Stop doing permacache mutations in a background thread

Using background threads causes annoying complications in general (like right
now if the worker thread is backed up but we restart the app we just lose the
work). But the particular problem at hand is that the permacache zlib patch
wants to do compression. That can cost a lot of CPU so we want to know how long
it takes which means that we need timers. But timers work by reading from
g.stats, and g isn't available in the background thread.

We could find some other way to do the timers, but I'd rather just stop using
the AMQP background thread for anything but actual AMQP stuff which will kill
both birds with one stone.

At a glance it looks like this would cost us roughly 25ms per update to these
listings. 99th percentile goes up to 480ms but seems to float nearer to 100ms.

Deletes has a crazier 99th percentile but a similar mean. Way fewer of them
happen.

r2/r2/lib/db/queries.py

@@ -942,35 +942,20 @@ def get_gilded_user(user):
     return [get_gilded_user_comments(user), get_gilded_user_links(user)]
 
 
-def add_queries(queries, insert_items=None, delete_items=None, foreground=False):
+def add_queries(queries, insert_items=None, delete_items=None):
     """Adds multiple queries to the query queue. If insert_items or
        delete_items is specified, the query may not need to be
        recomputed against the database."""
 
-    # we want to time how long background permacache mutations take, so we need
-    # to wrap g.stats up so that we can use it on the other thread
-    def mutation_timer(stats, timer_name, fn, *a):
-        def _fn():
-            with stats.get_timer(timer_name):
-                return fn(*a)
-        return _fn
-    mutation_timer = partial(mutation_timer, g.stats)
-
     for q in queries:
         if insert_items and q.can_insert():
             log.debug("Inserting %s into query %s" % (insert_items, q))
-            if foreground:
+            with g.stats.get_timer('permacache.foreground.insert'):
                 q.insert(insert_items)
-            else:
-                worker.do(mutation_timer('permacache.background.insert',
-                                         q.insert, insert_items))
         elif delete_items and q.can_delete():
             log.debug("Deleting %s from query %s" % (delete_items, q))
-            if foreground:
+            with g.stats.get_timer('permacache.foreground.delete'):
                 q.delete(delete_items)
-            else:
-                worker.do(mutation_timer('permacache.background.delete',
-                                         q.delete, delete_items))
         else:
             raise Exception("Cannot update query %r!" % (q,))
 
@@ -1137,7 +1122,7 @@ def new_vote(vote):
         elif isinstance(vote.thing, Comment):
             update_comment_votes([vote.thing])
 
-        add_queries(results, insert_items=vote.thing, foreground=True)
+        add_queries(results, insert_items=vote.thing)
     
     if isinstance(vote.thing, Link):
         with CachedQueryMutator() as m: