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diff --git a/apps/theming/lib/ImageManager.php b/apps/theming/lib/ImageManager.php
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+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2016 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+namespace OCA\Theming;
+
+use OCA\Theming\AppInfo\Application;
+use OCA\Theming\Service\BackgroundService;
+use OCP\Files\IAppData;
+use OCP\Files\NotFoundException;
+use OCP\Files\NotPermittedException;
+use OCP\Files\SimpleFS\ISimpleFile;
+use OCP\Files\SimpleFS\ISimpleFolder;
+use OCP\ICacheFactory;
+use OCP\IConfig;
+use OCP\ITempManager;
+use OCP\IURLGenerator;
+use Psr\Log\LoggerInterface;
+
+class ImageManager {
+	public const SUPPORTED_IMAGE_KEYS = ['background', 'logo', 'logoheader', 'favicon'];
+
+	public function __construct(
+		private IConfig $config,
+		private IAppData $appData,
+		private IURLGenerator $urlGenerator,
+		private ICacheFactory $cacheFactory,
+		private LoggerInterface $logger,
+		private ITempManager $tempManager,
+		private BackgroundService $backgroundService,
+	) {
+	}
+
+	/**
+	 * Get a globally defined image (admin theming settings)
+	 *
+	 * @param string $key the image key
+	 * @return string the image url
+	 */
+	public function getImageUrl(string $key): string {
+		$cacheBusterCounter = $this->config->getAppValue(Application::APP_ID, 'cachebuster', '0');
+		if ($this->hasImage($key)) {
+			return $this->urlGenerator->linkToRoute('theming.Theming.getImage', [ 'key' => $key ]) . '?v=' . $cacheBusterCounter;
+		} elseif ($key === 'backgroundDark' && $this->hasImage('background')) {
+			// Fall back to light variant
+			return $this->urlGenerator->linkToRoute('theming.Theming.getImage', [ 'key' => 'background' ]) . '?v=' . $cacheBusterCounter;
+		}
+
+		switch ($key) {
+			case 'logo':
+			case 'logoheader':
+			case 'favicon':
+				return $this->urlGenerator->imagePath('core', 'logo/logo.png') . '?v=' . $cacheBusterCounter;
+			case 'backgroundDark':
+			case 'background':
+				// Removing the background defines its mime as 'backgroundColor'
+				$mimeSetting = $this->config->getAppValue('theming', 'backgroundMime', '');
+				if ($mimeSetting !== 'backgroundColor') {
+					$image = BackgroundService::DEFAULT_BACKGROUND_IMAGE;
+					if ($key === 'backgroundDark') {
+						$image = BackgroundService::SHIPPED_BACKGROUNDS[$image]['dark_variant'] ?? $image;
+					}
+					return $this->urlGenerator->linkTo(Application::APP_ID, "img/background/$image");
+				}
+		}
+		return '';
+	}
+
+	/**
+	 * Get the absolute url. See getImageUrl
+	 */
+	public function getImageUrlAbsolute(string $key): string {
+		return $this->urlGenerator->getAbsoluteURL($this->getImageUrl($key));
+	}
+
+	/**
+	 * @param string $key
+	 * @param bool $useSvg
+	 * @return ISimpleFile
+	 * @throws NotFoundException
+	 * @throws NotPermittedException
+	 */
+	public function getImage(string $key, bool $useSvg = true): ISimpleFile {
+		$mime = $this->config->getAppValue('theming', $key . 'Mime', '');
+		$folder = $this->getRootFolder()->getFolder('images');
+
+		if ($mime === '' || !$folder->fileExists($key)) {
+			throw new NotFoundException();
+		}
+
+		if (!$useSvg && $this->shouldReplaceIcons()) {
+			if (!$folder->fileExists($key . '.png')) {
+				try {
+					$finalIconFile = new \Imagick();
+					$finalIconFile->setBackgroundColor('none');
+					$finalIconFile->readImageBlob($folder->getFile($key)->getContent());
+					$finalIconFile->setImageFormat('png32');
+					$pngFile = $folder->newFile($key . '.png');
+					$pngFile->putContent($finalIconFile->getImageBlob());
+					return $pngFile;
+				} catch (\ImagickException $e) {
