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Erik Johnston
5a8c4b3990
In `/sync` there is a mode called lazy loaded (LL) members. In this mode `/sync` does not return all memberships in the room, instead it only returns a) membership changes, and b) the memberships of senders of events in the timeline (if not previously sent). Synapse implements a lazy-loaded member cache to avoid repeatedly sending down the same memberships when a single user sends multiple events. It does this by storing for each user/device the memberships it has previously returned in an LRU cache. Note that it is fine for the server to return the same membership, clients handle that fine, but it is a waste of bandwidth. The current lazy-loaded membership cache has three intertwined flaws: 1. It does not handle retries or clients re-using old tokens. 2. It incorrectly applies the membership cache to membership changes, not just memberships added by lazy-loading. 3. It does not correctly handle multiple "sync streams" from the same device. In this PR we explicitly fix the first two issues in two commits, and the third issue is mitigated: 1. Alongside each membership in the LL cache we store the `next_batch` position of the response that carried it. A client that processed a response syncs from its `next_batch` next. When a request arrives with a `since` token, we delete every membership recorded after that token, as they were sent in responses the client never processed. Memberships recorded at or before `since` are kept. 2. We always return memberships that have changed, even if they appear in the LL member cache. These changes also half mitigate the issue of running multiple `/sync` loops per-device (though we do not see this in the wild), as we'd at least always return the changed membership states. Fixes #19978 and #20278, supersedes #20279