[ic] Cautious eval error handling

[Jon Jensen]

Interchange users,

A few months ago I had an adventure tracking down a very obscure bug in 
Interchange that was caused by a check for $@ too long after an eval was 
called. The biggest reason it was hard to find was that even a logging 
command had an eval that reset $@, making later checks fail. The fix was 
here:

http://git.icdevgroup.org/?p=interchange.git;a=commitdiff;h=9f9d7c578da699776722641ec3c1edd4b51f3a99

That reinforces that it's good hygiene to only check $@ immediately after 
the routine that sets it. And any routine expected to set it should 
guarantee that it's cleared when there's no error.

I decided it would be good to go through the whole Interchange codebase 
looking for any potential problems, and clean them up to save 
troubleshooting and unexpected corner case failures later.

I'm only about halfway through and am committing most changes separately 
so that they're small and have per-case explanations. I've pushed the work 
so far to a new GitHub branch:

http://github.com/jonjensen/interchange/commits/cautious-eval-errors/

Many of the changes should make no difference at all right now, but could 
prevent later problems and/or clarify the intent of the code.

Others change the way code checks for errors, trying to avoid $@. So far 
the only functions I think are valid for setting $@ are eval, Safe's 
reval, and Interchange's string_to_ref. When I come across others I'm 
trying to have Interchange use the return code & whatever API exists 
rather than rely on $@.

I'd appreciate any feedback or offers to help review & test, if any of you 
is interested.

Thanks,
Jon

-- 
Jon Jensen
End Point Corporation
http://www.endpoint.com/