+					$this->logger->info('The image was requested to be no SVG file, but converting it to PNG failed: ' . $e->getMessage());
+				}
+			} else {
+				return $folder->getFile($key . '.png');
+			}
+		}
+
+		return $folder->getFile($key);
+	}
+
+	public function hasImage(string $key): bool {
+		$mimeSetting = $this->config->getAppValue('theming', $key . 'Mime', '');
+		// Removing the background defines its mime as 'backgroundColor'
+		return $mimeSetting !== '' && $mimeSetting !== 'backgroundColor';
+	}
+
+	/**
+	 * @return array<string, array{mime: string, url: string}>
+	 */
+	public function getCustomImages(): array {
+		$images = [];
+		foreach (self::SUPPORTED_IMAGE_KEYS as $key) {
+			$images[$key] = [
+				'mime' => $this->config->getAppValue('theming', $key . 'Mime', ''),
+				'url' => $this->getImageUrl($key),
+			];
+		}
+		return $images;
+	}
+
+	/**
+	 * Get folder for current theming files
+	 *
+	 * @return ISimpleFolder
+	 * @throws NotPermittedException
+	 */
+	public function getCacheFolder(): ISimpleFolder {
+		$cacheBusterValue = $this->config->getAppValue('theming', 'cachebuster', '0');
+		try {
+			$folder = $this->getRootFolder()->getFolder($cacheBusterValue);
+		} catch (NotFoundException $e) {
+			$folder = $this->getRootFolder()->newFolder($cacheBusterValue);
+			$this->cleanup();
+		}
+		return $folder;
+	}
+
+	/**
+	 * Get a file from AppData
+	 *
+	 * @param string $filename
+	 * @throws NotFoundException
+	 * @return ISimpleFile
+	 * @throws NotPermittedException
+	 */
+	public function getCachedImage(string $filename): ISimpleFile {
+		$currentFolder = $this->getCacheFolder();
+		return $currentFolder->getFile($filename);
+	}
+
+	/**
+	 * Store a file for theming in AppData
+	 *
+	 * @param string $filename
+	 * @param string $data
+	 * @return ISimpleFile
+	 * @throws NotFoundException
+	 * @throws NotPermittedException
+	 */
+	public function setCachedImage(string $filename, string $data): ISimpleFile {
+		$currentFolder = $this->getCacheFolder();
+		if ($currentFolder->fileExists($filename)) {
+			$file = $currentFolder->getFile($filename);
+		} else {
+			$file = $currentFolder->newFile($filename);
+		}
+		$file->putContent($data);
+		return $file;
+	}
+
+	public function delete(string $key): void {
+		/* ignore exceptions, since we don't want to fail hard if something goes wrong during cleanup */
+		try {
+			$file = $this->getRootFolder()->getFolder('images')->getFile($key);
+			$file->delete();
+		} catch (NotFoundException $e) {
+		} catch (NotPermittedException $e) {
+		}
+		try {
+			$file = $this->getRootFolder()->getFolder('images')->getFile($key . '.png');
+			$file->delete();
+		} catch (NotFoundException $e) {
+		} catch (NotPermittedException $e) {
+		}
+
+		if ($key === 'logo') {
+			$this->config->deleteAppValue('theming', 'logoDimensions');
+		}
+	}
+
+	public function updateImage(string $key, string $tmpFile): string {
+		$this->delete($key);
+
+		try {
+			$folder = $this->getRootFolder()->getFolder('images');
+		} catch (NotFoundException $e) {
+			$folder = $this->getRootFolder()->newFolder('images');
+		}
+
+		$target = $folder->newFile($key);
+		$supportedFormats = $this->getSupportedUploadImageFormats($key);
+		$detectedMimeType = mime_content_type($tmpFile);
+		if (!in_array($detectedMimeType, $supportedFormats, true)) {
+			throw new \Exception('Unsupported image type: ' . $detectedMimeType);
+		}
+
+		if ($key === 'background') {
+			if ($this->shouldOptimizeBackgroundImage($detectedMimeType, filesize($tmpFile))) {
+				try {
+					// Optimize the image since some people may upload images that will be
+					// either to big or are not progressive rendering.
