HLS's tests sometimes trigger Language server unexpectedly terminated,
which means the pipe might be closed before we cancel the consuming thread.

I believe I find one of the cause.

runSession' :: ...
...
...
...
        finally (timeout msgTimeoutMs (runSession' exitServer))
                -- Make sure to kill the listener first, before closing
                -- handles etc via cleanupProcess
                (killThread tid >> cleanup)

Since killThread tid = throwTo tid ThreadKilled, it won't wait for the target thread to exit.
Fix can be trival, simply use async instead of forkIO and use cancel instead of killThread