The definition of threadWait is:

threadWait :: Event -> Fd -> IO ()
threadWait evt fd = mask_ $ do
  m <- newEmptyMVar
  mgr <- getSystemEventManager_
  reg <- registerFd mgr (\_ e -> putMVar m e) fd evt M.OneShot
  evt' <- takeMVar m `onException` unregisterFd_ mgr reg
  if evt' `eventIs` evtClose
    then ioError $ errnoToIOError "threadWait" eBADF Nothing Nothing
    else return ()

Although the entire function has asynchronous exceptions masked, the call to takeMVar uses onException to deal with the possibility of an exception. According to the docs in Control.Concurrent, takeMVar can throw exceptions. But my understand (which may be wrong) is that the only exception this could throw would be something like BlockedIndefinitelyOnMVar, which I don't believe can happen here.

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