Restore: a blob that does not decrypt as first read is read once more on a request of its own before the restore gives up

lib/Repository/Repository.php

@@ -895,8 +895,27 @@ final class Repository {
 			} catch (BackendException) {
 				throw $e;
 			}
+			$pack = $moved;
+		}
+		try {
+			return $this->decodeBlob($id, $encrypted);
+		} catch (\Exception $damaged) {
+			// Bytes that do not decrypt: damaged on the location, or a read that went wrong on the
+			// way (a stream that lost its place: an SMB restore stopped after 17 hours like this).
+			// Read exactly this blob once more, on a request of its own; only a second failure
+			// means the data itself is damaged.
+			$this->packReader->close();
+			try {
+				$again = $this->backend->getRange('packs/' . substr($pack, 0, 2) . '/' . $pack, $offset, $length);
+			} catch (\Exception) {
+				throw $damaged;
+			}
+			$data = $this->decodeBlob($id, $again);
+			if (function_exists('OCP\Log\logger')) {
+				\OCP\Log\logger('ng_backup')->warning('NG Backup: blob ' . $id . ' did not decrypt as first read from pack ' . $pack . '; read again, it is fine');
+			}
+			return $data;
 		}
-		return $this->decodeBlob($id, $encrypted);
 	}
 
 	/** Decrypts and checks a blob read from its pack; throws when it is corrupted or tampered with. */

tests/Unit/Repository/DamagedReadTest.php

@@ -0,0 +1,116 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OCA\NgBackup\Tests\Unit\Repository;
+
+use OCA\NgBackup\Backend\IBackend;
+use OCA\NgBackup\Backend\LocalBackend;
+use OCA\NgBackup\Crypto\KeyRing;
+use OCA\NgBackup\Repository\Repository;
+use OCA\NgBackup\Tests\Unit\TempDirTrait;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * A pack stream that hands out wrong bytes once (an SMB restore stopped after 17 hours with
+ * "Decryption failed"): the blob is read once more on a request of its own; only data that is
+ * damaged on the location stops the restore.
+ */
+class DamagedReadTest extends TestCase {
+	use TempDirTrait;
+
+	private string $root;
+	private KeyRing $keys;
+	private string $src;
+	private string $snapshot;
+
+	protected function setUp(): void {
+		$this->root = $this->tempDir();
+		$this->keys = KeyRing::generate();
+		$this->src = $this->tempDir();
+		for ($i = 0; $i < 6; $i++) {
+			file_put_contents("$this->src/f$i", random_bytes(3 * 1048576));
+		}
+		$repo = Repository::initWithKey(new LocalBackend($this->root), $this->keys, $this->keys->wrap('pw'));
+		$this->snapshot = $repo->backupDirectory($this->src)['snapshot'];
+	}
+
+	public function testABlobThatCameInDamagedOnceIsReadAgain(): void {
+		$backend = new GarblingBackend(new LocalBackend($this->root), streams: 1);
+		$target = $this->tempDir();
+
+		$this->assertSame(6, Repository::openWithKey($backend, $this->keys)->restore($this->snapshot, $target));
+
+		$this->assertSame(self::hashTree($this->src), self::hashTree($target));
+		$this->assertSame(0, $backend->streams, 'the damaged read happened');
+	}
+
+	public function testDataDamagedOnTheLocationStillStopsTheRestore(): void {
+		$backend = new GarblingBackend(new LocalBackend($this->root), streams: 1000, ranges: 1000);
+
+		$this->expectException(\Exception::class);
+		Repository::openWithKey($backend, $this->keys)->restore($this->snapshot, $this->tempDir());
+	}
+}
+
+/** Pack reads that flip a byte: the first $streams pack streams and $ranges range reads. */
+final class GarblingBackend implements IBackend {
+	use \OCA\NgBackup\Backend\GetsOneByOne;
+
+	public function __construct(
+		private IBackend $inner,
+		public int $streams = 0,
+		public int $ranges = 0,
+	) {
+	}
+
+	public function put(string $path, $stream): void {
+		$this->inner->put($path, $stream);
+	}
+
+	public function get(string $path) {
+		$fh = $this->inner->get($path);
+		if (!str_starts_with($path, 'packs/') || $this->streams <= 0) {
+			return $fh;
+		}
+		$this->streams--;
+		$data = stream_get_contents($fh);
+		fclose($fh);
+		for ($k = 100; $k < strlen($data); $k += 65536) {
+			$data[$k] = chr(ord($data[$k]) ^ 0x55);
+		}
+		$out = fopen('php://temp', 'w+b');
+		fwrite($out, $data);
+		rewind($out);
+		return $out;
+	}
+
+	public function getRange(string $path, int $offset, int $length): string {
+		$data = $this->inner->getRange($path, $offset, $length);
+		if (str_starts_with($path, 'packs/') && $this->ranges > 0) {
+			$this->ranges--;
+			$data[0] = chr(ord($data[0]) ^ 0x55);
+		}
+		return $data;
+	}
+
+	public function exists(string $path): bool {
+		return $this->inner->exists($path);
+	}
+
+	public function move(string $from, string $to): void {
+		$this->inner->move($from, $to);
+	}
+
+	public function list(string $prefix): array {
+		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);
+	}
+}