The patch https://github.com/matrix-org/matrix-rust-sdk/pull/2395 has
introduced `SlidingSyncInner::past_positions` as a mechanism to filter
duplicated responses. It was a problem because the sliding sync `ops`
could easily create corrupted states if they were applied more than
once.
Since https://github.com/matrix-org/matrix-rust-sdk/pull/3664/, `ops`
are ignored.
Now, `past_positions` create a problem with the sliding sync native
implementation inside Synapse because `pos` can stay the same between
multiple responses.
While `past_positions` was helpful to fix bugs in the past, it's no
longer necessary today. Moreover, it breaks an invariant about `pos`: we
must consider it as a blackbox. It means we must ignore if a `pos` value
has been received in the past or not. This invariant has been broken for
good reasons, but it now creates new issues.
This patch removes `past_positions`, along with the associated code
(like `Error::ResponseAlreadyReceived` for example).
Ivan Enderlin
d143c6198c