streams: Allow editing subscriptions in archived channels.

Since this does impact the ability to access the channel's content, it
makes sense to permit changing subscriptions, just like other
permissions settings on the archived channel.

zerver/tests/test_subs.py

@@ -3350,7 +3350,7 @@ class StreamAdminTest(ZulipTestCase):
 
         return stream
 
-    def archive_stream(self, stream: Stream) -> None:
+    def archive_stream(self, stream: Stream, *, expect_can_subscribe: bool = True) -> None:
         """
         Archive the stream and assess the result.
         """
@@ -3399,11 +3399,38 @@ class StreamAdminTest(ZulipTestCase):
         streams = [s["name"] for s in self.assert_json_success(result)["streams"]]
         self.assertIn(deactivated_stream_name, streams)
 
-        # You can't subscribe to archived stream.
+        # You can still subscribe to archived stream, if you have content access.
+        hamlet = self.example_user("hamlet")
         result = self.subscribe_via_post(
-            self.example_user("hamlet"), [deactivated_stream_name], allow_fail=True
+            hamlet,
+            [deactivated_stream_name],
+            allow_fail=not expect_can_subscribe,
         )
-        self.assert_json_error(result, f"Unable to access channel ({deactivated_stream_name}).")
+        if expect_can_subscribe:
+            self.assert_json_success(result)
+        else:
+            self.assert_json_error(result, "Unable to access channel (privstream).")
+
+            # now grant content access
+            setting_group_member_dict = UserGroupMembersDict(
+                direct_members=[hamlet.id], direct_subgroups=[]
+            )
+            do_change_stream_group_based_setting(
+                stream, "can_subscribe_group", setting_group_member_dict, acting_user=hamlet
+            )
+
+            result = self.subscribe_via_post(hamlet, [deactivated_stream_name])
+            self.assert_json_success(result)
+
+        # You can still unsubscribe from an archived stream.
+        result = self.client_delete(
+            "/json/users/me/subscriptions",
+            {
+                "subscriptions": orjson.dumps([deactivated_stream_name]).decode(),
+                "principals": orjson.dumps([hamlet.id]).decode(),
+            },
+        )
+        self.assert_json_success(result)
 
         # You cannot re-archive the stream
         with self.capture_send_event_calls(expected_num_events=0) as events:
@@ -3458,7 +3485,7 @@ class StreamAdminTest(ZulipTestCase):
         priv_stream = self.set_up_stream_for_archiving(
             "privstream", subscribed=False, invite_only=True
         )
-        self.archive_stream(priv_stream)
+        self.archive_stream(priv_stream, expect_can_subscribe=False)
 
     def attempt_unsubscribe_of_principal(
         self,