message: Support avoiding database queries in has_message_access.

If the caller has already fetched the Stream or subscription details
for the user, those can be passed to has_message_access to avoid extra
database queries.

zerver/lib/message.py

@@ -677,8 +677,21 @@ def access_message(
 
 
 def has_message_access(
-    user_profile: UserProfile, message: Message, user_message: Optional[UserMessage]
+    user_profile: UserProfile,
+    message: Message,
+    user_message: Optional[UserMessage],
+    *,
+    stream: Optional[Stream] = None,
+    is_subscribed: Optional[bool] = None,
 ) -> bool:
+    """
+    Returns whether a user has access to a given message.
+
+    * The user_message parameter must be provded if the user has a UserMessage
+      row for the target message.
+    * The optional stream parameter is validated; is_subscribed is not.
+    """
+
     # If you have a user_message object, you have access.
     if user_message is not None:
         return True
@@ -687,7 +700,11 @@ def has_message_access(
         # You can't access private messages you didn't receive
         return False
 
-    stream = Stream.objects.get(id=message.recipient.type_id)
+    if stream is None:
+        stream = Stream.objects.get(id=message.recipient.type_id)
+    else:
+        assert stream.recipient_id == message.recipient_id
+
     if stream.realm != user_profile.realm:
         # You can't access public stream messages in other realms
         return False
@@ -701,6 +718,9 @@ def has_message_access(
         return True
 
     # is_history_public_to_subscribers, so check if you're subscribed
+    if is_subscribed is not None:
+        return is_subscribed
+
     return Subscription.objects.filter(
         user_profile=user_profile, active=True, recipient=message.recipient
     ).exists()

zerver/tests/test_message_edit.py

@@ -1363,6 +1363,12 @@ class EditMessageTest(ZulipTestCase):
             has_message_access(guest_user, Message.objects.get(id=msg_id_to_test_acesss), None),
             True,
         )
+        self.assertEqual(
+            has_message_access(
+                guest_user, Message.objects.get(id=msg_id_to_test_acesss), None, stream=old_stream
+            ),
+            True,
+        )
         self.assertEqual(
             has_message_access(non_guest_user, Message.objects.get(id=msg_id_to_test_acesss), None),
             True,
@@ -1387,6 +1393,32 @@ class EditMessageTest(ZulipTestCase):
             has_message_access(non_guest_user, Message.objects.get(id=msg_id_to_test_acesss), None),
             True,
         )
+        self.assertEqual(
+            # If the guest user were subscribed to the new stream,
+            # they'd have access; has_message_access does not validate
+            # the is_subscribed parameter.
+            has_message_access(
+                guest_user,
+                Message.objects.get(id=msg_id_to_test_acesss),
+                None,
+                stream=new_stream,
+                is_subscribed=True,
+            ),
+            True,
+        )
+
+        self.assertEqual(
+            has_message_access(
+                guest_user, Message.objects.get(id=msg_id_to_test_acesss), None, stream=new_stream
+            ),
+            False,
+        )
+        with self.assertRaises(AssertionError):
+            # Raises assertion if you pass an invalid stream.
+            has_message_access(
+                guest_user, Message.objects.get(id=msg_id_to_test_acesss), None, stream=old_stream
+            )
+
         self.assertEqual(
             UserMessage.objects.filter(
                 user_profile_id=non_guest_user.id,