Interest in a SECURITY DEFINER function current_user stack access mechanism?

It'd be nice if SECURITY DEFINER functions could see what user invoked
them, but current_user is the DEFINER user, naturally, since that's how
this is done in fmgr_security_definer().

I was thinking that fmgr_security_definer() could keep a global pointer
to a linked list (with automatic nodes) of the save_userid values. Then
we could have a SQL function for accessing these, something like
pg_current_user(level int) returning text, where level 0 is
current_user, level 1 is "the previous current_user in the stack", and
so on, returning null when level is beyond the top-level.

This seems like a simple, small, easy patch, and since I [think I] need
it I suspect others probably do as well.

Thoughts?

Nico
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