[v9.2] Object access hooks with arguments support (v1)

The attached patch is a draft to support arguments in addition to
OAT_* enum and object identifiers.

The existing object_access_hook enables loadable modules to acquire
control when objects are referenced. The first guest of this hook is
contrib/sepgsql for assignment of default security label on newly
created objects. Right now, OAT_POST_CREATE is the all supported
object access type. However, we plan to enhance this mechanism onto
other widespread purpose; such as comprehensive DDL permissions
supported by loadable modules.

This patch is a groundwork to utilize this hook for object creation
permission checks, not only default labeling.
At the v9.1 development cycle, I proposed an idea that defines both
OAT_CREATE hook prior to system catalog modification and
OAT_POST_CREATE hook as currently we have. This design enables to
check permission next to the existing pg_xxx_aclcheck() or
pg_xxx_ownercheck(), and raise an error before system catalog updates.
However, it was painful to deliver private datum set on OAT_CREATE to
the OAT_POST_CREATE due to the code complexity.

The other idea tried to do all the necessary things in OAT_POST_CREATE
hook, and it had been merged, because loadable modules can pull
properties of the new object from system catalogs by the supplied
object identifiers. Thus, contrib/sepgsql assigns a default security
label on new object using OAT_POST_CREATE hook.
However, I have two concern on the existing hook to implement
permission check for object creation. The first one is the entry of
system catalog is not visible using SnaphotNow, so we had to open and
scan system catalog again, instead of syscache mechanism. The second
one is more significant. A part of information to make access control
decision is not available within contents of the system catalog
entries. For example, we hope to skip permission check when
heap_create_with_catalog() was launched by make_new_heap() because the
new relation is just used to swap later.

Thus, I'd like to propose to support argument of object_access_hook to
inform the loadable modules additional contextual information on its
invocations; to solve these concerns.

Regarding to the first concern, fortunately, most of existing
OAT_POST_CREATE hook is deployed just after insert or update of system
catalogs, but before CCI. So, it is helpful for the loadable modules
to deliver Form_pg_xxxx data to reference properties of the new
object, instead of open and scan the catalog again.
In the draft patch, I enhanced OAT_POST_CREATE hook commonly to take
an argument that points to the Form_pg_xxxx data of the new object.

Regarding to the second concern, I added a few contextual information
as second or later arguments in a part of object classes. Right now, I
hope the following contextual information shall be provided to
OAT_POST_CREATE hook to implement permission checks of object
creation.

* pg_class - TupleDesc structure of the new relation
I want to reference of pg_attribute, not only pg_class.

* pg_class - A flag to show whether the relation is defined for
rebuilding, or not.
I want not to apply permission check in the case when it is invoked
from make_new_heap(), because it just create a dummy table as a part
of internal process. All the necessary permission checks should be
done at ALTER TABLE or CLUSTER.

* pg_class - A flag to show whether the relation is created with
SELECT INTO, or not.
I want to check "insert" permission of the new table, created by
SELECT INTO, because DML hook is not available to check this case.

* pg_type - A flag to show whether the type is defined as implicit
array, or not.
I want not to apply permission check on creation of implicit array type.

* pg_database - Oid of the source (template) database.
I want to fetch security label of the source database to compute a
default label of the new database.

* pg_trigger - A flag to show whether the trigger is used to FK
constraint, or not.
I want not to apply permission check on creation of FK constraint. It
should be done in ALTER TABLE level.

Sorry for this long explanation. Right now, I tend to consider it is
the best way to implement permission checks on object creation with
least invasive way.

Thanks, Any comments welcome.
--
KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>