Create asset pipeline cache directory on demand. (#3525)

module/VuFindTheme/src/VuFindTheme/View/Helper/ConcatTrait.php

@@ -227,7 +227,7 @@ protected function filterItems()
 
     /**
      * Get the path to the directory where we can cache files generated by
-     * this trait.
+     * this trait. The directory will be created if it does not already exist.
      *
      * @return string
      */
@@ -238,7 +238,14 @@ protected function getResourceCacheDir()
                 'Asset pipeline feature depends on the LOCAL_CACHE_DIR constant.'
             );
         }
-        return LOCAL_CACHE_DIR . '/public/';
+        // TODO: it might be better to use \VuFind\Cache\Manager here.
+        $cacheDir = LOCAL_CACHE_DIR . '/public/';
+        if (!is_dir($cacheDir) && !file_exists($cacheDir)) {
+            if (!mkdir($cacheDir)) {
+                throw new \Exception("Unexpected problem creating cache directory: $cacheDir");
+            }
+        }
+        return $cacheDir;
     }
 
     /**