[Patch] kqueue(2) <-> procdesc(4): EVFILT_PROCDESC

[Ed Schouten]

Hi Robert,

The other day I was playing around with procdesc(4). Quite a nice
piece of work. I did notice, though, that there is no way you can use
kqueue(2) to monitor process descriptors. That's quite a shame,
because it would be awesome if we could just use EVFILT_PROC on
process descriptors directly. This is why I thought it would be nice
to introduce a variant called EVFILT_PROCDESC.

I initially tried altering kqueue(2) in such a way that
EVFILT_PROCDESC would use a mixture of filt_fileattach() to attach to
the descriptor and filt_proc() to watch for events, but this approach
didn't really work out all that well, for the reason that a kevent
either has a file descriptor or a process associated; not both.

In the end I decided to not make things more complex than needed and
just implement it like a regular file descriptor probe. This means
that we can get NOTE_EXIT to work, but NOTE_FORK, NOTE_EXEC and
NOTE_TRACK would require some more work.

What are your thoughts on the following patch?

http://80386.nl/pub/kqueue-evfilt-procdesc.txt

Some notes on this patch:

- I decided to just reuse the obsolete EVFILT_NETDEV. EVFILT_PROCDESC
will be used on completely different file descriptor types, so I can't
think of a way this would cause ABI issues.

- pd->pd_proc is protected by proctree_lock. It's a bit hard to pick
this up inside of the kqfilter, so simply make procdesc_exit() copy
out pd->pd_proc->p_xstat. We also want this copy, because we don't
want to run into a race condition where wait4() already reaps the
process before the kqfilter is called. This is works for EVFILT_PROC
anyway.

The nice thing about this patch is that even though pdwait4(2) is
still unimplemented, it does at least allow people to now extract the
exit code without accessing any global namespaces.

Thanks,
-- 
Ed Schouten <ed at 80386.nl>