core: check user record when deleting contact and display name (#3826)

* filter out on merge

* checl contact, ldn

* fix

* corrections

* fix

* refactor

* diff

* refactor2

* remove contact id from error

* Revert "remove contact id from error"

This reverts commit f58af3dcac.

* remove Maybe from error

---------

Co-authored-by: Evgeny Poberezkin <evgeny@poberezkin.com>

src/Simplex/Chat/Store/Shared.hs

@@ -110,6 +110,7 @@ data StoreError
   | SERemoteHostDuplicateCA
   | SERemoteCtrlNotFound {remoteCtrlId :: RemoteCtrlId}
   | SERemoteCtrlDuplicateCA
+  | SEProhibitedDeleteUser {userId :: UserId, contactId :: ContactId}
   deriving (Show, Exception)
 
 $(J.deriveJSON (sumTypeJSON $ dropPrefix "SE") ''StoreError)
@@ -401,3 +402,33 @@ createWithRandomBytes' size gVar create = tryCreate 3
 
 encodedRandomBytes :: TVar ChaChaDRG -> Int -> IO ByteString
 encodedRandomBytes gVar n = atomically $ B64.encode <$> C.randomBytes n gVar
+
+assertNotUser :: DB.Connection -> User -> Contact -> ExceptT StoreError IO ()
+assertNotUser db User {userId} Contact {contactId, localDisplayName} = do
+  r :: (Maybe Int64) <-
+    -- This query checks that the foreign keys in the users table
+    -- are not referencing the contact about to be deleted.
+    -- With the current schema it would cause cascade delete of user,
+    -- with mofified schema (in v5.6.0-beta.0) it would cause foreign key violation error.
+    liftIO . maybeFirstRow fromOnly $
+      DB.query
+        db
+        [sql|
+          SELECT 1 FROM users
+          WHERE (user_id = ? AND local_display_name = ?)
+             OR contact_id = ?
+          LIMIT 1
+        |]
+        (userId, localDisplayName, contactId)
+  when (isJust r) $ throwError $ SEProhibitedDeleteUser userId contactId
+
+safeDeleteLDN :: DB.Connection -> User -> ContactName -> IO ()
+safeDeleteLDN db User {userId} localDisplayName = do
+  DB.execute
+    db
+    [sql|
+      DELETE FROM display_names
+      WHERE user_id = ? AND local_display_name = ?
+        AND local_display_name NOT IN (SELECT local_display_name FROM users WHERE user_id = ?)
+    |]
+    (userId, localDisplayName, userId)