ExecutorCheckPerms() hook

In yesterday's development meeting, we talked about the possibility of
a basic SE-PostgreSQL implementation that checks permissions only for
DML.  Greg Smith offered the opinion that this could provide much of
the benefit of SE-PostgreSQL for many users, while being much simpler.
 In fact, SE-PostgreSQL would need to get control in just one place:
ExecCheckRTPerms.  This morning, Stephen Frost and I worked up a quick
patch showing how we could add a hook here to let a hypothetical
SE-PostgreSQL module get control in the relevant place.  The attached
patch also includes a toy contrib module showing how it could be used
to enforce arbitrary security policy.

I don't think that this by itself would be quite enough framework for
a minimal SE-PostgreSQL implementation - for that, you'd probably need
an object-labeling facility in core which SE-PostgreSQL could leverage
- or else some other way to determine which the label associated with
a given object - but I think that plus this would be enough.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company