[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.

Github-Pull: #16257
Rebased-From: 806b0052c3b45415862f74f20ba5f389e5b673de

7 files changed, 31 insertions, 10 deletions

diff --git a/doc/release-notes-0.18.1-16257.md b/doc/release-notes-0.18.1-16257.md
new file mode 100644
index 0000000000..21867b7fb2
--- /dev/null
+++ b/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.
diff --git a/src/policy/fees.cpp b/src/policy/fees.cpp
index aee6fbee1a..d9a5474056 100644
--- a/src/policy/fees.cpp
+++ b/src/policy/fees.cpp
@@ -38,7 +38,6 @@ 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);
 
diff --git a/src/policy/fees.h b/src/policy/fees.h
index 136fb481f7..d46738cefb 100644
--- a/src/policy/fees.h
+++ b/src/policy/fees.h
@@ -88,7 +88,6 @@ enum class FeeReason {
     PAYTXFEE,
     FALLBACK,
     REQUIRED,
-    MAXTXFEE,
 };
 
 std::string StringForFeeReason(FeeReason reason);
diff --git a/src/wallet/fees.cpp b/src/wallet/fees.cpp
index d620e25f2b..0cac42ba72 100644
--- a/src/wallet/fees.cpp
+++ b/src/wallet/fees.cpp
@@ -21,13 +21,7 @@ CAmount GetRequiredFee(const CWallet& wallet, unsigned int nTxBytes)
 
 CAmount GetMinimumFee(const CWallet& wallet, unsigned int nTxBytes, const CCoinControl& coin_control, const CTxMemPool& pool, const CBlockPolicyEstimator& estimator, FeeCalculation* feeCalc)
 {
-    CAmount fee_needed = GetMinimumFeeRate(wallet, coin_control, pool, estimator, feeCalc).GetFee(nTxBytes);
-    // Always obey the maximum
-    if (fee_needed > maxTxFee) {
-        fee_needed = maxTxFee;
-        if (feeCalc) feeCalc->reason = FeeReason::MAXTXFEE;
-    }
-    return fee_needed;
+    return GetMinimumFeeRate(wallet, coin_control, pool, estimator, feeCalc).GetFee(nTxBytes);
 }
 
 CFeeRate GetRequiredFeeRate(const CWallet& wallet)
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 8d52f7eeca..92dc0f5d31 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2671,6 +2671,11 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nC
         }
     }
 
+    if (nFeeRet > maxTxFee) {
+        strFailReason = _("Fee exceeds maximum configured by -maxtxfee");
+        return false;
+    }
+
     return true;
 }
 
diff --git a/test/functional/rpc_fundrawtransaction.py b/test/functional/rpc_fundrawtransaction.py
index 0c61e9ab62..019a9d4012 100755
--- a/test/functional/rpc_fundrawtransaction.py
+++ b/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)
diff --git a/test/functional/rpc_psbt.py b/test/functional/rpc_psbt.py
index ef895dd13a..24ac14162c 100755
--- a/test/functional/rpc_psbt.py
+++ b/test/functional/rpc_psbt.py
@@ -6,7 +6,15 @@
 """
 
 from test_framework.test_framework import BitcoinTestFramework
-from test_framework.util import assert_equal, assert_raises_rpc_error, find_output, disconnect_nodes, connect_nodes_bi, sync_blocks
+from test_framework.util import (
+    assert_equal,
+    assert_greater_than,
+    assert_raises_rpc_error,
+    connect_nodes_bi,
+    disconnect_nodes,
+    find_output,
+    sync_blocks,
+)
 
 import json
 import os
@@ -122,6 +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)