Related: #74

This is a minimal reproduction of the issue we're seeing.

  describe "withException" $ do
    it "should work when withAsync is in the handler" $ do
      let
        action =
          error "oops"
            `onException` do
              let
                timerAction n = do
                  threadDelay 1000000
                  when (n < 10) $ do
                    timerAction (n + 1)
              withAsync (timerAction 0) $ \a -> do
                cancel a
      action
        `shouldThrow`
          errorCall "oops"

This should finish instantaneously, but it actually waits for timerAction to complete - all 10 seconds of it. cancel is therefore broken inside of a withException or onException or even a bracket call.

What's very surprising about this behavior is that catch doesn't share it. catch works exactly like you'd expect, even though a very simple implementation of withException is withException action handler = catch action (\e -> handler e >> throwIO e).