We already have modifyMVarMasked and modifyMVarMasked_, and so I propose to add

{-|
  Like 'withMVar', but the @IO@ action in the second argument is executed
  with asynchronous exceptions masked.

  /Since: 4.7.0.0/
-}
{-# INLINE withMVarMasked_ #-}
withMVarMasked :: MVar a -> (a -> IO b) -> IO b
withMVarMasked m io =
  mask_ $ do
    a <- takeMVar m
    b <- io a `onException` putMVar m a
    putMVar m a
    return b

any arguments against this addition?