Restarting loop from exception handler breaks with-handlers, Ctrl+C?

[William J. Bowman]

I have a little model of a server below. The server handles all exceptions, and restarts itself after some errors. It also handles break?, to actually exit when requested.

While this server is running, if I press Ctrl+C before `(error 'foo)` is handled, the server exits correctly with 120. However, if I press Ctrl+C after `(error 'foo)` is handled, the `exn:break?` handler is never called, the server no longer responds to `Ctrl+C`, and (in this example) it eventually exits with status `1`.

I cannot figure out why. As far as I can tell, this should always handle `Ctrl+C` by exiting with 120.

> #lang racket/base
>
> (let loop ([n 0])
> (with-handlers ([exn:break? (lambda _ (exit 120))]
> [void
> (lambda (exn)
> (printf "handled error: ~a~n" (exn-message exn))
> (loop (add1 n)))])
> (displayln n)
> (sleep 1)
> (cond
> [(zero? n) (error 'foo)]
> [(= n 5) (exit 1)])
> (loop (add1 n))))

--
William J. Bowman

[Sorawee Porncharoenwase]

I think you want with-handler*? According to the docs:

with-handlers:

Before any predicate or handler procedure is invoked, the continuation of the entire with-handlers expression is restored, but also parameterize-breaked to disable breaks. Thus, breaks are disabled by default during the predicate and handler procedures (see Breaks), and the exception handler is the one from the continuation of the with-handlers expression.

with-handler*

Like with-handlers, but if a handler-expr procedure is called, breaks are not explicitly disabled, and the handler call is in tail position with respect to the with-handlers* form.

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[Sorawee Porncharoenwase]

Typo: with-handlers*

[William J. Bowman]

Ah of course, thanks!

On Thu, Oct 07, 2021 at 05:52:55PM -0700, Sorawee Porncharoenwase wrote:
> Typo: with-handlers*
>
> On Thu, Oct 7, 2021 at 5:52 PM Sorawee Porncharoenwase <
> sorawe...@gmail.com> wrote:
>
> > I think you want with-handler*? According to the docs:
> >
> > with-handlers:
> >
> > Before any predicate or handler procedure is invoked, the continuation of
> > the entire with-handlers

> > <https://docs.racket-lang.org/reference/exns.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._with-handlers%29%29>

> > expression is restored, but also parameterize-break

> > <https://docs.racket-lang.org/reference/breakhandler.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize-break%29%29>ed

> > to disable breaks. