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 https://github.com/matrix-org/synapse/issues/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.]
For inbound federation requests, if a given remote server makes too many
requests at once, we start stacking them up rather than processing them
immediatedly.
However, that means that there is a fair chance that the requesting server will
disconnect before we start processing the request. In that case, if it was a
read-only request (ie, a GET request), there is absolutely no point in
building a response (and some requests are quite expensive to handle).
Even in the case of a POST request, one of two things will happen:
* Most likely, the requesting server will retry the request and we'll get the
information anyway.
* Even if it doesn't, the requesting server has to assume that we didn't get
the memo, and act accordingly.
In short, we're better off aborting the request at this point rather than
ploughing on with what might be a quite expensive request.
It was correct at the time of our friend Jorik writing it (checking
git blame), but the world has moved now and it is no longer a
generator.
Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>
Add changelog
Save retrieved keys to the db
lint
Fix and de-brittle remote result dict processing
Use query_user_devices instead, assume only master, self_signing key types
Make changelog more useful
Remove very specific exception handling
Wrap get_verify_key_from_cross_signing_key in a try/except
Note that _get_e2e_cross_signing_verify_key can raise a SynapseError
lint
Add comment explaining why this is useful
Only fetch master and self_signing key types
Fix log statements, docstrings
Remove extraneous items from remote query try/except
lint
Factor key retrieval out into a separate function
Send device updates, modeled after SigningKeyEduUpdater._handle_signing_key_updates
Update method docstring
This changes the replication protocol so that the server does not send down `RDATA` for rows that happened before the client connected. Instead, the server will send a `POSITION` and clients then query the database (or master out of band) to get up to date.
