add -maxrelaytxfee option

src/init.cpp

@@ -703,6 +703,8 @@ void SetupServerArgs(ArgsManager& argsman)
                    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("-maxrelaytxfee=<amt>", strprintf("Fees (in %s/kvB) bigger than this are considered zero fee for relaying, mining and transaction creation (set to 0 to disable) (default: %s)",
+        CURRENCY_UNIT, DEFAULT_MAX_RELAY_TX_FEE), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-rejectparasites", strprintf("Refuse to relay or mine parasitic overlay protocols (default: %u)", DEFAULT_REJECT_PARASITES), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-rejecttokens",
                    strprintf("Refuse to relay or mine transactions involving non-bitcoin tokens (default: %u)",

src/interfaces/chain.h

@@ -295,6 +295,9 @@ class Chain
     //! Relay incremental fee setting (-incrementalrelayfee), reflecting cost of relay.
     virtual CFeeRate relayIncrementalFee() = 0;
 
+    //! Relay current maximum fee.
+    virtual CFeeRate relayMaxFee() = 0;
+
     //! Relay dust fee setting (-dustrelayfee), reflecting lowest rate it's economical to spend.
     virtual CFeeRate relayDustFee() = 0;
 

src/kernel/mempool_options.h

@@ -56,6 +56,7 @@ struct MemPoolOptions {
     CFeeRate incremental_relay_feerate{DEFAULT_INCREMENTAL_RELAY_FEE};
     /** A fee rate smaller than this is considered zero fee (for relaying, mining and transaction creation) */
     CFeeRate min_relay_feerate{DEFAULT_MIN_RELAY_TX_FEE};
+    CFeeRate max_relay_feerate{DEFAULT_MAX_RELAY_TX_FEE};
     CFeeRate dust_relay_feerate{DUST_RELAY_TX_FEE};
     CFeeRate dust_relay_feerate_floor{DUST_RELAY_TX_FEE};
     /** Negative for a target number of blocks, positive for target kB into current mempool. */

src/node/interfaces.cpp

@@ -779,6 +779,11 @@ class ChainImpl : public Chain
         if (!m_node.mempool) return CFeeRate{DEFAULT_INCREMENTAL_RELAY_FEE};
         return m_node.mempool->m_opts.incremental_relay_feerate;
     }
+    CFeeRate relayMaxFee() override
+    {
+        if (!m_node.mempool) return CFeeRate{DEFAULT_MAX_RELAY_TX_FEE};
+        return m_node.mempool->m_opts.max_relay_feerate;
+    }
     CFeeRate relayDustFee() override
     {
         if (!m_node.mempool) return CFeeRate{DUST_RELAY_TX_FEE};

src/node/mempool_args.cpp

@@ -122,6 +122,14 @@ util::Result<void> ApplyArgsManOptions(const ArgsManager& argsman, const CChainP
         LogPrintf("Increasing minrelaytxfee to %s to match incrementalrelayfee\n", mempool_opts.min_relay_feerate.ToString());
     }
 
+    if (argsman.IsArgSet("-maxrelaytxfee")) {
+        if (std::optional<CAmount> max_relay_feerate = ParseMoney(argsman.GetArg("-maxrelaytxfee", ""))) {
+            mempool_opts.max_relay_feerate = CFeeRate{max_relay_feerate.value()};
+        } else {
+            return util::Error{AmountErrMsg("maxrelayfee", argsman.GetArg("-maxrelaytxfee", ""))};
+        }
+    }
+
     // Feerate used to define dust.  Shouldn't be changed lightly as old
     // implementations may inadvertently create non-standard transactions
     if (argsman.IsArgSet("-dustrelayfee")) {

src/policy/policy.h

@@ -81,6 +81,8 @@ static const int DEFAULT_DUST_RELAY_MULTIPLIER{3'000};
 static const std::string DEFAULT_SPKREUSE{"allow"};
 /** Default for -minrelaytxfee, minimum relay fee for transactions */
 static constexpr unsigned int DEFAULT_MIN_RELAY_TX_FEE{1000};
+/** Default for -maxrelaytxfee, minimum relay fee for transactions */
+static constexpr unsigned int DEFAULT_MAX_RELAY_TX_FEE{0};
 /** Default for -limitancestorcount, max number of in-mempool ancestors */
 static constexpr unsigned int DEFAULT_ANCESTOR_LIMIT{25};
 /** Default for -limitancestorsize, maximum kilobytes of tx + all in-mempool ancestors */

src/validation.cpp

@@ -1086,6 +1086,12 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
     // feerate later.
     if (!args.m_package_feerates && !CheckFeeRate(ws.m_vsize, ws.m_modified_fees, state, args.m_ignore_rejects)) return false;
 
+
+    if ((ws.m_ptx->version != TRUC_VERSION || m_pool.m_opts.truc_policy != TRUCPolicy::Enforce) && ws.m_modified_fees > m_pool.m_opts.max_relay_feerate.GetFee(ws.m_vsize) && m_pool.m_opts.max_relay_feerate != CFeeRate{0}) {
+        return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "max relay fee met",
+                             strprintf("%d < %d", ws.m_modified_fees, m_pool.m_opts.max_relay_feerate.GetFee(ws.m_vsize)));
+    }
+
     std::set<Txid> conflicts_as_a_set;
     std::transform(ws.m_conflicts_incl_policy.begin(), ws.m_conflicts_incl_policy.end(),
                     std::inserter(conflicts_as_a_set, conflicts_as_a_set.end()),