Speed up resource updates when there are no matching webhooks (#7553)

The signal handling code in `cvat.apps.webhooks.signals` always fetches
the old version of the resource from the database and serializes it. But
if no webhook is defined for the current resource, this work is
pointless.

Select the webhooks first, and only then (if necessary) serialize the
old resource. Also, move the diffing of the old and new versions to the
`post_save` signal handler, to avoid serializing the new version a
second time. This leads to a considerable speedup for resources with
complicated serializers, such as tasks and jobs, when no webhooks are
attached. In my experiments, saving of annotations was sped up by 20-25%
with this patch.

This patch also makes the signal handlers more hygienic with the use of
temporary attributes. Both the attributes it uses are now prefixed with
`_webhooks` to avoid collisions, and removed after they are used.

changelog.d/20240305_171241_roman_webhooks_lazy_rendering.md

@@ -0,0 +1,4 @@
+### Changed
+
+- Sped up resource updates when there are no matching webhooks
+  (<https://github.com/opencv/cvat/pull/7553>)

cvat/apps/webhooks/signals.py

@@ -136,25 +136,32 @@ def get_sender(instance):
 @receiver(pre_save, sender=Invitation, dispatch_uid=__name__ + ":invitation:pre_save")
 @receiver(pre_save, sender=Membership, dispatch_uid=__name__ + ":membership:pre_save")
 def pre_save_resource_event(sender, instance, **kwargs):
-    try:
-        old_instance = sender.objects.get(pk=instance.pk)
-    except ObjectDoesNotExist:
-        return
+    instance._webhooks_selected_webhooks = []
 
-    old_serializer = get_serializer(instance=old_instance)
-    serializer = get_serializer(instance=instance)
-    diff = get_instance_diff(old_data=old_serializer.data, data=serializer.data)
+    if instance.pk is None:
+        created = True
+    else:
+        try:
+            old_instance = sender.objects.get(pk=instance.pk)
+            created = False
+        except ObjectDoesNotExist:
+            created = True
 
-    if not diff:
-        return
+    resource_name = instance.__class__.__name__.lower()
 
-    before_update = {
-        attr: value["old_value"]
-        for attr, value in diff.items()
-    }
+    event_type = event_name("create" if created else "update", resource_name)
+    if event_type not in map(lambda a: a[0], EventTypeChoice.choices()):
+        return
 
-    instance._before_update = before_update
+    instance._webhooks_selected_webhooks = select_webhooks(instance, event_type)
+    if not instance._webhooks_selected_webhooks:
+        return
 
+    if created:
+        instance._webhooks_old_data = None
+    else:
+        old_serializer = get_serializer(instance=old_instance)
+        instance._webhooks_old_data = old_serializer.data
 
 @receiver(post_save, sender=Project, dispatch_uid=__name__ + ":project:post_save")
 @receiver(post_save, sender=Task, dispatch_uid=__name__ + ":task:post_save")
@@ -164,31 +171,38 @@ def pre_save_resource_event(sender, instance, **kwargs):
 @receiver(post_save, sender=Organization, dispatch_uid=__name__ + ":organization:post_save")
 @receiver(post_save, sender=Invitation, dispatch_uid=__name__ + ":invitation:post_save")
 @receiver(post_save, sender=Membership, dispatch_uid=__name__ + ":membership:post_save")
-def post_save_resource_event(sender, instance, created, **kwargs):
-    resource_name = instance.__class__.__name__.lower()
+def post_save_resource_event(sender, instance, **kwargs):
+    selected_webhooks = instance._webhooks_selected_webhooks
+    del instance._webhooks_selected_webhooks
 
-    event_type = event_name("create" if created else "update", resource_name)
-    if event_type not in map(lambda a: a[0], EventTypeChoice.choices()):
+    if not selected_webhooks:
         return
 
-    filtered_webhooks = select_webhooks(instance, event_type)
-    if not filtered_webhooks:
-        return
+    old_data = instance._webhooks_old_data
+    del instance._webhooks_old_data
+
+    created = old_data is None
+
+    resource_name = instance.__class__.__name__.lower()
+    event_type = event_name("create" if created else "update", resource_name)
+
+    serializer = get_serializer(instance=instance)
 
     data = {
         "event": event_type,
-        resource_name: get_serializer(instance=instance).data,
+        resource_name: serializer.data,
         "sender": get_sender(instance),
     }
 
     if not created:
-        if before_update := getattr(instance, "_before_update", None):
-            data["before_update"] = before_update
-        else:
-            return
+        if diff := get_instance_diff(old_data=old_data, data=serializer.data):
+            data["before_update"] = {
+                attr: value["old_value"]
+                for attr, value in diff.items()
+            }
 
     transaction.on_commit(
-        lambda: batch_add_to_queue(filtered_webhooks, data),
+        lambda: batch_add_to_queue(selected_webhooks, data),
         robust=True,
     )