Formal comment #175 (defect)

returning exception handlers easily trigger infinite loops
Reported by: 	Robby Findler
Version:	5.92

As I understand the current informal spec (with help from Mike) and as
is shown in the operational semantics, exception handlers that return
to a `raise' very easily get into infinite loops. (Indeed, without
effects or dispatching on the exception itself, it seems impossible to
avoid an infinite loop when a handler returns.)

For example these programs:

  (with-exception-handler (lambda (x) x) (lambda () (raise 1)))
  (with-exception-handler (lambda (x) x) (lambda () (car '())))

both result in infinite loops. (Try it out! :)

I believe that there is a simple fix to this, namely to make the
implicit raise (the one that gets triggered when the handler returns)
be in the same context as call to the handler. That is, in the handler
context that does not contain the handler that just returned.

From the operational semantics point of view, it means changing the
begin expression in the 6xraise rule from this:

(begin (handlers v_1 ... (v_2 v_3)) (raise (condition "handler returned")))

to this:

(handlers v_1 ... (begin (v_2 v_3) (raise (condition "handler returned"))))

Robby


RESPONSE:

The specification of `raise' in section 6.1 of R5.92RS is actually
ambiguous on this issue, and allows the suggested semantics.  The next
revision of the report will tighten the specification to mandate the
suggested semantics.