wallet: error if an explicit fee rate was given but the needed fee rate differed

This avoids cases where a user requests a fee rate below the minimum and is silently overruled by the wallet.

Author: kallewoof

src/wallet/wallet.cpp

@@ -2729,6 +2729,12 @@ bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CTransac
 
             // Get the fee rate to use effective values in coin selection
             CFeeRate nFeeRateNeeded = GetMinimumFeeRate(*this, coin_control, &feeCalc);
+            // Do not, ever, assume that it's fine to change the fee rate if the user has explicitly
+            // provided one
+            if (coin_control.m_feerate && nFeeRateNeeded > *coin_control.m_feerate) {
+                error = strprintf(_("Fee rate (%s) is lower than the minimum fee rate setting (%s)"), coin_control.m_feerate->ToString(), nFeeRateNeeded.ToString());
+                return false;
+            }
 
             nFeeRet = 0;
             bool pick_new_inputs = true;