Re: 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 $$
-- say variable is just confirmed if it exists already in session?
DECLARE @user_is_auditor BOOLEAN PRIVATE;
RETURN @user_is_auditor;
$$ 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.