wallet: migration, fix watch-only and solvables wallets names

diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 987c2155..4db94020 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -4096,6 +4096,15 @@ bool CWallet::CanGrindR() const
     return !IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
 }
 
+// Returns wallet prefix for migration.
+// Used to name the backup file and newly created wallets.
+// E.g. a watch-only wallet is named "<prefix>_watchonly".
+static std::string MigrationPrefixName(CWallet& wallet)
+{
+    const std::string& name{wallet.GetName()};
+    return name.empty() ? "default_wallet" : name;
+}
+
 bool DoMigration(CWallet& wallet, WalletContext& context, bilingual_str& error, MigrationResult& res) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet)
 {
     AssertLockHeld(wallet.cs_wallet);
@@ -4127,7 +4136,7 @@ bool DoMigration(CWallet& wallet, WalletContext& context, bilingual_str& error,
 
             DatabaseStatus status;
             std::vector<bilingual_str> warnings;
-            std::string wallet_name = wallet.GetName() + "_watchonly";
+            std::string wallet_name = MigrationPrefixName(wallet) + "_watchonly";
             std::unique_ptr<WalletDatabase> database = MakeWalletDatabase(wallet_name, options, status, error);
             if (!database) {
                 error = strprintf(_("Wallet file creation failed: %s"), error);
@@ -4166,7 +4175,7 @@ bool DoMigration(CWallet& wallet, WalletContext& context, bilingual_str& error,
 
             DatabaseStatus status;
             std::vector<bilingual_str> warnings;
-            std::string wallet_name = wallet.GetName() + "_solvables";
+            std::string wallet_name = MigrationPrefixName(wallet) + "_solvables";
             std::unique_ptr<WalletDatabase> database = MakeWalletDatabase(wallet_name, options, status, error);
             if (!database) {
                 error = strprintf(_("Wallet file creation failed: %s"), error);
@@ -4281,7 +4290,7 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>
     // cases, but in the case where the wallet name is a path to a data file,
     // the name of the data file is used, and in the case where the wallet name
     // is blank, "default_wallet" is used.
-    const std::string backup_prefix = wallet_name.empty() ? "default_wallet" : [&] {
+    const std::string backup_prefix = wallet_name.empty() ? MigrationPrefixName(*local_wallet) : [&] {
         // fs::weakly_canonical resolves relative specifiers and remove trailing slashes.
         const auto legacy_wallet_path = fs::weakly_canonical(GetWalletDir() / fs::PathFromString(wallet_name));
         return fs::PathToString(legacy_wallet_path.filename());
diff --git a/test/functional/wallet_migration.py b/test/functional/wallet_migration.py
index 27caab8f..f802c186 100755
--- a/test/functional/wallet_migration.py
+++ b/test/functional/wallet_migration.py
@@ -3,6 +3,7 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test Migrating a wallet from legacy to descriptor."""
+from pathlib import Path
 import os.path
 import random
 import shutil
@@ -659,6 +660,14 @@ class WalletMigrationTest(BitcoinTestFramework):
 
         assert_equal(bals, wallet.getbalances())
 
+    def clear_default_wallet(self, backup_file):
+        # Test cleanup: Clear unnamed default wallet for subsequent tests
+        (self.old_node.wallets_path / "wallet.dat").unlink()
+        (self.master_node.wallets_path / "wallet.dat").unlink(missing_ok=True)
+        shutil.rmtree(self.master_node.wallets_path / "default_wallet_watchonly", ignore_errors=True)
+        shutil.rmtree(self.master_node.wallets_path / "default_wallet_solvables", ignore_errors=True)
+        backup_file.unlink()
+
     def test_default_wallet(self):
         self.log.info("Test migration of the wallet named as the empty string")
         wallet = self.create_legacy_wallet("")
@@ -676,6 +685,26 @@ class WalletMigrationTest(BitcoinTestFramework):
         assert os.path.basename(res["backup_path"]).startswith("default_wallet")
 
         wallet.unloadwallet()
+        self.clear_default_wallet(backup_file=Path(res["backup_path"]))
+
+    def test_default_wallet_watch_only(self):
+        self.log.info("Test unnamed (default) watch-only wallet migration")
+        master_wallet = self.master_node.get_wallet_rpc(self.default_wallet_name)
+        wallet = self.create_legacy_wallet("", blank=True)
+        wallet.importaddress(master_wallet.getnewaddress(address_type="legacy"))
+
+        res, wallet = self.migrate_and_get_rpc("")
+
+        info = wallet.getwalletinfo()
+        assert_equal(info["descriptors"], True)
+        assert_equal(info["format"], "sqlite")
+        assert_equal(info["private_keys_enabled"], False)
+        assert_equal(info["walletname"], "default_wallet_watchonly")
+        # Check the default wallet is not available anymore
+        assert not (self.master_node.wallets_path / "wallet.dat").exists()
+
+        wallet.unloadwallet()
+        self.clear_default_wallet(backup_file=Path(res["backup_path"]))
 
     def test_default_wallet_failure(self):
         self.log.info("Test failure during unnamed (default) wallet migration")
@@ -685,7 +714,7 @@ class WalletMigrationTest(BitcoinTestFramework):
 
         # Create wallet directory with the watch-only name and a wallet file.
         # Because the wallet dir exists, this will cause migration to fail.
-        watch_only_dir = self.master_node.wallets_path / "_watchonly"
+        watch_only_dir = self.master_node.wallets_path / "default_wallet_watchonly"
         os.mkdir(watch_only_dir)
         shutil.copyfile(self.old_node.wallets_path / "wallet.dat", watch_only_dir / "wallet.dat")
 
@@ -705,7 +734,7 @@ class WalletMigrationTest(BitcoinTestFramework):
         self.assert_is_bdb("")
 
         # Test cleanup: clear default wallet for next test
-        os.remove(self.old_node.wallets_path / "wallet.dat")
+        self.clear_default_wallet(backup_path)
 
     def test_direct_file(self):
         self.log.info("Test migration of a wallet that is not in a wallet directory")
@@ -1594,6 +1623,7 @@ class WalletMigrationTest(BitcoinTestFramework):
         self.test_wallet_with_path("path/that/ends/in/..")
         self.test_default_wallet_failure()
         self.test_default_wallet()
+        self.test_default_wallet_watch_only()
         self.test_direct_file()
         self.test_addressbook()
         self.test_migrate_raw_p2sh()