fix the multisite update background-css dir clearance

inc/Engine/Common/Cache/CacheInterface.php

@@ -35,9 +35,11 @@ public function get_root_path(): string;
 	public function generate_path( string $url ): string;
 
 	/**
-	 * Wipes the whole cache directory .
+	 * Wipes the whole cache directory.
+	 *
+	 * @param array $preserve_dirs List of directories to be preserved.
 	 *
 	 * @return bool True on success and false on failure.
 	 */
-	public function full_clear(): bool;
+	public function full_clear( array $preserve_dirs = [] ): bool;
 }

inc/Engine/Common/Cache/FilesystemCache.php

@@ -106,14 +106,16 @@ public function clear() {
 	/**
 	 * Clear the whole background-css directory.
 	 *
+	 * @param array $preserve_dirs List of directories to be preserved.
+	 *
 	 * @return bool True on success and false on failure.
 	 */
-	public function full_clear(): bool {
+	public function full_clear( array $preserve_dirs = [] ): bool {
 		$base_path = $this->get_base_path();
 		if ( ! $this->filesystem->exists( $base_path ) ) {
 			return false;
 		}
-		rocket_rrmdir( $base_path, [], $this->filesystem );
+		rocket_rrmdir( $base_path, $preserve_dirs, $this->filesystem );
 		return true;
 	}
 

inc/Engine/Media/Lazyload/CSS/Admin/Subscriber.php

@@ -92,7 +92,10 @@ public function clear_background_css_with_upgrade( $new_version, $old_version )
 		if ( empty( $old_version ) || version_compare( $old_version, '3.18', '>' ) ) {
 			return;
 		}
+
+		$preserve_dirs = is_multisite() ? get_sites( [ 'fields' => 'ids' ] ) : [ get_current_blog_id() ];
+
 		// Completely clear background-css directory.
-		$this->cache->full_clear();
+		$this->cache->full_clear( $preserve_dirs );
 	}
 }