Pre-beta review: safer destructive commands and hardening

- backup:restore:full refuses on an installation with more than one user
  and asks before it starts (--force skips both, also to rerun after a
  failure halfway), and warns when the snapshot's database dump was not
  one consistent transaction.
- backup:user:restore --mode=replace asks before deleting an existing
  account; non-interactive use needs --force.
- The recovery kit file is created owner-only and never over an existing
  file or symlink; backup:key:import shows the deletion delay it read from
  the kit.
- Adding a location through the web API needs password confirmation, like
  removing one.
- Key derivation parameters from a kit or repository config are bounded,
  and a stalled network stream no longer spins at full CPU.
- Floating-point columns are dumped exactly instead of with 14 digits.

lib/Command/RestoreUser.php

@@ -10,7 +10,10 @@ namespace OCA\NgBackup\Command;
 use OCA\NgBackup\Service\TargetService;
 use OCA\NgBackup\Service\UserRestoreService;
 use OCA\NgBackup\UserMigration\UserMigrationInstaller;
+use OCP\IUserManager;
 use Symfony\Component\Console\Command\Command;
+use Symfony\Component\Console\Helper\QuestionHelper;
+use Symfony\Component\Console\Question\ConfirmationQuestion;
 use Symfony\Component\Console\Input\InputArgument;
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Input\InputOption;
@@ -21,6 +24,7 @@ final class RestoreUser extends Command {
 		private TargetService $targets,
 		private UserRestoreService $userRestore,
 		private UserMigrationInstaller $installer,
+		private IUserManager $userManager,
 	) {
 		parent::__construct();
 	}
@@ -31,6 +35,7 @@ final class RestoreUser extends Command {
 			->addArgument('target', InputArgument::REQUIRED, 'Location name or id')
 			->addArgument('manifest', InputArgument::REQUIRED, 'Manifest path printed by backup:user:backup (users/<uid>/<exportId>)')
 			->addOption('mode', 'm', InputOption::VALUE_REQUIRED, 'replace or as-backup', UserRestoreService::MODE_REPLACE)
+			->addOption('force', null, InputOption::VALUE_NONE, 'Replace an existing account without asking (required when not interactive)')
 			->addOption('install-user-migration', null, InputOption::VALUE_NONE, 'Install and enable user_migration if it is missing (non-interactive consent)')
 			->setHelp(<<<'HELP'
 Never imports over a live account. Modes:
@@ -47,11 +52,26 @@ HELP);
 	}
 
 	protected function execute(InputInterface $input, OutputInterface $output): int {
+		$mode = (string)$input->getOption('mode');
+		$uid = explode('/', (string)$input->getArgument('manifest'))[1] ?? '';
+		if ($mode === UserRestoreService::MODE_REPLACE && $uid !== '' && $this->userManager->userExists($uid) && !$input->getOption('force')) {
+			// Replace deletes the live account (after a safety export). Shares to others and data of
+			// apps without a user_migration migrator do not come back, so never do it unasked.
+			if (!$input->isInteractive()) {
+				$output->writeln("<error>$uid exists: --mode=replace deletes and recreates it. Add --force, or use --mode=as-backup.</error>");
+				return 1;
+			}
+			$q = new ConfirmationQuestion("$uid exists and will be deleted and recreated from the backup (a safety export is made first; its shares to others and data of apps without a migrator are not restored). Continue? [y/N] ", false);
+			if (!(new QuestionHelper())->ask($input, $output, $q)) {
+				$output->writeln('Cancelled.');
+				return 1;
+			}
+		}
 		try {
 			$output->writeln('<comment>Note: on restore, user_migration only imports the app settings on its own allowlist (e.g. calendar view and reminder settings); other app settings of this user are skipped. Reason: NG Backup uses user_migration\'s importer; widening the list would mean building a settings importer of our own.</comment>');
 			$target = $this->targets->get($input->getArgument('target'));
 			$this->installer->ensureEnabled($input, $output);
-			$user = $this->userRestore->restoreUser($target, (string)$input->getArgument('manifest'), (string)$input->getOption('mode'));
+			$user = $this->userRestore->restoreUser($target, (string)$input->getArgument('manifest'), $mode);
 		} catch (\Throwable $e) {
 			$output->writeln('<error>' . $e->getMessage() . '</error>');
 			return 1;