I'm starting this now because it's starting to annoy me. 
013fc02a23e7861dfb879e4bea5cf02d0562a509 is the git commit that introduces this problem. The commit immediately preceding, 3424ed7d428b604616fabd2c73620a49348baa21 works as expected.
that's probably about waking main thread, something pthread_kill did in the past, can't think about anything else. I was not sure socket shutdown would work, yet neither buildbot, nor our macOS folks would complain. There is already some `termination_event_fd` for Linux for systemd by Daniel Black, to avoid socket shutdowns (bad for systemd "socket activation") . that thing works with eventfd. I think it is more portable way would be good old Unix self-pipe.