Merge bitcoin/bitcoin#26740: wallet: Migrate wallets that are not in a wallet dir

a1e653828b test: Add test for migrating default wallet and plain file wallet (Andrew Chow)
bdbe3fd76b wallet: Generated migrated wallet's path from walletdir and name (Andrew Chow)

Pull request description:

  This PR fixes an assertion error that is hit during the setup of the new database during migration of a wallet that was not contained in a wallet dir. Also added a test for this case as well as one for migrating the default wallet.

ACKs for top commit:
  ryanofsky:
    Code review ACK a1e653828b
  furszy:
    ACK a1e65382

Tree-SHA512: 96b218c0de8567d8650ec96e1bf58b0f8ca4c4726f5efc6362453979b56b9d569baea0bb09befb3a5aed8d16d29bf75ed5cd8ffc432bbd4cbcad3ac5574bc479

test/functional/wallet_migration.py

@@ -6,6 +6,7 @@
 
 import os
 import random
+import shutil
 from test_framework.descriptors import descsum_create
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
@@ -470,6 +471,40 @@ class WalletMigrationTest(BitcoinTestFramework):
 
         assert_equal(bals, wallet.getbalances())
 
+    def test_default_wallet(self):
+        self.log.info("Test migration of the wallet named as the empty string")
+        wallet = self.create_legacy_wallet("")
+
+        wallet.migratewallet()
+        info = wallet.getwalletinfo()
+        assert_equal(info["descriptors"], True)
+        assert_equal(info["format"], "sqlite")
+
+    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")
+        wallet.unloadwallet()
+
+        wallets_dir = os.path.join(self.nodes[0].datadir, "regtest", "wallets")
+        wallet_path = os.path.join(wallets_dir, "plainfile")
+        wallet_dat_path = os.path.join(wallet_path, "wallet.dat")
+        shutil.copyfile(wallet_dat_path, os.path.join(wallets_dir, "plainfile.bak"))
+        shutil.rmtree(wallet_path)
+        shutil.move(os.path.join(wallets_dir, "plainfile.bak"), wallet_path)
+
+        self.nodes[0].loadwallet("plainfile")
+        info = wallet.getwalletinfo()
+        assert_equal(info["descriptors"], False)
+        assert_equal(info["format"], "bdb")
+
+        wallet.migratewallet()
+        info = wallet.getwalletinfo()
+        assert_equal(info["descriptors"], True)
+        assert_equal(info["format"], "sqlite")
+
+        assert os.path.isdir(wallet_path)
+        assert os.path.isfile(wallet_dat_path)
+
     def run_test(self):
         self.generate(self.nodes[0], 101)
 
@@ -482,6 +517,8 @@ class WalletMigrationTest(BitcoinTestFramework):
         self.test_encrypted()
         self.test_unloaded()
         self.test_unloaded_by_path()
+        self.test_default_wallet()
+        self.test_direct_file()
 
 if __name__ == '__main__':
     WalletMigrationTest().main()