use silent payments change when sending

Author: josibake

src/wallet/spend.cpp

@@ -1180,12 +1180,12 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
     }
 
     // Create change script that will be used if we need change
-    CScript scriptChange;
+    CTxDestination change_dest;
     bilingual_str error; // possible error str
 
     // coin control: send change to custom address
     if (!std::get_if<CNoDestination>(&coin_control.destChange)) {
-        scriptChange = GetScriptForDestination(coin_control.destChange);
+        change_dest = coin_control.destChange;
     } else { // no coin control: send change to newly generated address
         // Note: We use a new key here to keep it from being obvious which side is the change.
         //  The drawback is that by not reusing a previous key, the change may be lost if a
@@ -1196,20 +1196,18 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
 
         // Reserve a new key pair from key pool. If it fails, provide a dummy
         // destination in case we don't need change.
-        CTxDestination dest;
         auto op_dest = reservedest.GetReservedDestination(true);
         if (!op_dest) {
             error = _("Transaction needs a change address, but we can't generate it.") + Untranslated(" ") + util::ErrorString(op_dest);
         } else {
-            dest = *op_dest;
-            scriptChange = GetScriptForDestination(dest);
+            change_dest = *op_dest;
         }
-        // A valid destination implies a change script (and
-        // vice-versa). An empty change script will abort later, if the
-        // change keypool ran out, but change is required.
-        CHECK_NONFATAL(IsValidDestination(dest) != scriptChange.empty());
     }
-    CTxOut change_prototype_txout(0, scriptChange);
+    CScript change_prototype_script = GetScriptForDestination(change_dest);
+    if (std::get_if<V0SilentPaymentDestination>(&change_dest)) {
+        change_prototype_script = GetScriptForDestination(WitnessV1Taproot());
+    }
+    CTxOut change_prototype_txout(0, change_prototype_script);
     coin_selection_params.change_output_size = GetSerializeSize(change_prototype_txout);
 
     // Get size of spending the change output
@@ -1314,17 +1312,33 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
             assert(out_idx < mutableVecSend.size());
             mutableVecSend[out_idx].dest = tr_dest;
         }
-
     }
     // vouts to the payees
     txNew.vout.reserve(vecSend.size() + 1); // + 1 because of possible later insert
-    for (const auto& recipient : mutableVecSend)
-    {
+    for (const auto& recipient : mutableVecSend) {
         txNew.vout.emplace_back(recipient.nAmount, GetScriptForDestination(recipient.dest));
     }
     const CAmount change_amount = result.GetChange(coin_selection_params.min_viable_change, coin_selection_params.m_change_fee);
     if (change_amount > 0) {
-        CTxOut newTxOut(change_amount, scriptChange);
+        // Give up if change keypool ran out as change is required
+        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());
+
+        CTxOut newTxOut(change_amount, change_script);
         if (!change_pos) {
             // Insert change txn at random position:
             change_pos = rng_fast.randrange(txNew.vout.size() + 1);
@@ -1465,11 +1479,6 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
         return util::Error{Untranslated(STR_INTERNAL_BUG("Fee needed > fee paid"))};
     }
 
-    // Give up if change keypool ran out and change is required
-    if (scriptChange.empty() && change_pos) {
-        return util::Error{error};
-    }
-
     if (sign && !wallet.SignTransaction(txNew)) {
         return util::Error{_("Signing transaction failed")};
     }

src/wallet/wallet.cpp

@@ -2258,13 +2258,16 @@ OutputType CWallet::TransactionChangeType(const std::optional<OutputType>& chang
         return OutputType::LEGACY;
     }
 
+    bool any_sp{false};
     bool any_tr{false};
     bool any_wpkh{false};
     bool any_sh{false};
     bool any_pkh{false};
 
     for (const auto& recipient : vecSend) {
-        if (std::get_if<WitnessV1Taproot>(&recipient.dest) || std::get_if<V0SilentPaymentDestination>(&recipient.dest)) {
+        if (std::get_if<V0SilentPaymentDestination>(&recipient.dest)) {
+            any_sp = true;
+        } else if (std::get_if<WitnessV1Taproot>(&recipient.dest)) {
             any_tr = true;
         } else if (std::get_if<WitnessV0KeyHash>(&recipient.dest)) {
             any_wpkh = true;
@@ -2275,6 +2278,10 @@ OutputType CWallet::TransactionChangeType(const std::optional<OutputType>& chang
         }
     }
 
+    const bool has_sp_spkman(GetScriptPubKeyMan(OutputType::SILENT_PAYMENTS, /*internal=*/true));
+    if (has_sp_spkman && any_sp) {
+        return OutputType::SILENT_PAYMENTS;
+    }
     const bool has_bech32m_spkman(GetScriptPubKeyMan(OutputType::BECH32M, /*internal=*/true));
     if (has_bech32m_spkman && any_tr) {
         // Currently tr is the only type supported by the BECH32M spkman