Simplification to Keyring.wait_for_previous_lookups. (#5250)

The list of server names was redundant, since it was equivalent to the keys on
the server_to_deferred map. This reduces the number of large lists being passed
around, and has the benefit of deduplicating the entries in `wait_on`.

changelog.d/5250.misc

@@ -0,0 +1 @@
+Simplification to Keyring.wait_for_previous_lookups.

synapse/crypto/keyring.py

@@ -180,9 +180,7 @@ def _start_key_lookups(self, verify_requests):
 
             # We want to wait for any previous lookups to complete before
             # proceeding.
-            yield self.wait_for_previous_lookups(
-                [rq.server_name for rq in verify_requests], server_to_deferred
-            )
+            yield self.wait_for_previous_lookups(server_to_deferred)
 
             # Actually start fetching keys.
             self._get_server_verify_keys(verify_requests)
@@ -215,12 +213,11 @@ def remove_deferreds(res, verify_request):
             logger.exception("Error starting key lookups")
 
     @defer.inlineCallbacks
-    def wait_for_previous_lookups(self, server_names, server_to_deferred):
+    def wait_for_previous_lookups(self, server_to_deferred):
         """Waits for any previous key lookups for the given servers to finish.
 
         Args:
-            server_names (list): list of server_names we want to lookup
-            server_to_deferred (dict): server_name to deferred which gets
+            server_to_deferred (dict[str, Deferred]): server_name to deferred which gets
                 resolved once we've finished looking up keys for that server.
                 The Deferreds should be regular twisted ones which call their
                 callbacks with no logcontext.
@@ -233,7 +230,7 @@ def wait_for_previous_lookups(self, server_names, server_to_deferred):
         while True:
             wait_on = [
                 (server_name, self.key_downloads[server_name])
-                for server_name in server_names
+                for server_name in server_to_deferred.keys()
                 if server_name in self.key_downloads
             ]
             if not wait_on:

tests/crypto/test_keyring.py

@@ -85,7 +85,7 @@ def test_wait_for_previous_lookups(self):
         # 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"], {"server1": lookup_1_deferred}
+            kr.wait_for_previous_lookups, {"server1": lookup_1_deferred}
         )
 
         # there were no previous lookups, so the deferred should be ready
@@ -94,7 +94,7 @@ def test_wait_for_previous_lookups(self):
         # 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"], {"server1": lookup_2_deferred}
+            kr.wait_for_previous_lookups, {"server1": lookup_2_deferred}
         )
 
         self.assertFalse(wait_2_deferred.called)