1.9.0rc1

changelog.d/6690.bugfix

-Fix a bug where we would assign a numeric userid if somebody tried registering with an empty username.

changelog.d/6691.misc

-Remove redundant RegistrationError class.

changelog.d/6697.misc

-Don't block processing of incoming EDUs behind processing PDUs in the same transaction.

changelog.d/6698.doc

-Add more endpoints to the documentation for Synapse workers.

changelog.d/6702.misc

-Remove duplicate check for the `session` query parameter on the `/auth/xxx/fallback/web` Client-Server endpoint.
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changelog.d/6706.misc

-Attempt to retry sending a transaction when we detect a remote server has come back online, rather than waiting for a transaction to be triggered by new data.

changelog.d/6711.bugfix

-Fix `purge_room` admin API.

changelog.d/6712.feature

-Add org.matrix.e2e_cross_signing to unstable_features in /versions as per [MSC1756](https://github.com/matrix-org/matrix-doc/pull/1756).

changelog.d/6714.bugfix

-Fix a bug causing Synapse to not always purge quiet rooms with a low `max_lifetime` in their message retention policies when running the automated purge jobs.

changelog.d/6715.misc

-Add StateMap type alias to simplify types.

changelog.d/6716.misc

-Add a `DeltaState` to track changes to be made to current state during event persistence.

changelog.d/6717.misc

-Add more logging around message retention policies support.

changelog.d/6718.bugfix

-Fix a bug causing the `synapse_port_db` script to return 0 in a specific error case.

changelog.d/6720.feature

-Add a new admin API to list and filter rooms on the server.
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changelog.d/6723.misc

-Updates to the SAML mapping provider API.

changelog.d/6724.misc

-When processing a SAML response, log the assertions for easier configuration.

changelog.d/6728.misc

-Add `local_current_membership` table for tracking local user membership state in rooms.

changelog.d/6730.bugfix

-Fix changing password via user admin API.

changelog.d/6731.bugfix

-Fix `/events/:event_id` deprecated API.

changelog.d/6732.misc

-Fixup `synapse.rest` to pass mypy.