fix(preview): regenerate previews whose stored file is gone

When a preview row exists in the database but the backing file is
missing, preview:generate failed with NotFoundException from
LocalPreviewStorage::readPreview. Drop the stale row and generate a
new preview instead so occ preview:generate and preview:generate-all
can repair as they run.

Fixes: #63349

Assisted-by: Grok:grok-4.6
Signed-off-by: Ray Vincent <rayhvincent@gmail.com>

Author: rayvincent2

lib/private/Preview/Generator.php

@@ -171,6 +171,11 @@ public function generatePreviews(File $file, array $specifications, ?string $mim
 					&& $preview->getHeight() === $height && $preview->getMimetype() === $maxPreview->getMimetype()
 					&& $preview->getVersion() === $previewVersion && $preview->isCropped() === $crop);
 
+				if ($preview !== null && !$this->storageFactory->previewExists($preview)) {
+					$this->dropStalePreview($previews, $preview, $file);
+					$preview = null;
+				}
+
 				if ($preview) {
 					$previewFile = new PreviewFile($preview, $this->storageFactory, $this->previewMapper);
 				} else {
@@ -316,13 +321,23 @@ public function getNumConcurrentPreviews(string $type): int {
 	 * @param Preview[] $previews
 	 * @throws NotFoundException
 	 */
-	private function getMaxPreview(array $previews, File $file, string $mimeType, ?string $version): Preview {
+	private function getMaxPreview(array &$previews, File $file, string $mimeType, ?string $version): Preview {
 		// We don't know the max preview size, so we can't use getCachedPreview.
 		// It might have been generated with a higher resolution than the current value.
 		foreach ($previews as $preview) {
-			if ($preview->isMax() && ($version === $preview->getVersion())) {
+			if (!$preview->isMax() || $version !== $preview->getVersion()) {
+				continue;
+			}
+
+			if ($this->storageFactory->previewExists($preview)) {
 				return $preview;
 			}
+
+			// The row outlived its file. Everything below assumes the max preview
+			// can be read, so drop the row and generate a new one instead of
+			// failing on this and every later request. It has to go from the
+			// caller's list too, which is still searched for cached previews.
+			$this->dropStalePreview($previews, $preview, $file);
 		}
 
 		$maxWidth = $this->config->getSystemValueInt('preview_max_x', 4096);
@@ -335,7 +350,7 @@ private function getMaxPreview(array $previews, File $file, string $mimeType, ?s
 				// Fetch again, likely two HTTP requests for the same file were done around the same time
 				[$file->getId() => $previews] = $this->previewMapper->getAvailablePreviews([$file->getId()]);
 				foreach ($previews as $preview) {
-					if ($preview->isMax() && ($version === $preview->getVersion())) {
+					if ($preview->isMax() && ($version === $preview->getVersion()) && $this->storageFactory->previewExists($preview)) {
 						return $preview;
 					}
 				}
@@ -344,6 +359,18 @@ private function getMaxPreview(array $previews, File $file, string $mimeType, ?s
 		}
 	}
 
+	/**
+	 * @param Preview[] $previews
+	 */
+	private function dropStalePreview(array &$previews, Preview $preview, File $file): void {
+		$this->logger->warning('Preview of file {path} named {name} is missing from storage, regenerating it.', [
+			'path' => $file->getPath(),
+			'name' => $preview->getName(),
+		]);
+		$this->previewMapper->delete($preview);
+		$previews = array_filter($previews, fn (Preview $candidate): bool => $candidate !== $preview);
+	}
+
 	/**
 	 * @throws DBException
 	 * @throws NotFoundException

lib/private/Preview/Storage/IPreviewStorage.php

@@ -48,6 +48,15 @@ public function deletePreview(Preview $preview): void;
 	 */
 	public function deleteUnreferencedPreview(Preview $preview): void;
 
+	/**
+	 * Whether the stored data of a preview is still present.
+	 *
+	 * A row can outlive its file, for instance after a partial write or when
+	 * the data directory is restored from an older backup. Every consumer of a
+	 * preview assumes it can be read.
+	 */
+	public function previewExists(Preview $preview): bool;
+
 	/**
 	 * Migration helper
 	 *

lib/private/Preview/Storage/LocalPreviewStorage.php

@@ -76,6 +76,11 @@ public function deleteUnreferencedPreview(Preview $preview): void {
 		// race. Deleting it would leave that one with a row but no file.
 	}
 
+	#[Override]
+	public function previewExists(Preview $preview): bool {
+		return is_file($this->constructPath($preview));
+	}
+
 	public function getRootFolder(): string {
 		return $this->config->getSystemValueString('datadirectory', OC::$SERVERROOT . '/data');
 	}

lib/private/Preview/Storage/ObjectStorePreviewStorage.php

@@ -174,6 +174,24 @@ public function deleteUnreferencedPreview(Preview $preview): void {
 		$this->deletePreview($preview);
 	}
 
+	#[Override]
+	public function previewExists(Preview $preview): bool {
+		if ($preview->getLocationId() === null) {
+			// Without a location the bucket cannot be resolved, which is the case
+			// for rows written before a move to an object store and for the dummy
+			// previews of the unit tests. Never report those as missing, or they
+			// would be dropped on every request.
+			return true;
+		}
+
+		[
+			'urn' => $urn,
+			'store' => $store,
+		] = $this->getObjectStoreInfoForExistingPreview($preview);
+
+		return $store->objectExists($urn);
+	}
+
 	public function getUrn(Preview $preview, array $config): string {
 		if ($preview->getOldFileId()) {
 			return ($config['arguments']['objectPrefix'] ?? 'urn:oid:') . $preview->getOldFileId();

lib/private/Preview/Storage/StorageFactory.php

@@ -42,6 +42,11 @@ public function deleteUnreferencedPreview(Preview $preview): void {
 		$this->getBackend()->deleteUnreferencedPreview($preview);
 	}
 
+	#[Override]
+	public function previewExists(Preview $preview): bool {
+		return $this->getBackend()->previewExists($preview);
+	}
+
 	private function getBackend(): IPreviewStorage {
 		if ($this->backend) {
 			return $this->backend;