[erikjohnston]

The cache key of _get_server_keys_json is a single argument which is itself a (server_name, key_id) tuple, and store_server_keys_response passed that nested tuple straight into the cache invalidation stream.
psycopg2 quietly serialises the inner tuple as a Postgres record, so the keys column of cache_invalidation_stream_by_instance ended up holding the record literal as a single string (e.g. {"(srv,ed25519:abc)"}) — which never matches the real cache key on the receiving side, i.e. the
invalidation has always been a silent no-op on workers. The native Rust backend's stricter parameter conversion turns the same nested tuple into a loud TypeError: unsupported parameter type for postgres: tuple.

Fix it the same way #18899 did for _get_e2e_cross_signing_signatures_for_device, which has the same nested-tuple key shape: invalidate the local cache directly, JSON-encode the key for the replication row, and decode it again in process_replication_rows.

Found as part of the the effort to port the database pool to Rust.

[Copilot]

Pull request overview

This PR fixes multi-worker cache invalidation for _get_server_keys_json, whose cache key is a single argument that is itself a (server_name, key_id) tuple. It changes the replication payload to JSON-encode that nested tuple (so it can be represented as text[] cleanly), and adds the corresponding JSON decode path on workers to ensure the correct cache entry is invalidated.

Changes:

• Invalidate _get_server_keys_json locally, but replicate invalidations using JSON-encoded (server_name, key_id) pairs to avoid nested tuple serialization issues.
• Teach cache invalidation replication processing to decode _get_server_keys_json keys from JSON and invalidate the correct entry.
• Add a regression test ensuring the invalidation round-trip works across replication, plus a Towncrier newsfragment.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/storage/databases/main/test_cache.py	Adds a multi-worker regression test validating _get_server_keys_json invalidation replication round-trip.
synapse/storage/databases/main/keys.py	Fixes invalidation emission by JSON-encoding nested cache keys for replication while still invalidating locally.
synapse/storage/databases/main/cache.py	Decodes JSON-encoded keys for _get_server_keys_json during replication processing and invalidates the proper cache entry.
changelog.d/19966.bugfix	Adds a newsfragment documenting the worker invalidation fix.

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[anoadragon453]

Seems like it'd be nice to just teach the replication tooling to recognise non-primitive data structures being passed and auto-serialise instead.

But I guess for now with only two instances of this, it's not necessary. Would be nice to prevent future footguns however.