PROC_SET_PDEATHSIG to get a signal when your parent exits

[Thomas Munro]

Hello,

In PostgreSQL we are planning to make use of Linux's
prctl(PR_SET_PDEATHSIG) facility which tells the kernel to signal you
when your parent process exits.  This will cut down on polling and
contention on a pipe in certain busy loops, where we need to be able
to exit quickly if something kills the main supervisor process but we
aren't waiting in kevent/epoll/poll-type primitive.  We considered
SIGIO but that'd require either one pipe per child process or a common
process group, neither of which works for us.  So I decided to try
adding a new command PROC_SET_PDEATHSIG to procctl() to work exactly
like Linux.

Please see attached early draft patch.  This is my first attempt at
writing a patch for FreeBSD, so I expect that it contains newbie
mistakes and style problems and I would be grateful for any feedback.
Is this design sane?  Is that the best syscall to use for this?  Did I
get the ptrace/orphan stuff right?  Should this somehow share
something with the Linux compat support for prctl(PR_SET_PDEATHSIG,
...)?  Would people want this?

Here's a quick and dirty userspace program I came up with while
developing the patch:

https://github.com/macdice/test-deathsig/blob/master/test-deathsig.c

That tests a few interesting cases I thought of, but note that I
haven't yet tested 32 bit support or the setuid/getuid logic.

Thanks for reading,

Thomas Munro
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