1. you disable a assert in compile time in dependency of enable_assertions variable. I don't think, so it is good idea. When somebody enables a assertions, then assertions will not work on all cached functions in session. You should to do check if assertions are enabled in execution time (there are no any significant advantage do it in compile time) or you should to clean cache.
2. a failed assert should to raise a exception, that should not be handled by any exception handler - similar to ERRCODE_QUERY_CANCELED - see exception_matches_conditions.
Attached is a patch for supporting assertions in PL/PgSQL. These are similar to the Assert() backend macro: they can be disabled during compile time, but when enabled, abort execution if the passed expression is not true.
A simple example:
CREATE FUNCTION delete_user(username text) RETURNS VOID AS $$ BEGIN DELETE FROM users WHERE users.username = delete_user.username; ASSERT FOUND; END $$ LANGUAGE plpgsql;
SELECT delete_user('mia'); ERROR: Assertion on line 4 failed CONTEXT: PL/pgSQL function delete_user(text) line 4 at ASSERT
Again, I'll add this to the open commitfest, but feedback is greatly appreciated.