
 
     // Check that the transaction hasn't been added to mempool.
     BOOST_CHECK_EQUAL(m_node.mempool->size(), initialPoolSize);
 
     // Check that the validation state reflects the unsuccessful attempt.
-    BOOST_CHECK(state.IsInvalid());
-    BOOST_CHECK_EQUAL(state.GetRejectReason(), "coinbase");
-    BOOST_CHECK(state.GetResult() == TxValidationResult::TX_CONSENSUS);
+    BOOST_CHECK(result.m_state.IsInvalid());
+    BOOST_CHECK_EQUAL(result.m_state.GetRejectReason(), "coinbase");
+    BOOST_CHECK(result.m_state.GetResult() == TxValidationResult::TX_CONSENSUS);
 }
 
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/test/txvalidationcache_tests.cpp b/src/test/txvalidationcache_tests.cpp
index bed2ba3608..af0090cc10 100644
--- a/src/test/txvalidationcache_tests.cpp
+++ b/src/test/txvalidationcache_tests.cpp
@@ -28,9 +28,9 @@ BOOST_FIXTURE_TEST_CASE(tx_mempool_block_doublespend, TestChain100Setup)
     const auto ToMemPool = [this](const CMutableTransaction& tx) {
         LOCK(cs_main);
 
-        TxValidationState state;
-        return AcceptToMemoryPool(*m_node.mempool, state, MakeTransactionRef(tx),
-            nullptr /* plTxnReplaced */, true /* bypass_limits */);
+        const MempoolAcceptResult result = AcceptToMemoryPool(*m_node.mempool, MakeTransactionRef(tx),
+            true /* bypass_limits */);
+        return result.m_result_type == MempoolAcceptResult::ResultType::VALID;
     };
 
     // Create a double-spend of mature coinbase txn:
diff --git a/src/test/validation_block_tests.cpp b/src/test/validation_block_tests.cpp
index ec45d9a434..6c18a9e1bb 100644
--- a/src/test/validation_block_tests.cpp
+++ b/src/test/validation_block_tests.cpp
@@ -283,15 +283,9 @@ BOOST_AUTO_TEST_CASE(mempool_locks_reorg)
         // Add the txs to the tx pool
         {
             LOCK(cs_main);
-            TxValidationState state;
-            std::list<CTransactionRef> plTxnReplaced;
             for (const auto& tx : txs) {
-                BOOST_REQUIRE(AcceptToMemoryPool(
-                    *m_node.mempool,
-                    state,
-                    tx,
-                    &plTxnReplaced,
-                    /* bypass_limits */ false));
+                const MempoolAcceptResult result = AcceptToMemoryPool(*m_node.mempool, tx, false /* bypass_limits */);
+                BOOST_REQUIRE(result.m_result_type == MempoolAcceptResult::ResultType::VALID);
             }
         }
 
diff --git a/src/validation.cpp b/src/validation.cpp
index 38df71b994..a1e57752d3 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -380,10 +380,8 @@ static void UpdateMempoolForReorg(CTxMemPool& mempool, DisconnectedBlockTransact
     auto it = disconnectpool.queuedTx.get<insertion_order>().rbegin();
     while (it != disconnectpool.queuedTx.get<insertion_order>().rend()) {
         // ignore validation errors in resurrected transactions
-        TxValidationState stateDummy;
         if (!fAddToMempool || (*it)->IsCoinBase() ||
-            !AcceptToMemoryPool(mempool, stateDummy, *it,
-                                nullptr /* plTxnReplaced */, true /* bypass_limits */)) {
+            AcceptToMemoryPool(mempool, *it, true /* bypass_limits */).m_result_type != MempoolAcceptResult::ResultType::VALID) {
             // If the transaction doesn't make it in to the mempool, remove any
             // transactions that depend on it (which would now be orphans).
             mempool.removeRecursive(**it, MemPoolRemovalReason::REORG);
@@ -465,7 +463,7 @@ public:
         const CChainParams& m_chainparams;
         TxValidationState &m_state;
         const int64_t m_accept_time;
-        std::list<CTransactionRef>* m_replaced_transactions;
+        std::list<CTransactionRef> m_replaced_transactions;
         const bool m_bypass_limits;
         /*
          * Return any outpoints which were not previously present in the coins
@@ -476,11 +474,11 @@ public:
          */
         std::vector<COutPoint>& m_coins_to_uncache;
         const bool m_test_accept;
-        CAmount* m_fee_out;
+        CAmount m_fee_out;
     };
 
     // Single transaction acceptance
-    bool AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs& args) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    MempoolAcceptResult AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs& args) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
 private:
     // All the intermediate state that gets passed between the various levels
@@ -688,10 +686,7 @@ 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;
-    }
+    args.m_fee_out = nFees;
 
