Never create a repository where backup data lies but no config could be read

CHANGELOG.md

@@ -23,6 +23,9 @@ stopped after 13 hours with "Blob not in index"; the backup itself was fine.
   backups record no ID counters, and they stayed at 1. Every counter of a restored table now goes
   after its data. Found in a disaster recovery simulation of 138 GB from MariaDB to PostgreSQL,
   where all 15,772 files and every count were identical.
+- Adding a location whose repository config could not be read, while backup data is there,
+  no longer creates a new repository over it: it stops with an explanation. A snapshot that
+  cannot be taken over when a location is added is now logged instead of left out silently.
 - An SFTP connection that a Hetzner Storage Box dropped without phpseclib noticing is now
   recognised and replaced: the check asks the server and counts its answer.
 - The extra connection a full restore reads ahead on is replaced when it breaks (up to 20 times),

lib/Repository/Repository.php

@@ -72,6 +72,14 @@ final class Repository {
 		if ($backend->exists('config')) {
 			throw new RepositoryException('A repository already exists at this location');
 		}
+		// No config, but backup data: the config is missing (or could not be seen just now, a
+		// connection that failed). A new repository here would write a config over the one that
+		// opens this data; refuse instead.
+		foreach (['index', 'snapshots'] as $dir) {
+			if ($backend->listFiles($dir) !== []) {
+				throw new RepositoryException("This location holds backup data ($dir/) but no repository config could be read: not creating a new repository over it. Check the connection and the folder; a missing config can be restored from the recovery kit.");
+			}
+		}
 		$id = bin2hex(random_bytes(16));
 		$config = ['format' => self::FORMAT, 'id' => $id, 'created' => gmdate('c'), 'key' => $wrappedKey,
 			'check' => base64_encode((new StreamCipher($keys))->encryptString($id, 'config-check'))];

tests/Unit/Repository/RepositoryTest.php

@@ -99,6 +99,20 @@ class RepositoryTest extends TestCase {
 		Repository::init($this->backend, 'pw');
 	}
 
+	public function testInitRefusesALocationWithBackupDataButNoConfig(): void {
+		$repo = Repository::init($this->backend, 'pw');
+		$repo->backupDirectory($this->src);
+		$this->backend->delete('config'); // missing, or not seen for a moment
+
+		try {
+			Repository::init($this->backend, 'pw');
+			$this->fail('a new repository over existing backup data');
+		} catch (RepositoryException $e) {
+			$this->assertStringContainsString('holds backup data', $e->getMessage());
+		}
+		$this->assertFalse($this->backend->exists('config'), 'no new config written');
+	}
+
 	public function testTamperedPackIsDetected(): void {
 		$repo = Repository::init($this->backend, 'pw');
 		$r = $repo->backupDirectory($this->src);