process_fts_updates: After getting notifies, process them all.

Every insert into `fts_update_log` triggers a `NOTIFY`; processing a
batch of 1000 for every NOTIFY that we get thus results in a lot of
extra processing of no rows.

Instead, we clear all notifies, and repeat the pattern of processing
rows until the batch comes up short.  This may still result in extra
wake-ups, in the event that a new NOTIFY occurs after we clear them,
and that new row is processed by us in `update_all_rows`.  However,
these wake-ups are necessary for correctness, and this change will
result in drastically fewer extra checks.

puppet/zulip/files/postgresql/process_fts_updates

@@ -91,6 +91,23 @@ def update_fts_columns(conn: psycopg2.extensions.connection) -> int:
         return len(ids)
 
 
+def update_all_rows(msg: str, conn: psycopg2.extensions.connection) -> None:
+    while True:
+        start_time = time.perf_counter()
+        rows_updated = update_fts_columns(conn)
+        if rows_updated:
+            logger.log(
+                logging.INFO,
+                "process_fts_updates: %s %d rows, %d rows/sec",
+                msg,
+                rows_updated,
+                rows_updated / (time.perf_counter() - start_time),
+            )
+
+        if rows_updated != BATCH_SIZE:
+            return
+
+
 def am_master(conn: psycopg2.extensions.connection) -> bool:
     with conn.cursor() as cursor:
         cursor.execute("SELECT pg_is_in_recovery()")
@@ -223,24 +240,14 @@ while True:
                 conn.commit()
 
             # Catch up on any historical columns
-            while True:
+            update_all_rows("Caught up", conn)
-                rows_updated = update_fts_columns(conn)
-                logger.log(
-                    logging.INFO if rows_updated > 0 else logging.DEBUG,
-                    "process_fts_updates: Processed %d rows catching up",
-                    rows_updated,
-                )
-
-                if rows_updated != BATCH_SIZE:
-                    # We're caught up, so proceed to the listening for updates phase.
-                    break
 
         # TODO: If we go back into recovery, we should stop processing updates
         if select.select([conn], [], [], 30) != ([], [], []):
             conn.poll()
-            while conn.notifies:
+            conn.notifies.clear()
-                conn.notifies.pop()
+            update_all_rows("Updated", conn)
-                update_fts_columns(cursor)
+
     except psycopg2.OperationalError as e:
         retries -= 1
         if retries <= 0: