subs: Send create event to new subscribers of invite-only streams.

This fixes a regression introduced by our migration to track
subscribers for all public streams, where now users who are added to
an invite-only stream were receiving a mark_subscribed event
for a stream their browser didn't know existed, causing an exception.

To fix this, we now send a stream create event to the browser just
before the user receives the notification that it was added to the
invite-only stream.

zerver/lib/actions.py

@@ -1669,12 +1669,31 @@ def bulk_add_subscriptions(streams, users):
         sub_tuples_by_user[sub.user_profile.id].append((sub, stream))
         new_streams.add((sub.user_profile.id, stream.id))
 
+    # We now send several types of events to notify browsers.  The
+    # first batch is notifications to users on invite-only streams
+    # that the stream exists.
+    for stream in streams:
+        new_users = [user for user in users if (user.id, stream.id) in new_streams]
+
+        # Users newly added to invite-only streams need a `create`
+        # notification, since they didn't have the invite-only stream
+        # in their browser yet.
+        if stream.invite_only:
+            event = dict(type="stream", op="create",
+                         streams=[stream.to_dict()])
+            send_event(event, [user.id for user in new_users])
+
+    # The second batch is events for the users themselves that they
+    # were subscribed to the new streams.
     for user_profile in users:
         if len(sub_tuples_by_user[user_profile.id]) == 0:
             continue
         sub_pairs = sub_tuples_by_user[user_profile.id]
         notify_subscriptions_added(user_profile, sub_pairs, fetch_stream_subscriber_emails)
 
+    # The second batch is events for other users who are tracking the
+    # subscribers lists of streams in their browser; everyone for
+    # public streams and only existing subscribers for private streams.
     for stream in streams:
         if stream.realm.is_zephyr_mirror_realm and not stream.invite_only:
             continue

zerver/tests/test_subs.py

@@ -40,7 +40,7 @@ from zerver.lib.actions import (
     gather_subscriptions_helper, bulk_add_subscriptions, bulk_remove_subscriptions,
     gather_subscriptions, get_default_streams_for_realm, get_realm, get_stream,
     get_user_profile_by_email, set_default_streams, get_subscription,
-    create_streams_if_needed, active_user_ids
+    create_stream_if_needed, create_streams_if_needed, active_user_ids
 )
 
 from zerver.views.streams import (
@@ -1548,6 +1548,49 @@ class SubscriptionAPITest(ZulipTestCase):
         self.assertEqual(add_peer_event['event']['op'], 'peer_add')
         self.assertEqual(add_peer_event['event']['user_id'], user_profile.id)
 
+    def test_private_stream_subscription(self):
+        # type: () -> None
+        realm = get_realm("zulip")
+
+        # Create a private stream with Hamlet subscribed
+        stream_name = "private"
+        (stream, _) = create_stream_if_needed(realm, stream_name, invite_only=True)
+
+        existing_email = "hamlet@zulip.com"
+        existing_user_profile = get_user_profile_by_email(existing_email)
+        bulk_add_subscriptions([stream], [existing_user_profile])
+
+        # Now subscribe Cordelia to the stream, capturing events
+        email = 'cordelia@zulip.com'
+        user_profile = get_user_profile_by_email(email)
+
+        events = []
+        with tornado_redirected_to_list(events):
+            bulk_add_subscriptions([stream], [user_profile])
+
+        self.assert_length(events, 3)
+        create_event, add_event, add_peer_event = events
+
+        self.assertEqual(create_event['event']['type'], 'stream')
+        self.assertEqual(create_event['event']['op'], 'create')
+        self.assertEqual(create_event['users'], [user_profile.id])
+        self.assertEqual(create_event['event']['streams'][0]['name'], stream_name)
+
+        self.assertEqual(add_event['event']['type'], 'subscription')
+        self.assertEqual(add_event['event']['op'], 'add')
+        self.assertEqual(add_event['users'], [user_profile.id])
+        self.assertEqual(
+            set(add_event['event']['subscriptions'][0]['subscribers']),
+            set([user_profile.email, existing_user_profile.email])
+        )
+
+        # We don't send a peer_add event to othello
+        self.assertNotIn(user_profile.id, add_peer_event['users'])
+        self.assertEqual(len(add_peer_event['users']), 1)
+        self.assertEqual(add_peer_event['event']['type'], 'subscription')
+        self.assertEqual(add_peer_event['event']['op'], 'peer_add')
+        self.assertEqual(add_peer_event['event']['user_id'], user_profile.id)
+
     def test_users_getting_add_peer_event(self):
         # type: () -> None
         """