Introduce `mempoolfullrbf` node setting.

This new node policy setting enables to accept replaced-by-fee
transaction without inspection of the replaceability signaling
as described in BIP125 "explicit signaling".

If turns on, the node mempool accepts transaction replacement
as described in `policy/mempool-replacements.md`.

The default setting value is `false`, implying opt-in RBF
is enforced.

7 files changed, 43 insertions, 1 deletions

diff --git a/src/init.cpp b/src/init.cpp
index ee90abcc93..e17728373b 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -558,6 +558,7 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-bytespersigop", strprintf("Equivalent bytes per sigop in transactions for relay and mining (default: %u)", DEFAULT_BYTES_PER_SIGOP), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarrier", strprintf("Relay and mine data carrier transactions (default: %u)", DEFAULT_ACCEPT_DATACARRIER), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarriersize", strprintf("Maximum size of data in data carrier transactions we relay and mine (default: %u)", MAX_OP_RETURN_RELAY), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
+    argsman.AddArg("-mempoolfullrbf", strprintf("Accept transaction replace-by-fee without requiring replaceability signaling (default: %u)", DEFAULT_MEMPOOL_FULL_RBF), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-minrelaytxfee=<amt>", strprintf("Fees (in %s/kvB) smaller than this are considered zero fee for relaying, mining and transaction creation (default: %s)",
         CURRENCY_UNIT, FormatMoney(DEFAULT_MIN_RELAY_TX_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-whitelistforcerelay", strprintf("Add 'forcerelay' permission to whitelisted inbound peers with default permissions. This will relay transactions even if the transactions were already in the mempool. (default: %d)", DEFAULT_WHITELISTFORCERELAY), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
diff --git a/src/kernel/mempool_options.h b/src/kernel/mempool_options.h
index a14abb6628..07953b443b 100644
--- a/src/kernel/mempool_options.h
+++ b/src/kernel/mempool_options.h
@@ -15,6 +15,8 @@ class CBlockPolicyEstimator;
 static constexpr unsigned int DEFAULT_MAX_MEMPOOL_SIZE_MB{300};
 /** Default for -mempoolexpiry, expiration time for mempool transactions in hours */
 static constexpr unsigned int DEFAULT_MEMPOOL_EXPIRY_HOURS{336};
+/** Default for -mempoolfullrbf, if the transaction replaceability signaling is ignored */
+static constexpr bool DEFAULT_MEMPOOL_FULL_RBF{false};
 
 namespace kernel {
 /**
@@ -31,6 +33,7 @@ struct MemPoolOptions {
     int check_ratio{0};
     int64_t max_size_bytes{DEFAULT_MAX_MEMPOOL_SIZE_MB * 1'000'000};
     std::chrono::seconds expiry{std::chrono::hours{DEFAULT_MEMPOOL_EXPIRY_HOURS}};
+    bool full_rbf{DEFAULT_MEMPOOL_FULL_RBF};
     MemPoolLimits limits{};
 };
 } // namespace kernel
diff --git a/src/mempool_args.cpp b/src/mempool_args.cpp
index e26cbe0275..77caa127e9 100644
--- a/src/mempool_args.cpp
+++ b/src/mempool_args.cpp
@@ -33,5 +33,7 @@ void ApplyArgsManOptions(const ArgsManager& argsman, MemPoolOptions& mempool_opt
 
     if (auto hours = argsman.GetIntArg("-mempoolexpiry")) mempool_opts.expiry = std::chrono::hours{*hours};
 
+    mempool_opts.full_rbf = argsman.GetBoolArg("-mempoolfullrbf", mempool_opts.full_rbf);
+
     ApplyArgsManOptions(argsman, mempool_opts.limits);
 }
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 69ae9fed99..aeaa10034e 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -458,6 +458,7 @@ CTxMemPool::CTxMemPool(const Options& opts)
       minerPolicyEstimator{opts.estimator},
       m_max_size_bytes{opts.max_size_bytes},
       m_expiry{opts.expiry},
+      m_full_rbf{opts.full_rbf},
       m_limits{opts.limits}
 {
     _clear(); //lock free clear
diff --git a/src/txmempool.h b/src/txmempool.h
index f44e78fde5..6e37f59f2e 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -568,6 +568,7 @@ public:
 
     const int64_t m_max_size_bytes;
     const std::chrono::seconds m_expiry;
+    const bool m_full_rbf;
 
     using Limits = kernel::MemPoolLimits;
 
diff --git a/src/validation.cpp b/src/validation.cpp
index 6b21d33871..4c694a2c21 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -740,7 +740,10 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
                 // Applications relying on first-seen mempool behavior should
                 // check all unconfirmed ancestors; otherwise an opt-in ancestor
                 // might be replaced, causing removal of this descendant.
-                if (!SignalsOptInRBF(*ptxConflicting)) {
+                //
+                // If replaceability signaling is ignored due to node setting,
+                // replacement is always allowed.
+                if (!m_pool.m_full_rbf && !SignalsOptInRBF(*ptxConflicting)) {
                     return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "txn-mempool-conflict");
                 }
 
diff --git a/test/functional/feature_rbf.py b/test/functional/feature_rbf.py
index 91dc222bab..5558c76138 100755
--- a/test/functional/feature_rbf.py
+++ b/test/functional/feature_rbf.py
@@ -94,6 +94,9 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         self.log.info("Running test replacement relay fee...")
         self.test_replacement_relay_fee()
 
+        self.log.info("Running test full replace by fee...")
+        self.test_fullrbf()
+
         self.log.info("Passed")
 
     def make_utxo(self, node, amount, confirmed=True, scriptPubKey=DUMMY_P2WPKH_SCRIPT):
@@ -714,5 +717,33 @@ class ReplaceByFeeTest(BitcoinTestFramework):
         tx.vout[0].nValue -= 1
         assert_raises_rpc_error(-26, "insufficient fee", self.nodes[0].sendrawtransaction, tx.serialize().hex())
 
+    def test_fullrbf(self):
+        txid = self.wallet.send_self_transfer(from_node=self.nodes[0])['txid']
+        self.generate(self.nodes[0], 1)
+        confirmed_utxo = self.wallet.get_utxo(txid=txid)
+
+        self.restart_node(0, extra_args=["-mempoolfullrbf=1"])
+
+        # Create an explicitly opt-out transaction
+        optout_tx = self.wallet.send_self_transfer(
+            from_node=self.nodes[0],
+            utxo_to_spend=confirmed_utxo,
+            sequence=SEQUENCE_FINAL,
+            fee_rate=Decimal('0.01'),
+        )
+        assert_equal(False, self.nodes[0].getmempoolentry(optout_tx['txid'])['bip125-replaceable'])
+
+        conflicting_tx = self.wallet.create_self_transfer(
+                utxo_to_spend=confirmed_utxo,
+                sequence=SEQUENCE_FINAL,
+                fee_rate=Decimal('0.02'),
+        )
+
+        # Send the replacement transaction, conflicting with the optout_tx.
+        self.nodes[0].sendrawtransaction(conflicting_tx['hex'], 0)
+
+        # Optout_tx is not anymore in the mempool.
+        assert optout_tx['txid'] not in self.nodes[0].getrawmempool()
+
 if __name__ == '__main__':
     ReplaceByFeeTest().main()