Comment by o11c

> Because the Linux kernel coalesces SIGCHLD (and other signals), the only way to reliably determine if a monitored process has exited, is to loop through all PIDs registered by any kqueue when we receive a SIGCHLD. This involves many calls to waitid(2) and may have a negative performance impact.

This is somewhat wrong. To speed things up in the happy case (where we are the only part of the program that is spawning children), you can just do a `WNOHANG` wait for any child first, and check if it's one of the children we care about. Only if it's an unknown child do you have to do the full loop (of course, if you only have a couple of children the loop may be better).