Modify StoreKeyFetcher to read from server_keys_json. (#15417)

Before this change:

* `PerspectivesKeyFetcher` and `ServerKeyFetcher` write to `server_keys_json`.
* `PerspectivesKeyFetcher` also writes to `server_signature_keys`.
* `StoreKeyFetcher` reads from `server_signature_keys`.

After this change:

* `PerspectivesKeyFetcher` and `ServerKeyFetcher` write to `server_keys_json`.
* `PerspectivesKeyFetcher` also writes to `server_signature_keys`.
* `StoreKeyFetcher` reads from `server_keys_json`.

This results in `StoreKeyFetcher` now using the results from `ServerKeyFetcher`
in addition to those from `PerspectivesKeyFetcher`, i.e. keys which are directly
fetched from a server will now be pulled from the database instead of refetched.

An additional minor change is included to avoid creating a `PerspectivesKeyFetcher`
(and checking it) if no `trusted_key_servers` are configured.

The overall impact of this should be better usage of cached results:

* If a server has no trusted key servers configured then it should reduce how often keys
  are fetched.
* if a server's trusted key server does not have a requested server's keys cached then it
  should reduce how often keys are directly fetched.

1 files changed, 3 insertions, 0 deletions

diff --git a/tests/utils.py b/tests/utils.py
index a0ac11bc5c..e73b46944b 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -131,6 +131,9 @@ def default_config(
         # the test signing key is just an arbitrary ed25519 key to keep the config
         # parser happy
         "signing_key": "ed25519 a_lPym qvioDNmfExFBRPgdTU+wtFYKq4JfwFRv7sYVgWvmgJg",
+        # Disable trusted key servers, otherwise unit tests might try to actually
+        # reach out to matrix.org.
+        "trusted_key_servers": [],
         "event_cache_size": 1,
         "enable_registration": True,
         "enable_registration_captcha": False,