The rooms row for a new room is inserted by the persist transaction,
Rust or Python, when the create event's context is flagged, instead of
by a store_room transaction of its own beforehand. The insert also
claims the room ID: a row already there fails the batch with
RoomIdCollisionError, and create_room runs the whole creation again
under a fresh ID, perturbing the create event's timestamp for v12+
rooms as the old loop did. The room version is prefilled on the upgrade
path too, as the row is no longer there to read.
The chain cover check skips its rooms read when the create event is
among the events, since the row may not be stored yet, and the
createRoom bench advances the fake clock per room so that rooms do not
collide.
Matthew Hodgson
d3cbb71cfc