[usocket-devel] Fwd: WITH-MAPPED-CONDITIONS and other WITH-* macros

[Hans Huebner]

Here is an email I received from Tobias in response to my suggestion
to wrap the body of invocations of WITH-CONNECTED-SOCKET with a
WITH-MAPPED-CONDITIONS.  We discussed his comment afterwards and came
to the conclusion that it will be problematic to get nested occurences
of WITH-CONNECTED-SOCKET to work if the all of the handlers for any of
the involved conditions declines.  For conditions signalled with ERROR
(which is what usocket needs to do anyway) this will be no problem, as
ERROR itself will invoke the debugger if all handlers decline.  With
SIGNAL, the behavior Tobias describes could be observed, which would
clearly be unwanted.

-Hans

---------- Forwarded message ----------
From: Tobias C. Rittweiler <tcr at freebits.de>
Date: 26.03.2008 13:55
Subject: Re: WITH-MAPPED-CONDITIONS and other WITH-* macros
To: Hans Hübner <hans.huebner at gmail.com>


"Hans Hübner" <hans-zv1TXXZFLoBAfugRpC6u6w at public.gmane.org> writes:

 > Hi,
 >
 > how would you feel about adding a WITH-MAPPED-CONDITIONS to the
 > expansion of WITH-CONNECTED-SOCKET?  The reason for that is that it is
 > possible that stream read/write operations on a socket stream generate
 > socket errors that need conversion.
 >
 > Thoughts?


I'm not familiar with the usocket source base, but I found out yesterday
 the hard way that remapping conditions that work properly when being
 nested is entirely not trivial.

  CL-USER> (handler-bind ((warning #'(lambda (w)
                                       (format t "<1> ~S~%" w)
                                       (signal w))))
             (handler-bind ((warning #'(lambda (w)
                                       (format t "<2> ~S~%" w)
                                       (signal (make-condition 'warning)))))
               (warn "FOO")))

  <2> #<SIMPLE-WARNING {B7060A1}> ; created by the (warn "FOO")
  <1> #<WARNING {B706CE1}>        ; created by the (make-condition 'warning)
  <1> #<SIMPLE-WARNING {B7060A1}> ; same as in <2> <-- WE DON'T WANT
THIS TO HAPPEN
  WARNING: FOO
  NIL

 The problem is that handler <2> signals the conditions _anew_, but if
 this newly signalled condition isn't handled anywhere upwards, the
 condition that was originally signalled continues its search for a
 handler further upwards (causing handler <1> to be run twice.)

 The problem is that there's no RESIGNAL operator (or a DECLINE) operator
 in Common Lisp, making this really troublesome.


 My point is that: If WITH-CONNECTED-SOCKET is supposed to work when
 being nested (or if multiple WITH-MAPPED-CONDITIONS can appear in the
 extent of a WITH-CONNECTED-SOCKET), you have to be careful not to screw
 up.

 HTH,

  -T.


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