Repository: list flat folders without a type check per entry

Opening a repository lists index/, which holds one file per pack.
StorageBackend::list() checked every entry with is_dir(), and on a
remote storage each check is a round trip. Over SFTP to a Hetzner Storage
Box (33 ms) with a 60 GiB repository, occ backup:run (30 s steps) spent
nearly every step on these requests and uploaded 45 KiB/s; with this
change it uploads 5.8 MB/s. The cost grows with the number of packs.

index/, snapshots/, catalog/ and trash/info/ only ever hold objects:
list them with the new IBackend::listFiles(), one listing without
per-entry checks.

tests/Unit/Backend/StorageBackendListTest.php

@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OCA\NgBackup\Tests\Unit\Backend;
+
+use OCA\NgBackup\Backend\StorageBackend;
+use OCA\NgBackup\Tests\Unit\TempDirTrait;
+use OCP\Files\Storage\IStorage;
+use PHPUnit\Framework\TestCase;
+
+/** On a remote storage every is_dir() is a round trip: listing index/ must not cost one per pack. */
+class StorageBackendListTest extends TestCase {
+	use TempDirTrait;
+
+	public function testListFilesChecksOnlyTheFolderItself(): void {
+		$dir = $this->tempDir();
+		foreach (['b1', 'a2', 'c3.part-0123abcd'] as $name) {
+			touch("$dir/$name");
+		}
+		$isDirCalls = 0;
+		$storage = $this->createMock(IStorage::class);
+		$storage->method('instanceOfStorage')->willReturn(false);
+		$storage->method('is_dir')->willReturnCallback(function (string $path) use (&$isDirCalls): bool {
+			$isDirCalls++;
+			return $path === 'repo' || $path === 'repo/index';
+		});
+		$storage->method('opendir')->willReturnCallback(fn (string $path) => $path === 'repo/index' ? opendir($dir) : false);
+
+		$backend = new StorageBackend($storage, 'repo');
+		$isDirCalls = 0; // the constructor checks the base folder
+		$this->assertSame(['index/a2', 'index/b1'], $backend->listFiles('index'));
+		$this->assertSame(1, $isDirCalls);
+
+		$isDirCalls = 0;
+		$this->assertSame(['index/a2', 'index/b1'], $backend->list('index'));
+		$this->assertSame(4, $isDirCalls, 'list() checks every entry: the reason listFiles() exists');
+	}
+
+	public function testListFilesOfAMissingFolderIsEmpty(): void {
+		$storage = $this->createMock(IStorage::class);
+		$storage->method('instanceOfStorage')->willReturn(false);
+		$storage->method('is_dir')->willReturnCallback(fn (string $path): bool => $path === 'repo');
+		$storage->expects($this->never())->method('opendir');
+		$this->assertSame([], (new StorageBackend($storage, 'repo'))->listFiles('snapshots'));
+	}
+}

tests/Unit/Repository/VerifyTest.php

@@ -166,6 +166,10 @@ final class FlakyRangeBackend implements IBackend {
 		return $this->inner->list($prefix);
 	}
 
+	public function listFiles(string $dir): array {
+		return $this->inner->listFiles($dir);
+	}
+
 	public function delete(string $path): void {
 		$this->inner->delete($path);
 	}

tests/bootstrap.php

@@ -3,3 +3,13 @@
 declare(strict_types=1);
 
 require __DIR__ . '/../vendor/autoload.php';
+
+// nextcloud/ocp ships the public interfaces without an autoloader; tests that mock them need one.
+spl_autoload_register(static function (string $class): void {
+	if (str_starts_with($class, 'OCP\\')) {
+		$file = __DIR__ . '/../vendor/nextcloud/ocp/' . str_replace('\\', '/', $class) . '.php';
+		if (is_file($file)) {
+			require $file;
+		}
+	}
+});