MDL-69779 core: Improved the X-Redirect-By header when debugging is on

lib/upgrade.txt

@@ -52,6 +52,7 @@ information provided here is intended especially for developers.
   renamed to `is_listed()` and `get_not_listed()` respectively.
 * Method `mustache_helper_collection::strip_blacklisted_helpers()` has been deprecated and renamed to
   `strip_disallowed_helpers()`.
+* Function redirect() now emits a line of backtrace into the X-Redirect-By header when debugging is one
 
 === 3.9 ===
 * Following function has been deprecated, please use \core\task\manager::run_from_cli().

lib/weblib.php

@@ -2954,9 +2954,17 @@ function redirect($url, $message='', $delay=null, $messagetype = \core\output\no
     \core\session\manager::write_close();
 
     if ($delay == 0 && !$debugdisableredirect && !headers_sent()) {
+
         // This helps when debugging redirect issues like loops and it is not clear
-        // which layer in the stack sent the redirect header.
-        @header('X-Redirect-By: Moodle');
+        // which layer in the stack sent the redirect header. If debugging is on
+        // then the file and line is also shown.
+        $redirectby = 'Moodle';
+        if (debugging('', DEBUG_DEVELOPER)) {
+            $origin = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 1)[0];
+            $redirectby .= ' /' . str_replace($CFG->dirroot . '/', '', $origin['file']) . ':' . $origin['line'];
+        }
+        @header("X-Redirect-By: $redirectby");
+
         // 302 might not work for POST requests, 303 is ignored by obsolete clients.
         @header($_SERVER['SERVER_PROTOCOL'] . ' 303 See Other');
         @header('Location: '.$url);