Limit the number of old messages the client requests

(imported from commit 6bff6aa0b48d46b98aa68c6e29eb569cf41f4989)

zephyr/static/js/zephyr.js

@@ -3,6 +3,10 @@ var message_dict = {};
 var subject_dict = {};
 var people_hash = {};
 
+// We only tell the server to backfill old messages
+// if we have fewer than this many messages total.
+var max_messages_for_backfill = 1000;
+
 var selected_message_class = 'selected_message';
 var viewport = $(window);
 var reloading_app = false;
@@ -14,7 +18,8 @@ var get_updates_params = {
     last:  -1,
     failures: 0,
     server_generation: -1, /* to be filled in on document.ready */
-    reload_pending: 0
+    reload_pending: 0,
+    want_old_messages: true
 };
 
 $(function () {
@@ -615,6 +620,7 @@ $(function () {
 var get_updates_xhr;
 var get_updates_timeout;
 function get_updates() {
+    get_updates_params.want_old_messages = (message_array.length < max_messages_for_backfill);
     get_updates_xhr = $.ajax({
         type:     'POST',
         url:      '/json/get_updates',

zephyr/views.py

@@ -239,6 +239,7 @@ def return_messages_immediately(request, handler, user_profile, **kwargs):
     first = request.POST.get("first")
     last = request.POST.get("last")
     failures = request.POST.get("failures")
+    want_old_messages = (request.POST.get("want_old_messages") == "true")
     client_server_generation = request.POST.get("server_generation")
     client_reload_pending = request.POST.get("reload_pending")
     if first is None or last is None:
@@ -264,7 +265,7 @@ def return_messages_immediately(request, handler, user_profile, **kwargs):
                   + list(query.filter(id__gte=ptr)[:200]))
     else:
         messages = query.filter(id__gt=last)[:400]
-        if not messages:
+        if want_old_messages and not messages:
             # No more messages in the future; try filling in from the past.
             messages = last_n(400, query.filter(id__lt=first))
             where = 'top'