wallet: override minfee checks (fOverrideFeeRate) for fee_rate

in RPCs fundrawtransaction and walletcreatefundedpsbt only.

This provides the existing feeRate (BTC/kvB) behavior in these two RPCs to the
new fee_rate (sat/vB) param also.

See these two GitHub review discussions for more info:
https://github.com/bitcoin/bitcoin/pull/10706/#discussion_r126560525
https://github.com/bitcoin/bitcoin/pull/20305#discussion_r520032533

3 files changed, 37 insertions, 31 deletions

diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index d20a819a02..25c6cf306d 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -197,17 +197,19 @@ static std::string LabelFromValue(const UniValue& value)
 /**
  * Update coin control with fee estimation based on the given parameters
  *
- * @param[in]     pwallet        Wallet pointer
- * @param[in,out] cc             Coin control which is to be updated
- * @param[in]     conf_target    UniValue integer, confirmation target in blocks, values between 1 and 1008 are valid per policy/fees.h;
- *                                   if a fee_rate is present, 0 is allowed here as a no-op positional placeholder
- * @param[in]     estimate_mode  UniValue string, fee estimation mode, valid values are "unset", "economical" or "conservative";
- *                                   if a fee_rate is present, "" is allowed here as a no-op positional placeholder
- * @param[in]     fee_rate       UniValue real, fee rate in sat/vB;
- *                                   if a fee_rate is present, both conf_target and estimate_mode must either be null, or no-op values
+ * @param[in]     pwallet           Wallet pointer
+ * @param[in,out] cc                Coin control to be updated
+ * @param[in]     conf_target       UniValue integer; confirmation target in blocks, values between 1 and 1008 are valid per policy/fees.h;
+ *                                      if a fee_rate is present, 0 is allowed here as a no-op positional placeholder
+ * @param[in]     estimate_mode     UniValue string; fee estimation mode, valid values are "unset", "economical" or "conservative";
+ *                                      if a fee_rate is present, "" is allowed here as a no-op positional placeholder
+ * @param[in]     fee_rate          UniValue real; fee rate in sat/vB;
+ *                                      if a fee_rate is present, both conf_target and estimate_mode must either be null, or no-op
+ * @param[in]     override_min_fee  bool; whether to set fOverrideFeeRate to true to disable minimum fee rate checks and instead
+ *                                      verify only that fee_rate is greater than 0
  * @throws a JSONRPCError if conf_target, estimate_mode, or fee_rate contain invalid values or are in conflict
  */
-static void SetFeeEstimateMode(const CWallet* pwallet, CCoinControl& cc, const UniValue& conf_target, const UniValue& estimate_mode, const UniValue& fee_rate)
+static void SetFeeEstimateMode(const CWallet* pwallet, CCoinControl& cc, const UniValue& conf_target, const UniValue& estimate_mode, const UniValue& fee_rate, bool override_min_fee)
 {
     if (!fee_rate.isNull()) {
         if (!conf_target.isNull() && conf_target.get_int() > 0) {
@@ -216,7 +218,14 @@ static void SetFeeEstimateMode(const CWallet* pwallet, CCoinControl& cc, const U
         if (!estimate_mode.isNull() && !estimate_mode.get_str().empty()) {
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot specify both estimate_mode and fee_rate");
         }
-        cc.m_feerate = CFeeRate(AmountFromValue(fee_rate), COIN);
+        CFeeRate fee_rate_in_sat_vb{CFeeRate(AmountFromValue(fee_rate), COIN)};
+        if (override_min_fee) {
+            if (fee_rate_in_sat_vb <= CFeeRate(0)) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid fee_rate %s (must be greater than 0)", fee_rate_in_sat_vb.ToString(FeeEstimateMode::SAT_VB)));
+            }
+            cc.fOverrideFeeRate = true;
+        }
+        cc.m_feerate = fee_rate_in_sat_vb;
         // Default RBF to true for explicit fee_rate, if unset.
         if (cc.m_signal_bip125_rbf == nullopt) cc.m_signal_bip125_rbf = true;
         return;
@@ -504,7 +513,7 @@ static RPCHelpMan sendtoaddress()
     // We also enable partial spend avoidance if reuse avoidance is set.
     coin_control.m_avoid_partial_spends |= coin_control.m_avoid_address_reuse;
 
-    SetFeeEstimateMode(pwallet, coin_control, /* conf_target */ request.params[6], /* estimate_mode */ request.params[7], /* fee_rate */ request.params[9]);
+    SetFeeEstimateMode(pwallet, coin_control, /* conf_target */ request.params[6], /* estimate_mode */ request.params[7], /* fee_rate */ request.params[9], /* override_min_fee */ false);
 
     EnsureWalletIsUnlocked(pwallet);
 
