The problem is that read_pg_options needs DataDir to read its file but

DataDir is set after read_pg_options if postgres is called
interactively.  If postgres is forked by postgres DataDir is read from
the PGDATA enviromnent variable set by the postmaster and this explains
while the bug disappears.  I have written this patch but I don't like
it. Any better idea?

Massimo Dal Zotto