wallet: use silent_payments as change if bech32m is not available

This will allow wallets with only silent payment descriptors to use silent payment outputs as a change

Author: Eunovo

src/wallet/spend.cpp

@@ -10,6 +10,7 @@
 #include <consensus/amount.h>
 #include <consensus/validation.h>
 #include <interfaces/chain.h>
+#include <key_io.h>
 #include <node/types.h>
 #include <numeric>
 #include <policy/policy.h>
@@ -1299,26 +1300,29 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
            result.GetSelectedValue());
 
     std::vector<CRecipient> mutableVecSend = vecSend;
-    if (coin_control.m_silent_payment) {
+    bool fSilentPayment = coin_control.m_silent_payment || std::holds_alternative<V0SilentPaymentDestination>(change_dest);
+    if (fSilentPayment) {
         // Get the silent payment destinations, generate the scriptPubKeys,
         // and update vecSend with the generated scriptPubKeys
         std::map<size_t, V0SilentPaymentDestination> sp_dests;
+        size_t sp_change_index = 0;
         for (size_t i = 0; i < mutableVecSend.size(); ++i) {
             if (const auto* sp = std::get_if<V0SilentPaymentDestination>(&mutableVecSend.at(i).dest)) {
                 // Keep track of the index in vecSend
                 sp_dests[i] = *sp;
+                sp_change_index += 1;
             }
         }
-        if (const auto* change_sp = std::get_if<V0SilentPaymentDestination>(&change_dest)) {
+        if (const auto* sp_change = std::get_if<V0SilentPaymentDestination>(&change_dest)) {
             // Generate output for change too
-            sp_dests[mutableVecSend.size()] = *change_sp;
+            sp_dests[sp_change_index] = *sp_change;
         }
         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()) {
+            if (out_idx == sp_change_index) {
                 change_dest = tr_dest;
                 continue;
             }

src/wallet/wallet.cpp

@@ -2298,6 +2298,11 @@ OutputType CWallet::TransactionChangeType(const std::optional<OutputType>& chang
     if (has_bech32m_spkman) {
         return OutputType::BECH32M;
     }
+    bool is_silent_payments_enabled{!IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS) && !IsWalletFlagSet(WALLET_FLAG_EXTERNAL_SIGNER)};
+    if (is_silent_payments_enabled && has_sp_spkman) {
+        // If silent payments are enabled, use the silent payments spkman for change
+        return OutputType::SILENT_PAYMENTS;
+    }
     if (has_bech32_spkman) {
         return OutputType::BECH32;
     }

test/functional/wallet_silentpayments_receiving.py

@@ -147,7 +147,29 @@ def do_nothing():
 
         self.log.info("Wallet persistence verified successfully")
 
-    def test_import_rescan(self):
+    def test_import_descriptors(self):
+        self.log.info("Check import silent payments descriptor into blank wallet")
+
+        self.nodes[0].createwallet(wallet_name="import_blank", blank=True, silent_payments=True)
+        wallet = self.nodes[0].get_wallet_rpc("import_blank")
+
+        descriptor = "sp([eea23daf/352h/0h/0h/1h/0]cRCGWnoELVHTr9oZWz1TUp7jmdBJPS3Kx8UCaHvfpxL6KiVSHH1A,[eea23daf/352h/0h/0h/0h/0]cUxxbQ67tepsEn3AUKSCWAKrvLwP6MPk55DAptcqxqDiXN6yKkNW)#q42ue5dg"
+        res = wallet.importdescriptors([{
+            "desc": descriptor,
+            "active": True,
+            "next_index": 0,
+            "timestamp": "now"
+        }])
+        assert_equal(res[0]["success"], True)
+
+        self.def_wallet.sendtoaddress(wallet.getnewaddress(address_type="silent-payments"), 10)
+        self.generate(self.nodes[0], 1)
+        assert_equal(wallet.getbalance(), 10)
+
+        wallet.send({self.def_wallet.getnewaddress(): 5})
+        self.generate(self.nodes[0], 1)
+        assert_approx(wallet.getbalance(), 5, 0.0001)
+
         self.log.info("Check import rescan works for silent payments")
 
         self.nodes[0].createwallet(wallet_name="alice", silent_payments=True)
@@ -298,7 +320,7 @@ def run_test(self):
         self.test_encrypting_unencrypted()
         self.test_basic()
         self.test_wallet_persistence()
-        self.test_import_rescan()
+        self.test_import_descriptors()
         self.test_createwallet_descriptor()
         self.test_backup_and_restore()
         self.test_getaddressinfo()