Allow all mempool txs to be replaced after a configurable timeout (default 6h)

src/init.cpp

@@ -485,6 +485,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-datacarrier", strprintf(_("Relay and mine data carrier transactions (default: %u)"), DEFAULT_ACCEPT_DATACARRIER));
     strUsage += HelpMessageOpt("-datacarriersize", strprintf(_("Maximum size of data in data carrier transactions we relay and mine (default: %u)"), MAX_OP_RETURN_RELAY));
     strUsage += HelpMessageOpt("-mempoolreplacement", strprintf(_("Enable transaction replacement in the memory pool (default: %u)"), DEFAULT_ENABLE_REPLACEMENT));
+    strUsage += HelpMessageOpt("-mempoolreplacementtimeout=<n>", strprintf(_("Number of seconds after which transactions in mempool can be replaced (default: %u)"), DEFAULT_REPLACEMENT_TIMEOUT));
     strUsage += HelpMessageOpt("-minrelaytxfee=<amt>", strprintf(_("Fees (in %s/kB) smaller than this are considered zero fee for relaying, mining and transaction creation (default: %s)"),
         CURRENCY_UNIT, FormatMoney(DEFAULT_MIN_RELAY_TX_FEE)));
     strUsage += HelpMessageOpt("-whitelistrelay", strprintf(_("Accept relayed transactions received from whitelisted peers even when not relaying transactions (default: %d)"), DEFAULT_WHITELISTRELAY));

src/txmempool.h

@@ -350,7 +350,8 @@ class SaltedTxidHasher
  * - a transaction which doesn't meet the minimum fee requirements.
  * - a new transaction that double-spends an input of a transaction already in
  * the pool where the new transaction does not meet the Replace-By-Fee
- * requirements as defined in BIP 125.
+ * requirements as defined in BIP 125 and the transaction(s) being replaced
+ * has not been in mempool for over a timeout defaulting to 6 hours.
  * - a non-standard transaction.
  *
  * CTxMemPool::mapTx, and CTxMemPoolEntry bookkeeping:

src/validation.cpp

@@ -20,6 +20,7 @@
 #include "init.h"
 #include "policy/fees.h"
 #include "policy/policy.h"
+#include "policy/rbf.h"
 #include "pow.h"
 #include "primitives/block.h"
 #include "primitives/transaction.h"
@@ -477,6 +478,7 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
 
     // Check for conflicts with in-memory transactions
     std::set<uint256> setConflicts;
+    const int64_t nReplacementTimeout = GetArg("-mempoolreplacementtimeout", DEFAULT_REPLACEMENT_TIMEOUT);
     {
     LOCK(pool.cs); // protect pool.mapNextTx
     for (const CTxIn &txin : tx.vin)
@@ -499,17 +501,14 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
                 // first-seen mempool behavior should be checking all
                 // unconfirmed ancestors anyway; doing otherwise is hopelessly
                 // insecure.
+                //
+                // All transactions in mempool become replaceable after the timeout.
                 bool fReplacementOptOut = true;
                 if (fEnableReplacement)
                 {
-                    for (const CTxIn &_txin : ptxConflicting->vin)
-                    {
-                        if (_txin.nSequence < std::numeric_limits<unsigned int>::max()-1)
-                        {
-                            fReplacementOptOut = false;
-                            break;
-                        }
-                    }
+                    const int64_t nConflictingTime = pool.info(ptxConflicting->GetHash()).nTime;
+                    const bool fConflictingPreTimeout = nAcceptTime - nConflictingTime < nReplacementTimeout;
+                    fReplacementOptOut = fConflictingPreTimeout && !SignalsOptInRBF(*ptxConflicting);
                 }
                 if (fReplacementOptOut) {
                     return state.Invalid(false, REJECT_DUPLICATE, "txn-mempool-conflict");

src/validation.h

@@ -135,6 +135,8 @@ static const unsigned int DEFAULT_BANSCORE_THRESHOLD = 100;
 static const bool DEFAULT_PERSIST_MEMPOOL = true;
 /** Default for -mempoolreplacement */
 static const bool DEFAULT_ENABLE_REPLACEMENT = true;
+/** Default for -mempoolreplacementtimeout in seconds after which transactions in mempool are replaceable (i.e. 6 hours) */
+static const unsigned int DEFAULT_REPLACEMENT_TIMEOUT = 6 * 60 * 60;
 /** Default for using fee filter */
 static const bool DEFAULT_FEEFILTER = true;