     // Check for non-standard pay-to-script-hash in inputs
     const auto& params = args.m_chainparams.GetConsensus();
@@ -1007,8 +1002,7 @@ bool MemPoolAccept::Finalize(ATMPArgs& args, Workspace& ws)
                 hash.ToString(),
                 FormatMoney(nModifiedFees - nConflictingFees),
                 (int)entry->GetTxSize() - (int)nConflictingSize);
-        if (args.m_replaced_transactions)
-            args.m_replaced_transactions->push_back(it->GetSharedTx());
+        args.m_replaced_transactions.push_back(it->GetSharedTx());
     }
     m_pool.RemoveStaged(allConflicting, false, MemPoolRemovalReason::REPLACED);
 
@@ -1031,14 +1025,14 @@ bool MemPoolAccept::Finalize(ATMPArgs& args, Workspace& ws)
     return true;
 }
 
-bool MemPoolAccept::AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs& args)
+MempoolAcceptResult MemPoolAccept::AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs& args)
 {
     AssertLockHeld(cs_main);
     LOCK(m_pool.cs); // mempool "read lock" (held through GetMainSignals().TransactionAddedToMempool())
 
     Workspace workspace(ptx);
 
-    if (!PreChecks(args, workspace)) return false;
+    if (!PreChecks(args, workspace)) return MempoolAcceptResult(args.m_state);
 
     // Only compute the precomputed transaction data if we need to verify
     // scripts (ie, other policy checks pass). We perform the inexpensive
@@ -1046,31 +1040,36 @@ bool MemPoolAccept::AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs
     // checks pass, to mitigate CPU exhaustion denial-of-service attacks.
     PrecomputedTransactionData txdata;
 
-    if (!PolicyScriptChecks(args, workspace, txdata)) return false;
+    if (!PolicyScriptChecks(args, workspace, txdata)) return MempoolAcceptResult(args.m_state);
 
-    if (!ConsensusScriptChecks(args, workspace, txdata)) return false;
+    if (!ConsensusScriptChecks(args, workspace, txdata)) return MempoolAcceptResult(args.m_state);
 
     // Tx was accepted, but not added
-    if (args.m_test_accept) return true;
+    if (args.m_test_accept) {
+        return MempoolAcceptResult(std::move(args.m_replaced_transactions), args.m_fee_out);
+    }
 
-    if (!Finalize(args, workspace)) return false;
+    if (!Finalize(args, workspace)) return MempoolAcceptResult(args.m_state);
 
     GetMainSignals().TransactionAddedToMempool(ptx, m_pool.GetAndIncrementSequence());
 
-    return true;
+    return MempoolAcceptResult(std::move(args.m_replaced_transactions), args.m_fee_out);
 }
 
 } // anon namespace
 
 /** (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, bool test_accept, CAmount* fee_out=nullptr) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+static MempoolAcceptResult AcceptToMemoryPoolWithTime(const CChainParams& chainparams, CTxMemPool& pool,
+                                                      const CTransactionRef &tx, int64_t nAcceptTime,
+                                                      bool bypass_limits, bool test_accept)
+                                                      EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
+    TxValidationState state;
     std::vector<COutPoint> coins_to_uncache;
-    MemPoolAccept::ATMPArgs args { chainparams, state, nAcceptTime, plTxnReplaced, bypass_limits, coins_to_uncache, test_accept, fee_out };
-    bool res = MemPoolAccept(pool).AcceptSingleTransaction(tx, args);
-    if (!res) {
+    MemPoolAccept::ATMPArgs args { chainparams, state, nAcceptTime, {}, bypass_limits, coins_to_uncache, test_accept, {} };
+
+    const MempoolAcceptResult result = MemPoolAccept(pool).AcceptSingleTransaction(tx, args);
+    if (result.m_result_type != MempoolAcceptResult::ResultType::VALID) {
         // Remove coins that were not present in the coins cache before calling ATMPW;
         // this is to prevent memory DoS in case we receive a large number of
         // invalid transactions that attempt to overrun the in-memory coins cache
@@ -1082,15 +1081,13 @@ static bool AcceptToMemoryPoolWithTime(const CChainParams& chainparams, CTxMemPo
     // After we've (potentially) uncached entries, ensure our coins cache is still within its size limits
     BlockValidationState state_dummy;
     ::ChainstateActive().FlushStateToDisk(chainparams, state_dummy, FlushStateMode::PERIODIC);
-    return res;
+    return result;
 }
 
