MDL-39846 prevent adding of snapshots after event trigger

lib/classes/event/base.php

@@ -508,10 +508,16 @@ abstract class base implements \IteratorAggregate {
      *
      * @param string $tablename
      * @param \stdClass $record
+     *
+     * @throws \coding_exception if used after ::trigger()
      */
     public function add_record_snapshot($tablename, $record) {
         global $DB;
 
+        if ($this->triggered) {
+            throw new \coding_exception('It is not possible to add snapshots after triggering of events');
+        }
+
         // NOTE: this might use some kind of MUC cache,
         //       hopefully we will not run out of memory here...
         if (debugging('', DEBUG_DEVELOPER)) { // This should be replaced by new $CFG->slowdebug flag if introduced.

lib/tests/event_test.php

@@ -639,6 +639,15 @@ class core_event_testcase extends advanced_testcase {
         $user = $event->get_record_snapshot('user', 1);
         $this->assertEquals(1, $user->id);
         $this->assertSame('guest', $user->username);
+
+        $event->add_record_snapshot('course', $course1);
+        $event->trigger();
+        try {
+            $event->add_record_snapshot('course', $course1);
+            $this->fail('Updating of snapshots after trigger is not ok');;
+        } catch (\moodle_exception $e) {
+            $this->assertInstanceOf('\coding_exception', $e);
+        }
     }
 
     public function test_iteration() {