[Bug 229741] kevent is not properly adding EVFILT_READ and EVFILT_WRITE for unix sockets when used in one call

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=229741

--- Comment #4 from commit-hook at freebsd.org ---
A commit references this bug:

Author: kevans
Date: Wed Apr 22 03:45:52 UTC 2020
New revision: 360182
URL: https://svnweb.freebsd.org/changeset/base/360182

Log:
  kqueue(2): add a note about EV_RECEIPT

  In the below-referenced PR, a case is attached of a simple reproducer that
  exhibits suboptimal behavior: EVFILT_READ and EVFILT_WRITE being set in the
  same kevent(2) call will only honor the first one. This is, in-fact, how
  it's supposed to work.

  A read of the manpage leads me to believe we could be more clear about this;
  right now there's a logical leap to make in the relevant statement: "When
  passed as input, it forces EV_ERROR to always be returned." -- the logical
  leap being that this indicates the caller should have allocated space for
  the change to be returned with EV_ERROR indicated in the events, or
  subsequent filters will get dropped on the floor.

  Another possible workaround that accomplishes similar effect without needing
  space for all events is just setting EV_RECEIPT on the final change being
  passed in; if any errored before it, the kqueue would not be drained. If we
  made it to the final change with EV_RECEIPT set, then we would return that
  one with EV_ERROR and still not drain the kqueue. This would seem to not be
  all that advisable.

  PR:           229741
  MFC after:    1 week

Changes:
  head/lib/libc/sys/kqueue.2

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