Backend: an existing folder that cannot be listed is an error, not empty

Read as empty, a catalog/ or index/ folder would make an existing repository look new.

lib/Backend/LocalBackend.php

@@ -114,7 +114,10 @@ final class LocalBackend implements IBackend {
 		}
 		$result = [];
 		$names = scandir($base);
-		foreach ($names === false ? [] : $names as $name) {
+		if ($names === false) {
+			throw new BackendException('Cannot list ' . $dir);
+		}
+		foreach ($names as $name) {
 			if ($name !== '.' && $name !== '..' && !StorageBackend::isTemporary($name)) {
 				$result[] = trim($dir, '/') . '/' . $name;
 			}

lib/Backend/StorageBackend.php

@@ -140,7 +140,8 @@ final class StorageBackend implements IBackend {
 		}
 		$dh = $this->storage->opendir($abs);
 		if ($dh === false) {
-			return [];
+			// An existing folder read as empty would look like a new repository.
+			throw new BackendException('Cannot list ' . $dir);
 		}
 		$cut = $this->base === '' ? 0 : strlen($this->base) + 1;
 		$result = [];

tests/Unit/Backend/StorageBackendListTest.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace OCA\NgBackup\Tests\Unit\Backend;
 
+use OCA\NgBackup\Backend\BackendException;
 use OCA\NgBackup\Backend\StorageBackend;
 use OCA\NgBackup\Tests\Unit\TempDirTrait;
 use OCP\Files\Storage\IStorage;
@@ -44,4 +45,13 @@ class StorageBackendListTest extends TestCase {
 		$storage->expects($this->never())->method('opendir');
 		$this->assertSame([], (new StorageBackend($storage, 'repo'))->listFiles('snapshots'));
 	}
+
+	public function testAnUnreadableFolderIsAnErrorNotEmpty(): void {
+		$storage = $this->createMock(IStorage::class);
+		$storage->method('instanceOfStorage')->willReturn(false);
+		$storage->method('is_dir')->willReturn(true);
+		$storage->method('opendir')->willReturn(false);
+		$this->expectException(BackendException::class);
+		(new StorageBackend($storage, 'repo'))->listFiles('catalog');
+	}
 }