[wallet] abort when attempting to fund a transaction above maxtxfee

FundTransaction calls GetMinimumFee which, when the fee rate is absurdly high, quietly reduced the fee to -maxtxfee. Becaue an absurdly high fee rate is usually the result of a fat finger, aborting seems safer behavior.

doc/release-notes-0.18.1-16257.md

@@ -0,0 +1,6 @@
+Wallet changes
+--------------
+When creating a transaction with a fee above `-maxtxfee` (default 0.1 BTC),
+the RPC commands `walletcreatefundedpsbt` and  `fundrawtransaction` will now fail
+instead of rounding down the fee. Beware that the `feeRate` argument is specified
+in BTC per kilobyte, not satoshi per byte.

src/policy/fees.h

@@ -43,7 +43,6 @@ enum class FeeReason {
     PAYTXFEE,
     FALLBACK,
     REQUIRED,
-    MAXTXFEE,
 };
 
 /* Used to determine type of fee estimation requested */

src/qt/walletmodel.cpp

@@ -221,9 +221,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
             return TransactionCreationFailed;
         }
 
-        // reject absurdly high fee. (This can never happen because the
-        // wallet caps the fee at m_default_max_tx_fee. This merely serves as a
-        // belt-and-suspenders check)
+        // Reject absurdly high fee
         if (nFeeRequired > m_wallet->getDefaultMaxTxFee())
             return AbsurdFee;
     }

src/util/error.cpp

@@ -27,6 +27,8 @@ std::string TransactionErrorString(const TransactionError err)
             return "PSBTs not compatible (different transactions)";
         case TransactionError::SIGHASH_MISMATCH:
             return "Specified sighash value does not match existing value";
+        case TransactionError::MAX_FEE_EXCEEDED:
+            return "Fee exceeds maximum configured by -maxtxfee";
         // no default case, so the compiler can warn about missing cases
     }
     assert(false);

src/util/error.h

@@ -27,6 +27,7 @@ enum class TransactionError {
     INVALID_PSBT,
     PSBT_MISMATCH,
     SIGHASH_MISMATCH,
+    MAX_FEE_EXCEEDED,
 };
 
 std::string TransactionErrorString(const TransactionError error);

src/util/fees.cpp

@@ -18,7 +18,6 @@ std::string StringForFeeReason(FeeReason reason) {
         {FeeReason::PAYTXFEE, "PayTxFee set"},
         {FeeReason::FALLBACK, "Fallback fee"},
         {FeeReason::REQUIRED, "Minimum Required Fee"},
-        {FeeReason::MAXTXFEE, "MaxTxFee limit"}
     };
     auto reason_string = fee_reason_strings.find(reason);
 

src/wallet/fees.cpp

@@ -18,14 +18,7 @@ CAmount GetRequiredFee(const CWallet& wallet, unsigned int nTxBytes)
 
 CAmount GetMinimumFee(const CWallet& wallet, unsigned int nTxBytes, const CCoinControl& coin_control, FeeCalculation* feeCalc)
 {
-    CAmount fee_needed = GetMinimumFeeRate(wallet, coin_control, feeCalc).GetFee(nTxBytes);
-    // Always obey the maximum
-    const CAmount max_tx_fee = wallet.m_default_max_tx_fee;
-    if (fee_needed > max_tx_fee) {
-        fee_needed = max_tx_fee;
-        if (feeCalc) feeCalc->reason = FeeReason::MAXTXFEE;
-    }
-    return fee_needed;
+    return GetMinimumFeeRate(wallet, coin_control, feeCalc).GetFee(nTxBytes);
 }
 
 CFeeRate GetRequiredFeeRate(const CWallet& wallet)

src/wallet/wallet.cpp

@@ -2694,6 +2694,11 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nC
         }
     }
 
+    if (nFeeRet > this->m_default_max_tx_fee) {
+        strFailReason = TransactionErrorString(TransactionError::MAX_FEE_EXCEEDED);
+        return false;
+    }
+
     return true;
 }
 

test/functional/rpc_fundrawtransaction.py

@@ -662,6 +662,7 @@ class RawTransactionsTest(BitcoinTestFramework):
         result = self.nodes[3].fundrawtransaction(rawtx) # uses min_relay_tx_fee (set by settxfee)
         result2 = self.nodes[3].fundrawtransaction(rawtx, {"feeRate": 2*min_relay_tx_fee})
         result3 = self.nodes[3].fundrawtransaction(rawtx, {"feeRate": 10*min_relay_tx_fee})
+        assert_raises_rpc_error(-4, "Fee exceeds maximum configured by -maxtxfee", self.nodes[3].fundrawtransaction, rawtx, {"feeRate": 1})
         result_fee_rate = result['fee'] * 1000 / count_bytes(result['hex'])
         assert_fee_amount(result2['fee'], count_bytes(result2['hex']), 2 * result_fee_rate)
         assert_fee_amount(result3['fee'], count_bytes(result3['hex']), 10 * result_fee_rate)

test/functional/rpc_psbt.py

@@ -9,6 +9,7 @@ from decimal import Decimal
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
+    assert_greater_than,
     assert_raises_rpc_error,
     connect_nodes_bi,
     disconnect_nodes,
@@ -129,6 +130,15 @@ class PSBTTest(BitcoinTestFramework):
         assert_equal(walletprocesspsbt_out['complete'], True)
         self.nodes[1].sendrawtransaction(self.nodes[1].finalizepsbt(walletprocesspsbt_out['psbt'])['hex'])
 
+        # feeRate of 0.1 BTC / KB produces a total fee slightly below -maxtxfee (~0.05280000):
+        res = self.nodes[1].walletcreatefundedpsbt([{"txid":txid,"vout":p2wpkh_pos},{"txid":txid,"vout":p2sh_p2wpkh_pos},{"txid":txid,"vout":p2pkh_pos}], {self.nodes[1].getnewaddress():29.99}, 0, {"feeRate": 0.1})
+        assert_greater_than(res["fee"], 0.05)
+        assert_greater_than(0.06, res["fee"])
+
+        # feeRate of 10 BTC / KB produces a total fee well above -maxtxfee
+        # previously this was silenty capped at -maxtxfee
+        assert_raises_rpc_error(-4, "Fee exceeds maximum configured by -maxtxfee", self.nodes[1].walletcreatefundedpsbt, [{"txid":txid,"vout":p2wpkh_pos},{"txid":txid,"vout":p2sh_p2wpkh_pos},{"txid":txid,"vout":p2pkh_pos}], {self.nodes[1].getnewaddress():29.99}, 0, {"feeRate": 10})
+
         # partially sign multisig things with node 1
         psbtx = self.nodes[1].walletcreatefundedpsbt([{"txid":txid,"vout":p2wsh_pos},{"txid":txid,"vout":p2sh_pos},{"txid":txid,"vout":p2sh_p2wsh_pos}], {self.nodes[1].getnewaddress():29.99})['psbt']
         walletprocesspsbt_out = self.nodes[1].walletprocesspsbt(psbtx)