`PersistReader` wraps a `SyncReader` and adds the reads the persist
controller makes before its write transaction: forward and state DAG
extremities, the prev event walks of `_get_events_which_are_prevs` and
`_get_prevs_before_rejected`, `have_seen_events`, memberships by event ID,
`is_local_host_in_room_ignoring_users`, the room version, the sliding sync
joined room check, `get_last_event_pos_in_room` and the soft failed sticky
events. None of them is cached. Each runs once per chunk.
The chain cover reads are free functions over a `Querier`, not reader
methods. The chain cover step reads, allocates chain IDs and writes in one
transaction, so they must run on that transaction's connection. To allow
that, `WriteTxn` now implements `Querier`. A `WriteError` that passes
through `anyhow` on the way out comes back as itself, so a conflict is still
retried.
The persister builds its own `SyncStore` over the shared caches.
`tests/storage/test_persist_reader.py` runs each read against the Python
method or statement it ports, on the native driver.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UHKYPSc37kWqGACn9aKxKL
