This change generalizes the implementation of a `resource` to many
different kinds of resources, though there is still only one
kind (`data`). In the future, we also expect resource kinds for roles,
user-defined functions (UDFs), and possibly on particular REST
end-points.
I considered several approaches to generalizing to different kinds of
resources.
One approach is to have a base class that is inherited from by different
resource kinds. The common functionality would be accessed through
virtual member functions and kind-specific functions would exist in
sub-classes. I rejected this approach because dealing with different
kinds of resources uniformly requires storage and life-time management
through something like `std::unique_ptr<auth::resource>`, which means
that we lose value semantics (including comparison) and must deal with
complications around ownership.
Another option was to use `boost::variant` (or, in future,
`std::variant`). This is closer to what we want, since there a static
set of resource kinds that we support. I rejected this approach for two
reasons. The first is that all resource kinds share the same data (a
list of segments and a root identifier), which would be duplicated in
each type that composed the variant. The second is that the complexity
and source-code overhead of `boost::variant` didn't seem warranted.
The solution I ended up with is home-grown variant. All resources are
described in the same `final` class: `auth::resource`. This class has
value semantics, supports equality comparison, and has a strict
ordering. All resources have in common a tag ("kind") and a list of
parts. Most operations on resources don't care about the kind of
resource (like getting its name, parsing a name, querying for the
parent, etc). These are just member functions of the class.
When we care about a kind-specific interpretation of a resource, we can
produce a "view" of the resource. For example, `data_resource_view`
allows for accessing the (optional) keyspace and table names.
I anticipate in the future to add functions for creating role
resources (`auth::resource::role`) and also `role_resource_view`.
The functional behaviour of the system should be unchanged with this
patch.
I've added new unit tests in `auth_resource_test.cc` and removed the old
test from `auth_test.cc`.
Fixes#3027.
1bb22bb190