Features

--------

-   Add blacklist & whitelist of servers allowed to send events to a room via `m.room.server_acl` event.
-   Cache factor override system for specific caches ([\#3334](https://github.com/matrix-org/synapse/issues/3334))
-   Add metrics to track appservice transactions ([\#3344](https://github.com/matrix-org/synapse/issues/3344))
-   Try to log more helpful info when a sig verification fails ([\#3372](https://github.com/matrix-org/synapse/issues/3372))
-   Synapse now uses the best performing JSON encoder/decoder according to your runtime (simplejson on CPython, stdlib json on PyPy). ([\#3462](https://github.com/matrix-org/synapse/issues/3462))
-   Add optional ip\_range\_whitelist param to AS registration files to lock AS IP access ([\#3465](https://github.com/matrix-org/synapse/issues/3465))
-   Reject invalid server names in federation requests ([\#3480](https://github.com/matrix-org/synapse/issues/3480))
-   Reject invalid server names in homeserver.yaml ([\#3483](https://github.com/matrix-org/synapse/issues/3483))

Bugfixes
--------

-   Strip access\_token from outgoing requests ([\#3327](https://github.com/matrix-org/synapse/issues/3327))
-   Redact AS tokens in logs ([\#3349](https://github.com/matrix-org/synapse/issues/3349))
-   Fix federation backfill from SQLite servers ([\#3355](https://github.com/matrix-org/synapse/issues/3355))
-   Fix event-purge-by-ts admin API ([\#3363](https://github.com/matrix-org/synapse/issues/3363))
-   Fix event filtering in get\_missing\_events handler ([\#3371](https://github.com/matrix-org/synapse/issues/3371))
-   Synapse is now stricter regarding accepting events which it cannot retrieve the prev\_events for. ([\#3456](https://github.com/matrix-org/synapse/issues/3456))
-   Fix bug where synapse would explode when receiving unicode in HTTP User-Agent header ([\#3470](https://github.com/matrix-org/synapse/issues/3470))
-   Invalidate cache on correct thread to avoid race ([\#3473](https://github.com/matrix-org/synapse/issues/3473))

Improved Documentation
----------------------

-   `doc/postgres.rst`: fix display of the last command block. Thanks to \@ArchangeGabriel! ([\#3340](https://github.com/matrix-org/synapse/issues/3340))

Deprecations and Removals
-------------------------

-   Remove was\_forgotten\_at ([\#3324](https://github.com/matrix-org/synapse/issues/3324))

Misc
----

-   [\#3332](https://github.com/matrix-org/synapse/issues/3332), [\#3341](https://github.com/matrix-org/synapse/issues/3341), [\#3347](https://github.com/matrix-org/synapse/issues/3347), [\#3348](https://github.com/matrix-org/synapse/issues/3348),
[\#3356](https://github.com/matrix-org/synapse/issues/3356), [\#3385](https://github.com/matrix-org/synapse/issues/3385), [\#3446](https://github.com/matrix-org/synapse/issues/3446), [\#3447](https://github.com/matrix-org/synapse/issues/3447),
[\#3467](https://github.com/matrix-org/synapse/issues/3467), [\#3474](https://github.com/matrix-org/synapse/issues/3474)
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