Attempt to complete inbox requests synchronously

When an inbox activity comes in from another fediverse instance, the
behavior prior to this commit was always to immediately give a 200
response to the external server and then create a celery activity
(usually in the MEDIUM_PRIORITY queue) to complete it.

Instead, this would receive a request and try to complete it without
making any http requests (which would make the request take too long to
process). If an external request is required to complete the activity, a
task is created and added to the queue.

Ideally, this will cause some tasks to happen very promptly, and reduce
the load on celery, which would help queued tasks happen more quickly as
well.

One downside is that this will make completing http requests from
external servers slowing (since it's doing a bunch of thinking before
responding).

bookwyrm/views/inbox.py

@@ -64,7 +64,7 @@ class Inbox(View):
         high = ["Follow", "Accept", "Reject", "Block", "Unblock", "Undo"]
 
         priority = HIGH if activity_json["type"] in high else MEDIUM
-        activity_task.apply_async(args=(activity_json,), queue=priority)
+        sometimes_async_activity_task(activity_json, queue=priority)
         return HttpResponse()
 
 
@@ -102,6 +102,19 @@ def raise_is_blocked_activity(activity_json):
         raise PermissionDenied()
 
 
+def sometimes_async_activity_task(activity_json, queue=MEDIUM):
+    """Sometimes we can effectively respond to a request without queuing a new task,
+    and whever that is possible, we should do it."""
+    activity = activitypub.parse(activity_json)
+
+    # try resolving this activity without making any http requests
+    try:
+        activity.action(allow_external_connections=False)
+    except activitypub.ActivitySerializerError:
+        # if that doesn't work, run it asynchronously
+        activity_task.apply_async(args=(activity_json,), queue=queue)
+
+
 @app.task(queue=MEDIUM)
 def activity_task(activity_json):
     """do something with this json we think is legit"""