wallet: fix silent payment change output not found during scanning

The bug was caused by generating change output separately, causing `CreateSilentPaymentOutputs` to use `k = 0` for multiple outputs

Author: Eunovo

src/wallet/spend.cpp

@@ -1304,11 +1304,20 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
                 sp_dests[i] = *sp;
             }
         }
+        if (const auto* change_sp = std::get_if<V0SilentPaymentDestination>(&change_dest)) {
+            // Generate output for change too
+            sp_dests[mutableVecSend.size()] = *change_sp;
+        }
         const auto& silent_payment_tr_spks = CreateSilentPaymentOutputs(wallet, sp_dests, result.GetInputSet(), error);
         if (!silent_payment_tr_spks.has_value()) {
             return util::Error{error};
         }
         for (const auto& [out_idx, tr_dest] : *silent_payment_tr_spks) {
+            if (out_idx == mutableVecSend.size()) {
+                change_dest = tr_dest;
+                continue;
+            }
+
             assert(out_idx < mutableVecSend.size());
             mutableVecSend[out_idx].dest = tr_dest;
         }
@@ -1324,16 +1333,6 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
         if (!IsValidDestination(change_dest)) {
             return util::Error{error};
         }
-        if (const auto* sp = std::get_if<V0SilentPaymentDestination>(&change_dest)) {
-            // Since we will be getting the final destination
-            // for only the change output, the index doesn't matter.
-            std::map<size_t, V0SilentPaymentDestination> change_sp_dest;
-            change_sp_dest[0] = *sp;
-            auto taproot_change = CreateSilentPaymentOutputs(wallet, change_sp_dest, result.GetInputSet(), error);
-            if (taproot_change.has_value()) {
-                change_dest = taproot_change->at(0);
-            }
-        }
 
         CScript change_script = GetScriptForDestination(change_dest);
         Assert(!change_script.empty());
@@ -1537,7 +1536,8 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
         if (wallet.IsMine(*tx, spent_coins))
             wallet.WalletLogPrintf("Detected Silent Payments self-transfer: %s", tx->GetHash().ToString());
     }
-    return CreatedTransactionResult(tx, current_fee, change_pos, feeCalc);
+    LogPrintf("change_type: %s", FormatOutputType(change_type));
+    return CreatedTransactionResult(tx, current_fee, change_pos, change_type, feeCalc);
 }
 
 util::Result<CreatedTransactionResult> CreateTransaction(
@@ -1572,7 +1572,9 @@ util::Result<CreatedTransactionResult> CreateTransaction(
         tmp_cc.m_avoid_partial_spends = true;
 
         // Reuse the change destination from the first creation attempt to avoid skipping BIP44 indexes
-        if (txr_ungrouped.change_pos) {
+        // Do not reuse the change dest if it came from a silent payments destination
+        // silent payments change dest must be generated with the other silent payment outputs
+        if (txr_ungrouped.change_pos && txr_ungrouped.change_type != OutputType::SILENT_PAYMENTS) {
             ExtractDestination(txr_ungrouped.tx->vout[*txr_ungrouped.change_pos].scriptPubKey, tmp_cc.destChange);
         }
 

src/wallet/spend.h

@@ -208,9 +208,10 @@ struct CreatedTransactionResult
     CAmount fee;
     FeeCalculation fee_calc;
     std::optional<unsigned int> change_pos;
+    OutputType change_type;
 
-    CreatedTransactionResult(CTransactionRef _tx, CAmount _fee, std::optional<unsigned int> _change_pos, const FeeCalculation& _fee_calc)
-        : tx(_tx), fee(_fee), fee_calc(_fee_calc), change_pos(_change_pos) {}
+    CreatedTransactionResult(CTransactionRef _tx, CAmount _fee, std::optional<unsigned int> _change_pos, const OutputType change_type, const FeeCalculation& _fee_calc)
+        : tx(_tx), fee(_fee), fee_calc(_fee_calc), change_pos(_change_pos), change_type(change_type) {}
 };
 
 /**