[Lispweb] small issue in get-multipart-header

[Steve Haflich]

   From: Kamen TOMOV <kamen at evrocom.net>
   
   I haven't test it on SBCL but on CMUCL/x86 there isn't such note:
   
   CL-USER> (handler-case t (t ()))
   T
   
This isn't directly a comment on the original question, but this
handler-case is not good CL style, and may even be dangerously so.

It is customary to think of the condition system as having to do with
errors and warnings, but it was carefully defined to be a general
facility in the execution model.  The handler-case form above handles
_any_ otherwise-unhandled condition signalled within the execution of
the expression.  It is a really poor idea to handle conditions
promiscuously, that is, to handle condition types about which you know
nothing and about whose existence you might not even be aware.  The
implementation or some other loaded module independent of your code
might use some condition subclass disjoint from any of the public
condition classes in order to serve some private communication need.
Code like the above could break that code in was hard to debug, and
perhaps even hard to detect.

It is possible, for instance, that a mp threading system could use
signals behind the programmer's back, or signals could even be used to
handle the first reference to a particular dynamic variable on a new
thread.  A defsys facility might signal conditions to communicate
upward information about actions it takes building subsystems.  I know
of no implementations that do any of these things, but someday there
could be, and it is unwise to write code that is gratuitously
incompatible.

The moral is that CL code should handle only publically documented
condition classes that have a well-established meaning, such as error
and warning, or conditions that the code defines itself and which are
otherwise disjoint subclasses only of the class condition itself.

The following shows a silly use of signals that is not transparent to
the promiscuous handler-case:

cl-user(3): (define-condition tuesday () ())
tuesday
cl-user(4): (defparameter *tuesday* (make-condition 'tuesday))
*tuesday*
cl-user(5): (defun tuesdayp ()
	      (= 1 (nth-value 6 (decode-universal-time (get-universal-time)))))
tuesdayp
cl-user(6): (defun my-random (n)
              ;; Let monitors know if we are called on a Tuesday.
	      (when (tuesdayp) (signal *tuesday*))
	      (random n))
my-random
cl-user(7): (handler-case (my-random 100)
	      (t nil))
nil
cl-user(8): (handler-case (random 100)
	      (t nil))
21