Pair matching when attribute not present

[Brian Candler]

There was a discussion previously about merging the different versions of
paircompare logic, and I'd just like to mention a related issue.

I was expecting that the following rlm_sql authorize_check_query response
would work if the attribute was not present:

    'Calling-Station-Id', '=~', '^$|^0845'

However it doesn't. It looks like there are two sets of logic which might be
used:

src/lib/valuepair.c: paircmp() expands the argument using vp_prints_value. 
If it's missing then it expands to empty string. In that case, I think the
match would succeed.

src/main/valuepair.c: paircompare() fails immediately if the argument is
non-existent, except when using the !* operator (T_OP_CMP_FALSE).

I haven't yet worked out exactly in which circumstances paircmp() is used
and in which circumstances paircompare() is used :-(

Another usage case:

    'Reply-Message', '!=', 'SPECIALOPTION'

(i.e. I would like this to match if either Reply-Message is not present, or
if its value is not SPECIALOPTION)

It's a matter of discussion as to which is the desired behaviour, and is
maybe something that could be considered for freeradius 3.x

Regards,

Brian.