wallet: Move fee underpayment check to after fee setting

It doesn't make sense to be checking whether the fee paid is underpaying
before we've finished setting the fees. So do that after we have done
the reduction for SFFO and change adjustment for fee overpayment.

src/primitives/transaction.h

@@ -17,6 +17,7 @@
 #include <ios>
 #include <limits>
 #include <memory>
+#include <numeric>
 #include <string>
 #include <tuple>
 #include <utility>
@@ -280,6 +281,12 @@ inline void SerializeTransaction(const TxType& tx, Stream& s) {
     s << tx.nLockTime;
 }
 
+template<typename TxType>
+inline CAmount CalculateOutputValue(const TxType& tx)
+{
+    return std::accumulate(tx.vout.cbegin(), tx.vout.cend(), CAmount{0}, [](CAmount sum, const auto& txout) { return sum + txout.nValue; });
+}
+
 
 /** The basic transaction that is broadcasted on the network and contained in
  * blocks.  A transaction can contain multiple inputs and outputs.

src/wallet/spend.cpp

@@ -8,6 +8,7 @@
 #include <interfaces/chain.h>
 #include <numeric>
 #include <policy/policy.h>
+#include <primitives/transaction.h>
 #include <script/signingprovider.h>
 #include <util/check.h>
 #include <util/fees.h>
@@ -959,19 +960,18 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
         return util::Error{_("Missing solving data for estimating transaction size")};
     }
     CAmount fee_needed = coin_selection_params.m_effective_feerate.GetFee(nBytes);
-    CAmount current_fee = result->GetSelectedValue() - recipients_sum - change_amount;
-
-    // The only time that fee_needed should be less than the amount available for fees is when
-    // we are subtracting the fee from the outputs. If this occurs at any other time, it is a bug.
-    if (!coin_selection_params.m_subtract_fee_outputs && fee_needed > current_fee) {
-        return util::Error{Untranslated(STR_INTERNAL_BUG("Fee needed > fee paid"))};
-    }
+    const CAmount output_value = CalculateOutputValue(txNew);
+    Assume(recipients_sum + change_amount == output_value);
+    CAmount current_fee = result->GetSelectedValue() - output_value;
 
     // If there is a change output and we overpay the fees then increase the change to match the fee needed
     if (nChangePosInOut != -1 && fee_needed < current_fee) {
         auto& change = txNew.vout.at(nChangePosInOut);
         change.nValue += current_fee - fee_needed;
-        current_fee = fee_needed;
+        current_fee = result->GetSelectedValue() - CalculateOutputValue(txNew);
+        if (fee_needed != current_fee) {
+            return util::Error{Untranslated(STR_INTERNAL_BUG("Change adjustment: Fee needed != fee paid"))};
+        }
     }
 
     // Reduce output values for subtractFeeFromAmount
@@ -1007,7 +1007,16 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
             }
             ++i;
         }
-        current_fee = fee_needed;
+        current_fee = result->GetSelectedValue() - CalculateOutputValue(txNew);
+        if (fee_needed != current_fee) {
+            return util::Error{Untranslated(STR_INTERNAL_BUG("SFFO: Fee needed != fee paid"))};
+        }
+    }
+
+    // fee_needed should now always be less than or equal to the current fees that we pay.
+    // If it is not, it is a bug.
+    if (fee_needed > current_fee) {
+        return util::Error{Untranslated(STR_INTERNAL_BUG("Fee needed > fee paid"))};
     }
 
     // Give up if change keypool ran out and change is required