Fix rounding errors in calculation of minimum change size

Author: morcos

src/wallet/wallet.cpp

@@ -2861,13 +2861,15 @@ bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CWalletT
                     // new inputs. We now know we only need the smaller fee
                     // (because of reduced tx size) and so we should add a
                     // change output. Only try this once.
-                    CAmount fee_needed_for_change = GetMinimumFee(change_prototype_size, coin_control, ::mempool, ::feeEstimator, nullptr);
-                    CAmount minimum_value_for_change = GetDustThreshold(change_prototype_txout, discard_rate);
-                    CAmount max_excess_fee = fee_needed_for_change + minimum_value_for_change;
-                    if (nFeeRet > nFeeNeeded + max_excess_fee && nChangePosInOut == -1 && nSubtractFeeFromAmount == 0 && pick_new_inputs) {
-                        pick_new_inputs = false;
-                        nFeeRet = nFeeNeeded + fee_needed_for_change;
-                        continue;
+                    if (nChangePosInOut == -1 && nSubtractFeeFromAmount == 0 && pick_new_inputs) {
+                        unsigned int tx_size_with_change = nBytes + change_prototype_size + 2; // Add 2 as a buffer in case increasing # of outputs changes compact size
+                        CAmount fee_needed_with_change = GetMinimumFee(tx_size_with_change, coin_control, ::mempool, ::feeEstimator, nullptr);
+                        CAmount minimum_value_for_change = GetDustThreshold(change_prototype_txout, discard_rate);
+                        if (nFeeRet >= fee_needed_with_change + minimum_value_for_change) {
+                            pick_new_inputs = false;
+                            nFeeRet = fee_needed_with_change;
+                            continue;
+                        }
                     }
 
                     // If we have change output already, just increase it