[validation] Add CChainState::ProcessTransaction()

This just calls through to AcceptToMemoryPool() internally, and is currently unused.

Also add a new transaction validation failure reason TX_NO_MEMPOOL to
indicate that there is no mempool.

src/consensus/validation.h

@@ -53,6 +53,7 @@ enum class TxValidationResult {
      */
     TX_CONFLICT,
     TX_MEMPOOL_POLICY,        //!< violated mempool's fee/size/descendant/RBF/etc limits
+    TX_NO_MEMPOOL,            //!< this node does not have a mempool so can't validate the transaction
 };
 
 /** A "reason" why a block was invalid, suitable for determining whether the

src/net_processing.cpp

@@ -1409,6 +1409,7 @@ bool PeerManagerImpl::MaybePunishNodeForTx(NodeId nodeid, const TxValidationStat
     case TxValidationResult::TX_WITNESS_STRIPPED:
     case TxValidationResult::TX_CONFLICT:
     case TxValidationResult::TX_MEMPOOL_POLICY:
+    case TxValidationResult::TX_NO_MEMPOOL:
         break;
     }
     if (message != "") {

src/validation.cpp

@@ -3421,6 +3421,17 @@ bool ChainstateManager::ProcessNewBlock(const CChainParams& chainparams, const s
     return true;
 }
 
+MempoolAcceptResult ChainstateManager::ProcessTransaction(const CTransactionRef& tx, bool test_accept)
+{
+    CChainState& active_chainstate = ActiveChainstate();
+    if (!active_chainstate.m_mempool) {
+        TxValidationState state;
+        state.Invalid(TxValidationResult::TX_NO_MEMPOOL, "no-mempool");
+        return MempoolAcceptResult::Failure(state);
+    }
+    return AcceptToMemoryPool(active_chainstate, *active_chainstate.m_mempool, tx, /*bypass_limits=*/ false, test_accept);
+}
+
 bool TestBlockValidity(BlockValidationState& state,
                        const CChainParams& chainparams,
                        CChainState& chainstate,

src/validation.h

@@ -994,6 +994,15 @@ public:
      */
     bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& block, BlockValidationState& state, const CChainParams& chainparams, const CBlockIndex** ppindex = nullptr) LOCKS_EXCLUDED(cs_main);
 
+    /**
+     * Try to add a transaction to the memory pool.
+     *
+     * @param[in]  tx              The transaction to submit for mempool acceptance.
+     * @param[in]  test_accept     When true, run validation checks but don't submit to mempool.
+     */
+    [[nodiscard]] MempoolAcceptResult ProcessTransaction(const CTransactionRef& tx, bool test_accept=false)
+        EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+
     //! Load the block tree and coins database from disk, initializing state if we're running with -reindex
     bool LoadBlockIndex() EXCLUSIVE_LOCKS_REQUIRED(cs_main);