Function Permissions - Graphql Engine

Before we supported volatile functions, the permissions for functions are automatically inferred. We allowed functions when these conditions are met:

The function has to be stable/immutable
The function should return a table type

A stable/immutable function f returning a table type t is exposed for a role r if there is a select permission defined on table t for role r. The rationale behind this decision is that a stable/immutable function does not modify the database and the data returned by the function is filtered using the permissions that are specified precisely for that data.

Now consider mutable/volatile functions, we can't automatically infer whether or not these functions should be exposed for the sole reason that they can modify the database. This necessitates a permission system for functions.

Permissions on functions

Going forward, a function f, stable or volatile is only exposed for a role r if there is a permission defined on the function f for the role r. A permission should only be allowed if there is a select permission on the table type.

Function permission

yaml

type: create_function_permission
args:
  function: <function_name>
  role: <role_name>
  definition: {}

Inside the metadata, it be as follows:

yaml

version: 3
sources:
  - name: default
    functions:
    - function: some_function
      configuration: {}
      permissions:
      - role: some_role
        definition: {}

definition is empty for now but we'll have more options as we extend the feature set.

Backwards compatibility

Currently stable/immutable functions are exposed automatically, so we need to preserve this behaviour. So, we can introduce a new flag --infer-function-permissions, the presence of which is an explicit indication from the user that they want stable/immutable functions which return a table type to be exposed automatically like before. Note that if the user specifies permission for a stable function, that is preferred over auto inferred permission (currently since the permission definition doesn't have anything, this amounts to the same thing).

Future work

Relax the restrictions on the functions that can be tracked. We can allow tracking functions that return a scalar in addition to functions that return a table type.
Argument presets, can be configured per role for each function. The definition will look like this:
yaml
```
 definition:
   argument_presets:
     arg1: ...
     arg2: ...
```
Ability to override the filter of the select permission defined on functions returning table types. <TODO: motivate the use case>