Handles Azure blob storage failure for large files

- The change updates the Azure Blob storage file system to use standard readfile() simplifying the code and aligning to latest PHP capabilities
- get_remote_path_from_storedfile() function calls is_file_readable_locally_by_storedfile() hence calling it again is not needed
- This allows site admin to control whether it should check for local or external first
- The readfile() function in PHP is memory-efficient for large files since 2016 and readfile_allow_large might not be needed

Author: sumaiyamannan

classes/azure_blob_storage_file_system.php

@@ -30,4 +30,21 @@
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 class azure_blob_storage_file_system extends file_system {
+
+     /**
+     * Output the content of the specified stored file.
+     *
+     * Note, this is different to get_content() as it uses the built-in php
+     * readfile function which is more efficient.
+     *
+     * @param stored_file $file The file to serve.
+     * @return void
+     */
+    public function readfile(\stored_file $file) {
+        $path = $this->get_remote_path_from_storedfile($file);
+        if (readfile($path) === false) {
+            throw new \file_exception('storedfilecannotreadfile', $file->get_filename());
+        }
+    }
+
 }