List preview directory only once

getCachedPreview used to call `getFile`, and this calls `getDirectoryListing` (or underlying function that list directory) to find the file. This was done for every preview spec.
Now, this is done only once at the beginning of the loop, and the array is just iterated when needed to find the correct entry.

Signed-off-by: Glandos <bugs-github@antipoul.fr>

Author: Glandos

lib/private/Preview/Generator.php

@@ -171,6 +171,8 @@ public function generatePreviews(File $file, array $specifications, $mimeType =
 		[$maxWidth, $maxHeight] = $this->getPreviewSize($maxPreview, $previewVersion);
 
 		$preview = null;
+		// List every existing preview first instead of trying to find them one by one
+		$previewFiles = $previewFolder->getDirectoryListing();
 
 		foreach ($specifications as $specification) {
 			$width = $specification['width'] ?? -1;
@@ -197,7 +199,7 @@ public function generatePreviews(File $file, array $specifications, $mimeType =
 			// Try to get a cached preview. Else generate (and store) one
 			try {
 				try {
-					$preview = $this->getCachedPreview($previewFolder, $width, $height, $crop, $maxPreview->getMimeType(), $previewVersion);
+					$preview = $this->getCachedPreview($previewFiles, $width, $height, $crop, $maxPreview->getMimeType(), $previewVersion);
 				} catch (NotFoundException $e) {
 					if (!$this->previewManager->isMimeSupported($mimeType)) {
 						throw new NotFoundException();
@@ -208,6 +210,8 @@ public function generatePreviews(File $file, array $specifications, $mimeType =
 					}
 
 					$preview = $this->generatePreview($previewFolder, $maxPreviewImage, $width, $height, $crop, $maxWidth, $maxHeight, $previewVersion);
+					// New file, augment our array
+					$previewFiles[] = $preview;
 				}
 			} catch (\InvalidArgumentException $e) {
 				throw new NotFoundException("", 0, $e);
@@ -649,7 +653,7 @@ private function generatePreview(ISimpleFolder $previewFolder, IImage $maxPrevie
 	}
 
 	/**
-	 * @param ISimpleFolder $previewFolder
+	 * @param array $files Array of FileInfo, as the result of getDirectoryListing()
 	 * @param int $width
 	 * @param int $height
 	 * @param bool $crop
@@ -659,10 +663,14 @@ private function generatePreview(ISimpleFolder $previewFolder, IImage $maxPrevie
 	 *
 	 * @throws NotFoundException
 	 */
-	private function getCachedPreview(ISimpleFolder $previewFolder, $width, $height, $crop, $mimeType, $prefix) {
+	private function getCachedPreview($files, $width, $height, $crop, $mimeType, $prefix) {
 		$path = $this->generatePath($width, $height, $crop, $mimeType, $prefix);
-
-		return $previewFolder->getFile($path);
+		foreach($files as $file) {
+			if ($file->getName() === $path) {
+				return $file;
+			}
+		}
+		throw new NotFoundException();
 	}
 
 	/**