MDL-85323 core: Don't cache the login page

lib/weblib.php

@@ -1564,8 +1564,7 @@ function send_headers($contenttype, $cacheable = true) {
         @header('Expires: ');
     } else {
         // Do everything we can to always prevent clients and proxies caching.
-        @header('Cache-Control: no-store, no-cache, must-revalidate');
-        @header('Cache-Control: post-check=0, pre-check=0, no-transform', false);
+        @header('Cache-Control: no-store, no-cache, must-revalidate, no-transform');
         @header('Pragma: no-cache');
         @header('Expires: Mon, 20 Aug 1969 09:23:00 GMT');
         @header('Last-Modified: ' . gmdate('D, d M Y H:i:s') . ' GMT');

login/index.php

@@ -50,6 +50,7 @@ $context = context_system::instance();
 $PAGE->set_url("$CFG->wwwroot/login/index.php");
 $PAGE->set_context($context);
 $PAGE->set_pagelayout('login');
+$PAGE->set_cacheable(false);
 
 /// Initialize variables
 $errormsg = '';