Changes to utility for adding a label to a superevent

We use get_or_create now to add labels to a superevent in order
to prevent IntegrityErrors when a user makes the same request
too rapidly and both sneak by the initial check.

gracedb/superevents/utils.py

@@ -112,7 +112,7 @@ def create_superevent(submitter, t_start, t_0, t_end, preferred_event,
 
     # Add labels
     for label in labels:
-        l = add_label_to_superevent(s, label, submitter,
+        labelling, _ = add_label_to_superevent(s, label, submitter,
             add_log_message=True, issue_alert=issue_alert)
 
     # Create superevent data directory
@@ -415,20 +415,32 @@ def remove_event_from_superevent(superevent, event, user, add_event_log=True,
 def add_label_to_superevent(superevent, label, user, add_log_message=True,
     issue_alert=True):
 
-    # Create Labelling object
-    labelling = Labelling.objects.create(label=label, creator=user,
-        superevent=superevent)
-
+    # Create Labelling object - we use get_or_create due to the possibility
+    # of IntegrityErrors. Sometimes two requests can get through the
+    # before the first one is actually applied so get_or_create allows
+    # us to safeguard against that.
+    labelling, created = Labelling.objects.get_or_create(label=label,
+        creator=user, superevent=superevent)
+
+    # If the label is actually created (i.e. this isn't a secondary request
+    # that squeaked through), then we optionally create a log message and
+    # issue an alert.  If it's *not* created, then that stuff should have been
+    # done already on a separate request and we don't need to repeat it here.
+    # This is not ideal because users will get a response like they created
+    # the label on that request but I don't think that is a significant
+    # problem. This should be a rare occurrence and is basically here to
+    # prevent server errors when users are doing dumb things.
     log_for_label_addition = None
-    if add_log_message:
-        # Record label addition in superevent logs
-        comment = 'Added label: {label_name}'.format(label_name=label.name)
-        log_for_label_addition = create_log(user, comment, superevent,
-            issue_alert=False, autogenerated=True)
-
-    if issue_alert:
-        SupereventLabelAlertIssuer(labelling, alert_type='label_added') \
-            .issue_alerts()
+    if created:
+        if add_log_message:
+            # Record label addition in superevent logs
+            comment = 'Added label: {label_name}'.format(label_name=label.name)
+            log_for_label_addition = create_log(user, comment, superevent,
+                issue_alert=False, autogenerated=True)
+
+        if issue_alert:
+            SupereventLabelAlertIssuer(labelling, alert_type='label_added') \
+                .issue_alerts()
 
     return labelling, log_for_label_addition