Merge #18275: wallet: error if an explicit fee rate was given but the needed fee rate differed

44cc75f wallet: error if an explicit fee rate was given but the needed fee rate differed (Karl-Johan Alm)

Pull request description:

  This ensures that the code doesn't silently ignore too low fee reates. It will now trigger an error in the QT client, if the user provides a fee rate below the minimum, and becomes a necessary check for #11413.

ACKs for top commit:
  Sjors:
    utACK 44cc75f (rebased)
  fjahr:
    re-ACK 44cc75f

Tree-SHA512: cd5a60ee496e64f7ab37aaa53f7748a7393357b1629ccd9660839d366c6191b6413b871ce3aa7293fce1539336222c300ef6f86304f30a1ae8fe361b02310483

Author: MarcoFalke

src/wallet/wallet.cpp

@@ -2742,6 +2742,12 @@ 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;