+					$newImage = @imagecreatefromstring(file_get_contents($tmpFile));
+					if ($newImage === false) {
+						throw new \Exception('Could not read background image, possibly corrupted.');
+					}
+
+					// Preserve transparency
+					imagesavealpha($newImage, true);
+					imagealphablending($newImage, true);
+
+					$imageWidth = imagesx($newImage);
+					$imageHeight = imagesy($newImage);
+
+					/** @var int */
+					$newWidth = min(4096, $imageWidth);
+					$newHeight = intval($imageHeight / ($imageWidth / $newWidth));
+					$outputImage = imagescale($newImage, $newWidth, $newHeight);
+					if ($outputImage === false) {
+						throw new \Exception('Could not scale uploaded background image.');
+					}
+
+					$newTmpFile = $this->tempManager->getTemporaryFile();
+					imageinterlace($outputImage, true);
+					// Keep jpeg images encoded as jpeg
+					if (str_contains($detectedMimeType, 'image/jpeg')) {
+						if (!imagejpeg($outputImage, $newTmpFile, 90)) {
+							throw new \Exception('Could not recompress background image as JPEG');
+						}
+					} else {
+						if (!imagepng($outputImage, $newTmpFile, 8)) {
+							throw new \Exception('Could not recompress background image as PNG');
+						}
+					}
+					$tmpFile = $newTmpFile;
+					imagedestroy($outputImage);
+				} catch (\Exception $e) {
+					if (isset($outputImage) && is_resource($outputImage) || $outputImage instanceof \GdImage) {
+						imagedestroy($outputImage);
+					}
+
+					$this->logger->debug($e->getMessage());
+				}
+			}
+
+			// For background images we need to announce it
+			$this->backgroundService->setGlobalBackground($tmpFile);
+		}
+
+		$target->putContent(file_get_contents($tmpFile));
+
+		if ($key === 'logo') {
+			$content = file_get_contents($tmpFile);
+			$newImage = @imagecreatefromstring($content);
+			if ($newImage !== false) {
+				$this->config->setAppValue('theming', 'logoDimensions', imagesx($newImage) . 'x' . imagesy($newImage));
+			} elseif (str_starts_with($detectedMimeType, 'image/svg')) {
+				$matched = preg_match('/viewbox=["\']\d* \d* (\d*\.?\d*) (\d*\.?\d*)["\']/i', $content, $matches);
+				if ($matched) {
+					$this->config->setAppValue('theming', 'logoDimensions', $matches[1] . 'x' . $matches[2]);
+				} else {
+					$this->logger->warning('Could not read logo image dimensions to optimize for mail header');
+					$this->config->deleteAppValue('theming', 'logoDimensions');
+				}
+			} else {
+				$this->logger->warning('Could not read logo image dimensions to optimize for mail header');
+				$this->config->deleteAppValue('theming', 'logoDimensions');
+			}
+		}
+
+		return $detectedMimeType;
+	}
+
+	/**
+	 * Decide whether an image benefits from shrinking and reconverting
+	 *
+	 * @param string $mimeType the mime type of the image
+	 * @param int $contentSize size of the image file
+	 * @return bool
+	 */
+	private function shouldOptimizeBackgroundImage(string $mimeType, int $contentSize): bool {
+		// Do not touch SVGs
+		if (str_contains($mimeType, 'image/svg')) {
+			return false;
+		}
+		// GIF does not benefit from converting
+		if (str_contains($mimeType, 'image/gif')) {
+			return false;
+		}
+		// WebP also does not benefit from converting
+		// We could possibly try to convert to progressive image, but normally webP images are quite small
+		if (str_contains($mimeType, 'image/webp')) {
+			return false;
+		}
+		// As a rule of thumb background images should be max. 150-300 KiB, small images do not benefit from converting
+		return $contentSize > 150000;
+	}
+
+	/**
+	 * Returns a list of supported mime types for image uploads.
+	 * "favicon" images are only allowed to be SVG when imagemagick with SVG support is available.
+	 *
+	 * @param string $key The image key, e.g. "favicon"
+	 * @return string[]
+	 */
+	public function getSupportedUploadImageFormats(string $key): array {
+		$supportedFormats = ['image/jpeg', 'image/png', 'image/gif', 'image/webp'];
+
+		if ($key !== 'favicon' || $this->shouldReplaceIcons() === true) {
+			$supportedFormats[] = 'image/svg+xml';
+			$supportedFormats[] = 'image/svg';
+		}
+
+		if ($key === 'favicon') {
+			$supportedFormats[] = 'image/x-icon';
+			$supportedFormats[] = 'image/vnd.microsoft.icon';
+		}
+
+		return $supportedFormats;
+	}
+
+	/**
+	 * remove cached files that are not required any longer
+	 *
+	 * @throws NotPermittedException
+	 * @throws NotFoundException
+	 */
+	public function cleanup() {
+		$currentFolder = $this->getCacheFolder();
+		$folders = $this->getRootFolder()->getDirectoryListing();
+		foreach ($folders as $folder) {
+			if ($folder->getName() !== 'images' && $folder->getName() !== $currentFolder->getName()) {
+				$folder->delete();
+			}
+		}
+	}
+
+	/**
+	 * Check if Imagemagick is enabled and if SVG is supported
+	 * otherwise we can't render custom icons
+	 *
+	 * @return bool
+	 */
+	public function shouldReplaceIcons() {
+		$cache = $this->cacheFactory->createDistributed('theming-' . $this->urlGenerator->getBaseUrl());
+		if ($value = $cache->get('shouldReplaceIcons')) {
+			return (bool)$value;
+		}
+		$value = false;
+		if (extension_loaded('imagick')) {
+			if (count(\Imagick::queryFormats('SVG')) >= 1) {
+				$value = true;
+			}
+		}
+		$cache->set('shouldReplaceIcons', $value);
+		return $value;
+	}
+
+	private function getRootFolder(): ISimpleFolder {
+		try {
+			return $this->appData->getFolder('global');
+		} catch (NotFoundException $e) {
+			return $this->appData->newFolder('global');
+		}
+	}
+}