[validation] de-duplicate package transactions already in mempool

As node operators are free to set their mempool policies however they
please, it's possible for package transaction(s) to already be in the
mempool. We definitely don't want to reject the entire package in that
case (as that could be a censorship vector).

We should still return the successful result to the caller, so add
another result type to MempoolAcceptResult.

Author: glozow

src/rpc/rawtransaction.cpp

@@ -974,6 +974,8 @@ static RPCHelpMan testmempoolaccept()
             continue;
         }
         const auto& tx_result = it->second;
+        // Package testmempoolaccept doesn't allow transactions to already be in the mempool.
+        CHECK_NONFATAL(tx_result.m_result_type != MempoolAcceptResult::ResultType::MEMPOOL_ENTRY);
         if (tx_result.m_result_type == MempoolAcceptResult::ResultType::VALID) {
             const CAmount fee = tx_result.m_base_fees.value();
             // Check that fee does not exceed maximum fee

src/validation.cpp

@@ -916,6 +916,10 @@ bool MemPoolAccept::PackageMempoolChecks(const std::vector<CTransactionRef>& txn
     AssertLockHeld(cs_main);
     AssertLockHeld(m_pool.cs);
 
+    // CheckPackageLimits expects the package transactions to not already be in the mempool.
+    assert(std::all_of(txns.cbegin(), txns.cend(), [this](const auto& tx)
+                       { return !m_pool.exists(GenTxid::Txid(tx->GetHash()));}));
+
     std::string err_string;
     if (!m_pool.CheckPackageLimits(txns, m_limit_ancestors, m_limit_ancestor_size, m_limit_descendants,
                                    m_limit_descendant_size, err_string)) {
@@ -1238,7 +1242,49 @@ PackageMempoolAcceptResult MemPoolAccept::AcceptPackage(const Package& package,
     m_view.SetBackend(m_dummy);
 
     LOCK(m_pool.cs);
-    return AcceptMultipleTransactions(package, args);
+    std::map<const uint256, const MempoolAcceptResult> results;
+    // As node operators are free to set their mempool policies however they please, it's possible
+    // for package transaction(s) to already be in the mempool, and we don't want to reject the
+    // entire package in that case (as that could be a censorship vector).  Filter the transactions
+    // that are already in mempool and add their information to results, since we already have them.
+    std::vector<CTransactionRef> txns_new;
+    for (const auto& tx : package) {
+        const auto& wtxid = tx->GetWitnessHash();
+        const auto& txid = tx->GetHash();
+        // There are 3 possibilities: already in mempool, same-txid-diff-wtxid already in mempool,
+        // or not in mempool. An already confirmed tx is treated as one not in mempool, because all
+        // we know is that the inputs aren't available.
+        if (m_pool.exists(GenTxid::Wtxid(wtxid))) {
+            // Exact transaction already exists in the mempool.
+            auto iter = m_pool.GetIter(wtxid);
+            assert(iter != std::nullopt);
+            results.emplace(wtxid, MempoolAcceptResult::MempoolTx(iter.value()->GetTxSize(), iter.value()->GetFee()));
+        } else if (m_pool.exists(GenTxid::Txid(txid))) {
+            // Transaction with the same non-witness data but different witness (same txid,
+            // different wtxid) already exists in the mempool.
+            //
+            // We don't allow replacement transactions right now, so just swap the package
+            // transaction for the mempool one. Note that we are ignoring the validity of the
+            // package transaction passed in.
+            // TODO: allow witness replacement in packages.
+            auto iter = m_pool.GetIter(wtxid);
+            assert(iter != std::nullopt);
+            results.emplace(txid, MempoolAcceptResult::MempoolTx(iter.value()->GetTxSize(), iter.value()->GetFee()));
+        } else {
+            // Transaction does not already exist in the mempool.
+            txns_new.push_back(tx);
+        }
+    }
+
+    // Nothing to do if the entire package has already been submitted.
+    if (txns_new.empty()) return PackageMempoolAcceptResult(package_state, std::move(results));
+    // Validate the (deduplicated) transactions as a package.
+    auto submission_result = AcceptMultipleTransactions(txns_new, args);
+    // Include already-in-mempool transaction results in the final result.
+    for (const auto& [wtxid, mempoolaccept_res] : results) {
+        submission_result.m_tx_results.emplace(wtxid, mempoolaccept_res);
+    }
+    return submission_result;
 }
 
 } // anon namespace

src/validation.h

@@ -161,17 +161,19 @@ struct MempoolAcceptResult {
     enum class ResultType {
         VALID, //!> Fully validated, valid.
         INVALID, //!> Invalid.
+        MEMPOOL_ENTRY, //!> Valid, transaction was already in the mempool.
     };
     const ResultType m_result_type;
     const TxValidationState m_state;
 
-    // The following fields are only present when m_result_type = ResultType::VALID
+    // The following fields are only present when m_result_type = ResultType::VALID or MEMPOOL_ENTRY
     /** Mempool transactions replaced by the tx per BIP 125 rules. */
     const std::optional<std::list<CTransactionRef>> m_replaced_transactions;
     /** Virtual size as used by the mempool, calculated using serialized size and sigops. */
     const std::optional<int64_t> m_vsize;
     /** Raw base fees in satoshis. */
     const std::optional<CAmount> m_base_fees;
+
     static MempoolAcceptResult Failure(TxValidationState state) {
         return MempoolAcceptResult(state);
     }
@@ -180,6 +182,10 @@ struct MempoolAcceptResult {
         return MempoolAcceptResult(std::move(replaced_txns), vsize, fees);
     }
 
+    static MempoolAcceptResult MempoolTx(int64_t vsize, CAmount fees) {
+        return MempoolAcceptResult(vsize, fees);
+    }
+
 // Private constructors. Use static methods MempoolAcceptResult::Success, etc. to construct.
 private:
     /** Constructor for failure case */
@@ -192,6 +198,10 @@ struct MempoolAcceptResult {
     explicit MempoolAcceptResult(std::list<CTransactionRef>&& replaced_txns, int64_t vsize, CAmount fees)
         : m_result_type(ResultType::VALID),
         m_replaced_transactions(std::move(replaced_txns)), m_vsize{vsize}, m_base_fees(fees) {}
+
+    /** Constructor for already-in-mempool case. It wouldn't replace any transactions. */
+    explicit MempoolAcceptResult(int64_t vsize, CAmount fees)
+        : m_result_type(ResultType::MEMPOOL_ENTRY), m_vsize{vsize}, m_base_fees(fees) {}
 };
 
 /**
@@ -201,7 +211,7 @@ struct PackageMempoolAcceptResult
 {
     const PackageValidationState m_state;
     /**
-    * Map from wtxid to finished MempoolAcceptResults. The client is responsible
+    * Map from (w)txid to finished MempoolAcceptResults. The client is responsible
     * for keeping track of the transaction objects themselves. If a result is not
     * present, it means validation was unfinished for that transaction. If there
     * was a package-wide error (see result in m_state), m_tx_results will be empty.