-bool AcceptToMemoryPool(CTxMemPool& pool, TxValidationState &state, const CTransactionRef &tx,
-                        std::list<CTransactionRef>* plTxnReplaced,
-                        bool bypass_limits, bool test_accept, CAmount* fee_out)
+MempoolAcceptResult AcceptToMemoryPool(CTxMemPool& pool, const CTransactionRef &tx, bool bypass_limits, bool test_accept)
 {
     const CChainParams& chainparams = Params();
-    return AcceptToMemoryPoolWithTime(chainparams, pool, state, tx, GetTime(), plTxnReplaced, bypass_limits, test_accept, fee_out);
+    return AcceptToMemoryPoolWithTime(chainparams, pool, tx, GetTime(), bypass_limits, test_accept);
 }
 
 CTransactionRef GetTransaction(const CBlockIndex* const block_index, const CTxMemPool* const mempool, const uint256& hash, const Consensus::Params& consensusParams, uint256& hashBlock)
@@ -5029,13 +5026,10 @@ bool LoadMempool(CTxMemPool& pool)
             if (amountdelta) {
                 pool.PrioritiseTransaction(tx->GetHash(), amountdelta);
             }
-            TxValidationState state;
             if (nTime > nNow - nExpiryTimeout) {
                 LOCK(cs_main);
-                AcceptToMemoryPoolWithTime(chainparams, pool, state, tx, nTime,
-                                           nullptr /* plTxnReplaced */, false /* bypass_limits */,
-                                           false /* test_accept */);
-                if (state.IsValid()) {
+                if (AcceptToMemoryPoolWithTime(chainparams, pool, tx, nTime, false /* bypass_limits */,
+                                               false /* test_accept */).m_result_type == MempoolAcceptResult::ResultType::VALID) {
                     ++count;
                 } else {
                     // mempool may contain the transaction already, e.g. from
diff --git a/src/validation.h b/src/validation.h
index fc7add85b7..c737c113ad 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -12,6 +12,7 @@
 
 #include <amount.h>
 #include <coins.h>
+#include <consensus/validation.h>
 #include <crypto/common.h> // for ReadLE64
 #include <fs.h>
 #include <optional.h>
@@ -23,6 +24,7 @@
 #include <txdb.h>
 #include <versionbits.h>
 #include <serialize.h>
+#include <util/check.h>
 #include <util/hasher.h>
 
 #include <atomic>
@@ -46,7 +48,6 @@ class CConnman;
 class CScriptCheck;
 class CTxMemPool;
 class ChainstateManager;
-class TxValidationState;
 struct ChainTxData;
 
 struct DisconnectedBlockTransactions;
@@ -181,12 +182,46 @@ void UnlinkPrunedFiles(const std::set<int>& setFilesToPrune);
 /** Prune block files up to a given height */
 void PruneBlockFilesManual(int nManualPruneHeight);
 
-/** (try to) add transaction to memory pool
- * 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, bool test_accept=false, CAmount* fee_out=nullptr) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+/**
+* Validation result for a single transaction mempool acceptance.
+*/
+struct MempoolAcceptResult {
+    /** Used to indicate the results of mempool validation,
+    * including the possibility of unfinished validation.
+    */
+    enum class ResultType {
+        VALID, //!> Fully validated, valid.
+        INVALID, //!> Invalid.
+    };
+    ResultType m_result_type;
+    TxValidationState m_state;
+
+    // The following fields are only present when m_result_type = ResultType::VALID
+    /** Mempool transactions replaced by the tx per BIP 125 rules. */
+    std::optional<std::list<CTransactionRef>> m_replaced_transactions;
+    /** Raw base fees. */
+    std::optional<CAmount> m_base_fees;
+
+    /** Constructor for failure case */
+    explicit MempoolAcceptResult(TxValidationState state)
+        : m_result_type(ResultType::INVALID),
+        m_state(state), m_replaced_transactions(nullopt), m_base_fees(nullopt) {
+            Assume(!state.IsValid()); // Can be invalid or error
+        }
+
+    /** Constructor for success case */
+    explicit MempoolAcceptResult(std::list<CTransactionRef>&& replaced_txns, CAmount fees)
+        : m_result_type(ResultType::VALID), m_state(TxValidationState{}),
+        m_replaced_transactions(std::move(replaced_txns)), m_base_fees(fees) {}
+};
+
+/**
+ * (Try to) add a transaction to the memory pool.
+ * @param[in]  bypass_limits   When true, don't enforce mempool fee limits.
+ * @param[in]  test_accept     When true, run validation checks but don't submit to mempool.
+ */
+MempoolAcceptResult AcceptToMemoryPool(CTxMemPool& pool, const CTransactionRef& tx,
+                                       bool bypass_limits, bool test_accept=false) 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);