Re: [HACKERS] proposal: session server side variables

My 0.02€ to try to illustrate a possible private session variable based 
implementation for this use case:

> * Session starts

\c app

> * app does SELECT setup_user('user-auth-key-data', 'some-other-blob')

SELECT setup_user('fjshdfjkshfjks', 'jklfsjfklsjfk');

> **  setup_user is SECURITY DEFINER to 'appadmin'

-- appadmin did:
CREATE FUNCTION setup_user(TEXT, TEXT)  RETURNS BOOLEAN SECURITY DEFINER AS $$

> **  'appadmin' owns a variable IS_AUDITOR. Other roles have only read
> access to it.
  not sure how it is used afterwards... is it the same as USER_IS_AUDITOR?

> **  setup_user(...) does whatever expensive/slow work it has to do
    ... checks, updates, whatever

> **   setup_user sets USER_IS_AUDITOR var
    -- declare a private session variable    DECLARE @user_is_auditor BOOLEAN PRIVATE;    -- set its value to whatever
appropriate   SET @user_is_auditor = ???;    --- returns its value    RETURN @user_is_auditor;
 
$$ LANGUAGE xxx;

> * Later RLS policies simply reference USER_IS_AUDITOR var. They don't
> need to know the 'user-auth-key-data', or do whatever expensive
> processing that it does.

-- appadmin did:
CREATE FUNCTION isUserAuditor()   RETURNS BOOLEAN SECURITY DEFINER AS $$   -- say variable is just confirmed if it
existsalready in session?   DECLARE @user_is_auditor BOOLEAN PRIVATE;   RETURN @user_is_auditor;
 
$$ LANGUAGE xxx;

> * Other later triggers, etc, also reference USER_IS_AUDITOR

The variable is not directly referenced, one would have to call 
isUserAuditor() to access the private session value, but then you can 
GRANT/REVOKE whatever you want on the access function.

> * User cannot make themselves an auditor by SETting USER_IS_AUDITOR

Indeed, the user cannot access the private variable, only appadmin can, 
and probably root could.

The user could create its own private session variable @user_is_auditor, 
or a public session variable of the same name. That would be distinct 
variables which would not influence isUserAuditor which would use its own.

-- 
Fabien.