refactor: Simplify call to bulk_get_subscriber_user_ids.

The way we were computing the dictionary was very
convoluted--all we need is a set of subscribed user
ids.

zerver/lib/actions.py

@@ -2628,16 +2628,19 @@ def validate_user_access_to_subscribers_helper(
 def bulk_get_subscriber_user_ids(
     stream_dicts: Iterable[Mapping[str, Any]],
     user_profile: UserProfile,
-    sub_dict: Mapping[int, bool],
+    subscribed_stream_ids: Set[int],
 ) -> Dict[int, List[int]]:
     """sub_dict maps stream_id => whether the user is subscribed to that stream."""
     target_stream_dicts = []
     for stream_dict in stream_dicts:
+        stream_id = stream_dict["id"]
+        is_subscribed = stream_id in subscribed_stream_ids
+
         try:
             validate_user_access_to_subscribers_helper(
                 user_profile,
                 stream_dict,
-                lambda user_profile: sub_dict[stream_dict["id"]],
+                lambda user_profile: is_subscribed,
             )
         except JsonableError:
             continue
@@ -5023,19 +5026,16 @@ def gather_subscriptions_helper(user_profile: UserProfile,
     unsubscribed = []
     never_subscribed = []
 
-    # Deactivated streams aren't in stream_hash.
-    streams = [stream_hash[sub["stream_id"]] for sub in sub_dicts
-               if sub["stream_id"] in stream_hash]
-    streams_subscribed_map = {sub["stream_id"]: sub["active"] for sub in sub_dicts}
-
-    # Add never subscribed streams to streams_subscribed_map
-    streams_subscribed_map.update({stream['id']: False for stream in all_streams if stream not in streams})
+    # The highly optimized bulk_get_subscriber_user_ids wants to know which
+    # streams we are subscribed to, for validation purposes, and it uses that
+    # info to know if it's allowed to find OTHER subscribers.
+    subscribed_stream_ids = {sub["stream_id"] for sub in sub_dicts if sub["active"]}
 
     if include_subscribers:
         subscriber_map: Mapping[int, Optional[List[int]]] = bulk_get_subscriber_user_ids(
             all_streams,
             user_profile,
-            streams_subscribed_map,
+            subscribed_stream_ids,
         )
     else:
         # If we're not including subscribers, always return None,

zerver/tests/test_subs.py

@@ -101,7 +101,7 @@ class TestMiscStuff(ZulipTestCase):
         result = bulk_get_subscriber_user_ids(
             stream_dicts=[],
             user_profile=user_profile,
-            sub_dict={},
+            subscribed_stream_ids=set(),
         )
         self.assertEqual(result, {})