* Robert Haas (robertmh...@gmail.com) wrote:
> On Sun, Jan 17, 2016 at 6:58 PM, Stephen Frost <sfr...@snowman.net> wrote:
> > I'm not against that idea, though I continue to feel that there are
> > common sets of privileges which backup tools could leverage.
> >
> > The other issue that I'm running into, again, while considering how to
> > move back to ACL-based permissions for these objects is that we can't
> > grant out the actual permissions which currently exist.  That means we
> > either need to break backwards compatibility, which would be pretty
> > ugly, in my view, or come up with new functions and then users will have
> > to know which functions to use when.
> >
> > As I don't think we really want to break backwards compatibility or
> > remove existing functionality, the only approach which is going to make
> > sense is to add additional functions in some cases.  In particular, we
> > will need alternate versions of pg_terminate_backend and
> > pg_cancel_backend.  One thought I had was to make that
> > 'pg_signal_backend', but that sounds like we'd allow any signal sent by
> > a user with that right, which seems a bit much to me...
> So, this seems like a case where a built-in role would be
> well-justified.  I don't really believe in built-in roles as a way of
> bundling related permissions; I know you do, but I don't.  I'd rather
> see the individual function permissions granted individually.  But
> here you are talking about a variable level of access to the same
> function, depending on role.  And for that it seems to me that a
> built-in role has a lot more to recommend it in that case than it does
> in the other.  If you have been granted pg_whack, you can signal any
> process on the system; otherwise just your own.  Those checks are
> internal to pg_terminate_backend/pg_cancel_backend so GRANT is not a
> substitute.

If we extend this into the future then it seems to potentially fall
afoul of Noah's concern that we're going to end up with two different
ways of saying GRANT EXECUTE X ON Y.  Consider the more complicated case
of pg_stat_activity, where values are shown or hidden based on who the
current role is.  The policy system only supports whole-row, today, but
the question has already come up, both on the lists and off, of support
for hiding individual column values for certain rows, exactly as we do
today for pg_stat_activity.  Once we reach that point, we'll have a way
to GRANT out these rights and a default role which just has them.

Personally, I don't have any particular issue having both, but the
desire was stated that it would be better to have the regular
GRANT EXECUTE ON catalog_func() working before we consider having
default roles for same.  That moves the goal posts awful far though, if
we're to stick with that and consider how we might extend the GRANT
system in the future.

What got me thinking along these lines was a question posed by Bruce
(Bruce, feel free to chime in if I've misunderstood) to me at SCALE
where we were chatting a bit about this, which was- how could we extend
GRANT to support the permissions that we actually wish
pg_terminate_backend() and pg_cancel_backend() to have, instead of using
a default role?  I didn't think too much on it at the time as it strikes
me as a pretty narrow use-case and requiring quite a bit of complexity
to get right, but there again, I'd certainly view a system where the
user is allowed to have pg_start_backup() rights but not
pg_stop_backup() as being quite a small use-case also, yet that's the
direction we're largely going in with this discussion.



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