Fix stack overflow in Keyring (#5724)

* Refactor Keyring._start_key_lookups

There's an awful lot of deferreds and dictionaries flying around here. The
whole thing can be made much simpler and achieve the same effect.

* Add a delay to key lookup lock release to fix stack overflow

A tactical call_later here should fix #5723

* changelog

1 files changed, 0 insertions, 29 deletions

diff --git a/tests/crypto/test_keyring.py b/tests/crypto/test_keyring.py

index 795703967d..8d94a503d6 100644
--- a/tests/crypto/test_keyring.py
+++ b/tests/crypto/test_keyring.py
@@ -86,35 +86,6 @@ class KeyringTestCase(unittest.HomeserverTestCase):
             getattr(LoggingContext.current_context(), "request", None), expected
         )
 
-    def test_wait_for_previous_lookups(self):
-        kr = keyring.Keyring(self.hs)
-
-        lookup_1_deferred = defer.Deferred()
-        lookup_2_deferred = defer.Deferred()
-
-        # we run the lookup in a logcontext so that the patched inlineCallbacks can check
-        # it is doing the right thing with logcontexts.
-        wait_1_deferred = run_in_context(
-            kr.wait_for_previous_lookups, {"server1": lookup_1_deferred}
-        )
-
-        # there were no previous lookups, so the deferred should be ready
-        self.successResultOf(wait_1_deferred)
-
-        # set off another wait. It should block because the first lookup
-        # hasn't yet completed.
-        wait_2_deferred = run_in_context(
-            kr.wait_for_previous_lookups, {"server1": lookup_2_deferred}
-        )
-
-        self.assertFalse(wait_2_deferred.called)
-
-        # let the first lookup complete (in the sentinel context)
-        lookup_1_deferred.callback(None)
-
-        # now the second wait should complete.
-        self.successResultOf(wait_2_deferred)
-
     def test_verify_json_objects_for_server_awaits_previous_requests(self):
         key1 = signedjson.key.generate_signing_key(1)