Commits on Jul 21, 2020

1. Put a cache on /state_ids …

   If we send out an event which refers to `prev_events` which other servers in
   the federation are missing, then (after a round or two of backfill attempts),
   they will end up asking us for `/state_ids` at a particular point in the DAG.
   
   As per #7893, this is quite
   expensive, and we tend to see lots of very similar requests around the same
   time.
   
   We can therefore handle this much more efficiently by using a cache, which (a)
   ensures that if we see the same request from multiple servers (or even the same
   server, multiple times), then they share the result, and (b) any other servers
   that miss the initial excitement can also benefit from the work.
   
   [It's interesting to note that `/state` has a cache for exactly this
   reason. `/state` is now essentially unused and replaced with `/state_ids`, but
   evidently when we replaced it we forgot to add a cache to the new endpoint.]

   

   richvdh committed Jul 21, 2020
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