@@ -932,7 +941,7 @@ static RPCHelpMan sendmany()
         coin_control.m_signal_bip125_rbf = request.params[5].get_bool();
     }
 
-    SetFeeEstimateMode(pwallet, coin_control, /* conf_target */ request.params[6], /* estimate_mode */ request.params[7], /* fee_rate */ request.params[8]);
+    SetFeeEstimateMode(pwallet, coin_control, /* conf_target */ request.params[6], /* estimate_mode */ request.params[7], /* fee_rate */ request.params[8], /* override_min_fee */ false);
 
     std::vector<CRecipient> recipients;
     ParseRecipients(sendTo, subtractFeeFromAmount, recipients);
@@ -3049,7 +3058,7 @@ static RPCHelpMan listunspent()
     };
 }
 
-void FundTransaction(CWallet* const pwallet, CMutableTransaction& tx, CAmount& fee_out, int& change_position, const UniValue& options, CCoinControl& coinControl)
+void FundTransaction(CWallet* const pwallet, CMutableTransaction& tx, CAmount& fee_out, int& change_position, const UniValue& options, CCoinControl& coinControl, bool override_min_fee)
 {
     // Make sure the results are valid at least up to the most recent block
     // the user could have gotten from another RPC command prior to now
@@ -3154,7 +3163,7 @@ void FundTransaction(CWallet* const pwallet, CMutableTransaction& tx, CAmount& f
         if (options.exists("replaceable")) {
             coinControl.m_signal_bip125_rbf = options["replaceable"].get_bool();
         }
-        SetFeeEstimateMode(pwallet, coinControl, options["conf_target"], options["estimate_mode"], options["fee_rate"]);
+        SetFeeEstimateMode(pwallet, coinControl, options["conf_target"], options["estimate_mode"], options["fee_rate"], override_min_fee);
       }
     } else {
         // if options is null and not a bool
@@ -3275,7 +3284,7 @@ static RPCHelpMan fundrawtransaction()
     CCoinControl coin_control;
     // Automatically select (additional) coins. Can be overridden by options.add_inputs.
     coin_control.m_add_inputs = true;
-    FundTransaction(pwallet, tx, fee, change_position, request.params[1], coin_control);
+    FundTransaction(pwallet, tx, fee, change_position, request.params[1], coin_control, /* override_min_fee */ true);
 
     UniValue result(UniValue::VOBJ);
     result.pushKV("hex", EncodeHexTx(CTransaction(tx)));
@@ -3482,7 +3491,7 @@ static RPCHelpMan bumpfee_helper(std::string method_name)
         if (options.exists("replaceable")) {
             coin_control.m_signal_bip125_rbf = options["replaceable"].get_bool();
         }
-        SetFeeEstimateMode(pwallet, coin_control, conf_target, options["estimate_mode"], options["fee_rate"]);
+        SetFeeEstimateMode(pwallet, coin_control, conf_target, options["estimate_mode"], options["fee_rate"], /* override_min_fee */ false);
     }
 
     // Make sure the results are valid at least up to the most recent block
@@ -4146,7 +4155,7 @@ static RPCHelpMan send()
             // Automatically select coins, unless at least one is manually selected. Can
             // be overridden by options.add_inputs.
             coin_control.m_add_inputs = rawTx.vin.size() == 0;
-            FundTransaction(pwallet, rawTx, fee, change_position, options, coin_control);
+            FundTransaction(pwallet, rawTx, fee, change_position, options, coin_control, /* override_min_fee */ false);
 
             bool add_to_wallet = true;
             if (options.exists("add_to_wallet")) {
@@ -4433,7 +4442,7 @@ static RPCHelpMan walletcreatefundedpsbt()
     // Automatically select coins, unless at least one is manually selected. Can
     // be overridden by options.add_inputs.
     coin_control.m_add_inputs = rawTx.vin.size() == 0;
-    FundTransaction(pwallet, rawTx, fee, change_position, request.params[3], coin_control);
+    FundTransaction(pwallet, rawTx, fee, change_position, request.params[3], coin_control, /* override_min_fee */ true);
 
     // Make a blank psbt
     PartiallySignedTransaction psbtx(rawTx);
diff --git a/test/functional/rpc_fundrawtransaction.py b/test/functional/rpc_fundrawtransaction.py
index d515b4a7d3..f987867e61 100755
--- a/test/functional/rpc_fundrawtransaction.py
+++ b/test/functional/rpc_fundrawtransaction.py
@@ -752,7 +752,7 @@ class RawTransactionsTest(BitcoinTestFramework):
                     node.fundrawtransaction, rawtx, {"estimate_mode": mode, "conf_target": n, "add_inputs": True})
 
         self.log.info("Test invalid fee rate settings")
-        assert_raises_rpc_error(-4, "Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)",
+        assert_raises_rpc_error(-8, "Invalid fee_rate 0.000 sat/vB (must be greater than 0)",
             node.fundrawtransaction, rawtx, {"fee_rate": 0, "add_inputs": True})
         assert_raises_rpc_error(-8, "Invalid feeRate 0.00000000 BTC/kvB (must be greater than 0)",
             node.fundrawtransaction, rawtx, {"feeRate": 0, "add_inputs": True})
@@ -766,12 +766,9 @@ class RawTransactionsTest(BitcoinTestFramework):
             assert_raises_rpc_error(-3, "Invalid amount",
                 node.fundrawtransaction, rawtx, {"fee_rate": "", "add_inputs": True})
 
