pop filters from kqueue

[Dirk Engling]

Dear fellow FreeBSD hackers,

while writing a daemon that uses a kqueue to keep track of forked 
processes and pipes to userland client code etc, I noticed a lack of 
features to implement a proper shutdown without holding data redundantly.

When my daemon quits, I can not ask the kqueue for my installed filters 
and get back the udata I passed to the kqueue.

This is unfortunate, because I like the idea of having only one owner per 
memory allocation. The most obvious use would be a per-fd-state held in a 
memory block. When passing it to kevent() via the udata entry, I would 
make this filter the owner of my allocation.

However, when gracefully shutting down, my daemon has no way of retrieving 
all the values passed to the filters. For most cases that may be okay:
memory allocations will just be thrown away on exit(), anyway.

But once I need to clean up external state, like sending signals to 
processes I installed an EVFILT_PROC for etc, I need to keep a redundant 
list of pids and the associated udata. This violates the rule of strict 
ownership and introduces room for inconsistencies.

Is there a specific reason I have overlooked that would forbid popping 
untriggered filters from my kqueue? Or is there even a way to do so that I 
have missed?

Regards,

   erdgeist