tornado: Cache messages internally in tornado process.

This allows us to handle the return_messages_immediately part of
get_updates requests without having to talk to the database.

(imported from commit ed0b7742d359efb21a0a4960f4fc25f4337e9ad4)

zephyr/lib/message_cache.py

@@ -0,0 +1,25 @@
+from zephyr.models import Message
+from zephyr.lib.cache import cache_with_key, djcache
+
+def message_cache_key(message_id):
+    return "message:%d" % (message_id,)
+
+def cache_save_message(message):
+    djcache.set(message_cache_key(message.id), (message,), timeout=3600*24)
+
+@cache_with_key(message_cache_key)
+def cache_get_message(message_id):
+    return Message.objects.select_related("client", "sender").get(id=message_id)
+
+# Called on Tornado startup to ensure our message cache isn't empty
+def populate_message_cache(count):
+    max_message_id = 0
+    min_message_id = 0
+    messages_for_memcached = {}
+    for m in Message.objects.select_related("sender", "client").all().order_by("-id")[0:count]:
+        max_message_id = max(m.id, max_message_id)
+        min_message_id = min(m.id, min_message_id)
+        messages_for_memcached[message_cache_key(m.id)] = (m,)
+
+    djcache.set_many(messages_for_memcached, timeout=3600*24)
+    return max_message_id, min_message_id