This is definitely unsafe: we're operating on `conn` (which we don't own) without any lock to prevent someone else doing so at the same time. Namely, by my reading, both the reaper and a new-connection-seeking thread can get here simultaneously -- though a second connection-seeker cannot, because in that path, we hold `@acquire_lock`. (In fact, as reaping involves network traffic, we potentially hold it for some time. Though that shouldn't matter much in practice: *when we started* reaping there wasn't any free connection capacity anyway.)