fix(submission): allow not modifying submissionUsers

src/Handler/Submission.hs

@@ -265,6 +265,9 @@ makeSubmissionForm cid msmid uploadMode grouping isLecturer prefillUsers = ident
 
           postProcess :: Map ListPosition (Either UserEmail UserId, ()) -> Set (Either UserEmail UserId)
           postProcess valMap
+            | is _Just msmid
+            , resultUsers == prefillUsers
+            = resultUsers
             | Just maxSize' <- maxSize
             , fromIntegral maxSize' >= Set.size resultUsers
             = resultUsers
@@ -399,11 +402,19 @@ submissionHelper tid ssh csh shn mcid = do
         }
 
   mCID <- fmap join . msgSubmissionErrors . runDBJobs $ do
+    -- Determine old submission users
+    subUsersOld <- if
+      | Just smid <- msmid -> Set.union
+          <$> (setOf (folded . _entityVal . _submissionUserUser . to Right) <$> selectList [SubmissionUserSubmission ==. smid] [])
+          <*> (sourceInvitationsF smid <&> Set.fromList . map (\(email, InvDBDataSubmissionUser) -> Left email))
+      | otherwise -> return Set.empty
+
     res' <- case res of
       FormMissing              -> return   FormMissing
       (FormFailure failmsgs)   -> return $ FormFailure failmsgs
       -- #227 Part II: no longer ignore submitter, if the user is lecturer or admin (allow lecturers to submit for their students)
       (FormSuccess res'@(_, groupMembers))
+        | groupMembers == subUsersOld -> return $ FormSuccess res'
         | Arbitrary{..} <- sheetGrouping -> do -- Validate AdHoc Group Members
           -- , length gEMails < maxParticipants -> do -- < since submitting user is already accounted for
           let (gEMails, gIds) = partitionEithers $ Set.toList groupMembers
@@ -479,13 +490,6 @@ submissionHelper tid ssh csh shn mcid = do
                 -- SubmissionUser for all group members (pre-registered & ad-hoc)
                 return $ groupUids `Set.union` adhocMembers
 
-          -- Determine old submission users
-          subUsersOld <- if
-            | isJust msmid -> Set.union
-                <$> (setOf (folded . _entityVal . _submissionUserUser . to Right) <$> selectList [SubmissionUserSubmission ==. smid] [])
-                <*> (sourceInvitationsF smid <&> Set.fromList . map (\(email, InvDBDataSubmissionUser) -> Left email))
-            | otherwise -> return Set.empty  -- optimization (do not perform selection if submission was freshly created)
-
           -- Since invitations carry no data we only need to consider changes to
           -- the set of users/invited emails
           -- Otherwise we would have to update old invitations (via