Merge bitcoin/bitcoin#29586: wallet: default wallet migration, modify inconvenient backup filename

a951dba wallet: default wallet migration, modify inconvenient backup filename (furszy)

Pull request description:

  Fixes #29584

  On default legacy wallets, the backup filename starts with an "-" due to the wallet name being empty. This is inconvenient for systems who treat what follows the initial "-" character as flags.

  Note:
  As the user can freely set the wallet name to anything, we could also guard the backup filename against other inconvenient characters in the future (we need to be careful here, because the wallet name could also be a path).

ACKs for top commit:
  achow101:
    ACK a951dba
  brunoerg:
    utACK a951dba
  vasild:
    ACK  a951dba

Tree-SHA512: 6347bb12cfb50526a4baad96e4f1df9d82b493f79f0a0f7e0a1c8335a86a1e8e147c7b7f95cec6ede6f4507506a7eaf7972bd35131a2d5ed4cbbf38d94f0a9ca

Author: achow101

src/wallet/wallet.cpp

@@ -4336,7 +4336,7 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
 
     // Make a backup of the DB
     fs::path this_wallet_dir = fs::absolute(fs::PathFromString(local_wallet->GetDatabase().Filename())).parent_path();
-    fs::path backup_filename = fs::PathFromString(strprintf("%s-%d.legacy.bak", wallet_name, GetTime()));
+    fs::path backup_filename = fs::PathFromString(strprintf("%s_%d.legacy.bak", (wallet_name.empty() ? "default_wallet" : wallet_name), GetTime()));
     fs::path backup_path = this_wallet_dir / backup_filename;
     if (!local_wallet->BackupWallet(fs::PathToString(backup_path))) {
         if (was_loaded) {

test/functional/wallet_migration.py

@@ -529,11 +529,20 @@ def test_default_wallet(self):
         self.log.info("Test migration of the wallet named as the empty string")
         wallet = self.create_legacy_wallet("")
 
-        self.migrate_wallet(wallet)
+        # Set time to verify backup existence later
+        curr_time = int(time.time())
+        wallet.setmocktime(curr_time)
+
+        res = self.migrate_wallet(wallet)
         info = wallet.getwalletinfo()
         assert_equal(info["descriptors"], True)
         assert_equal(info["format"], "sqlite")
 
+        # Check backup existence and its non-empty wallet filename
+        backup_path = self.nodes[0].wallets_path / f'default_wallet_{curr_time}.legacy.bak'
+        assert backup_path.exists()
+        assert_equal(str(backup_path), res['backup_path'])
+
     def test_direct_file(self):
         self.log.info("Test migration of a wallet that is not in a wallet directory")
         wallet = self.create_legacy_wallet("plainfile")