-        # Test setting explicit fee rate just below the minimum.
-        self.log.info("- raises RPC error 'fee rate too low' if fee_rate of 0.99999999 sat/vB is passed")
-        msg = "Fee rate (0.999 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)"
-        assert_raises_rpc_error(-4, msg, node.fundrawtransaction, rawtx, {"fee_rate": 0.99999999, "add_inputs": True})
-        # This feeRate test only passes if `coinControl.fOverrideFeeRate = true` in wallet/rpcwallet.cpp::FundTransaction is removed.
-        # assert_raises_rpc_error(-4, msg, node.fundrawtransaction, rawtx, {"feeRate": 0.00000999, "add_inputs": True})
+        self.log.info("Test min fee rate checks are bypassed with fundrawtxn, e.g. a fee_rate under 1 sat/vB is allowed")
+        node.fundrawtransaction(rawtx, {"fee_rate": 0.99999999, "add_inputs": True})
+        node.fundrawtransaction(rawtx, {"feeRate": 0.00000999, "add_inputs": True})
 
         self.log.info("- raises RPC error if both feeRate and fee_rate are passed")
         assert_raises_rpc_error(-8, "Cannot specify both fee_rate (sat/vB) and feeRate (BTC/kvB)",
diff --git a/test/functional/rpc_psbt.py b/test/functional/rpc_psbt.py
index 313611ca17..498197b5e5 100755
--- a/test/functional/rpc_psbt.py
+++ b/test/functional/rpc_psbt.py
@@ -192,9 +192,14 @@ class PSBTTest(BitcoinTestFramework):
         assert_approx(res1["fee"], 0.055, 0.005)
         res2 = self.nodes[1].walletcreatefundedpsbt(inputs, outputs, 0, {"feeRate": 0.1, "add_inputs": True})
         assert_approx(res2["fee"], 0.055, 0.005)
+        self.log.info("Test min fee rate checks with walletcreatefundedpsbt are bypassed, e.g. a fee_rate under 1 sat/vB is allowed")
+        res3 = self.nodes[1].walletcreatefundedpsbt(inputs, outputs, 0, {"fee_rate": 0.99999999, "add_inputs": True})
+        assert_approx(res3["fee"], 0.00000381, 0.0000001)
+        res4 = self.nodes[1].walletcreatefundedpsbt(inputs, outputs, 0, {"feeRate": 0.00000999, "add_inputs": True})
+        assert_approx(res4["fee"], 0.00000381, 0.0000001)
 
         self.log.info("Test invalid fee rate settings")
-        assert_raises_rpc_error(-4, "Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)",
+        assert_raises_rpc_error(-8, "Invalid fee_rate 0.000 sat/vB (must be greater than 0)",
             self.nodes[1].walletcreatefundedpsbt, inputs, outputs, 0, {"fee_rate": 0, "add_inputs": True})
         assert_raises_rpc_error(-8, "Invalid feeRate 0.00000000 BTC/kvB (must be greater than 0)",
             self.nodes[1].walletcreatefundedpsbt, inputs, outputs, 0, {"feeRate": 0, "add_inputs": True})
@@ -244,11 +249,6 @@ class PSBTTest(BitcoinTestFramework):
                 assert_raises_rpc_error(-8, "Invalid conf_target, must be between 1 and 1008",  # max value of 1008 per src/policy/fees.h
                     self.nodes[1].walletcreatefundedpsbt, inputs, outputs, 0, {"estimate_mode": mode, "conf_target": n, "add_inputs": True})
 
-        # Test setting explicit fee rate just below the minimum.
-        self.log.info("- raises RPC error 'fee rate too low' if feerate_sat_vb of 0.99999999 is passed")
-        assert_raises_rpc_error(-4, "Fee rate (0.999 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)",
-            self.nodes[1].walletcreatefundedpsbt, inputs, outputs, 0, {"fee_rate": 0.99999999, "add_inputs": True})
-
         self.log.info("Test walletcreatefundedpsbt with too-high fee rate produces total fee well above -maxtxfee and raises RPC error")
         # previously this was silently capped at -maxtxfee
         for bool_add, outputs_array in {True: outputs, False: [{self.nodes[1].getnewaddress(): 1}]}.items():