Savepoint support

postgresql-simple has it, an example manually combining it with Persistent can be found here: https://www.shimweasel.com/haskell/2016/07/21/yolo-oriented-database-programming

data Result
  = NotPresent
  | AlreadyAdded
  | NowAdded
  deriving (Eq,Show)

doAThing :: UUID -> UUID -> DB Result
doAThing barU fooU =
  handle uniqViolation $ do
    startTransaction
    inserted <-  insertSelectCount $
       from $
       \(foos,bars) ->
         do where_ (foos ^. FooUuid ==. val fooU)
            where_ (bars ^. BarUuid ==. val barU)
            return $ Baz <# (foos ^. FooId) <&> (bars ^. BarId)
    commitTransaction
    case inserted of
      0 -> return NotPresent
      1 -> return NowAdded
      n -> throwM (InvalidInsertedCount n)
  where
    uniqViolation
      :: SqlError -> DB Result
    uniqViolation _e = do
      rollbackTransaction
      return AlreadyAdded

    rollbackTransaction, startTransaction, commitTransaction :: DB ()
    startTransaction    = rawExecute "SAVEPOINT             savepointname" []
    commitTransaction   = rawExecute "RELEASE SAVEPOINT     savepointname" []
    rollbackTransaction = rawExecute "ROLLBACK TO SAVEPOINT savepointname" []

No repository files or tests are named. Start by reviewing Persistent's transaction APIs and the linked PostgreSQL example, then determine how savepoints should be exposed and supported across storage methods. Done means Persistent provides documented savepoint support with coverage for the relevant PostgreSQL behavior.