If the session wasn't valid anymore by the time the linter finishes,
writing to "$dir"/fifo would hang forever leaving temporary files in
/tmp/kak-lint.XXX and the process alive. This commit fixes that by
not writing to the fifo if the session was not valid.
throwawayaccount12345-1 Copyright Waiver
I dedicate any and all copyright interest in this software to the
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present and future rights to this software under copyright law.