wallet: Fix migration of wallets with pathnames.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>

src/wallet/wallet.cpp

@@ -4229,8 +4229,19 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>
         return util::Error{_("Error: This wallet is already a descriptor wallet")};
     }
 
-    // Make a backup of the DB
+    // Make a backup of the DB in the wallet's directory with a unique filename
-    fs::path backup_filename = fs::PathFromString(strprintf("%s_%d.legacy.bak", (wallet_name.empty() ? "default_wallet" : wallet_name), GetTime()));
+    // using the wallet name and current timestamp. The backup filename is based
+    // on the name of the parent directory containing the wallet data in most
+    // 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" : [&] {
+        // 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());
+    }();
+
+    fs::path backup_filename = fs::PathFromString(strprintf("%s_%d.legacy.bak", backup_prefix, GetTime()));
     fs::path backup_path = fsbridge::AbsPathJoin(GetWalletDir(), backup_filename);
     if (!local_wallet->BackupWallet(fs::PathToString(backup_path))) {
         return util::Error{_("Error: Unable to make a backup of your wallet")};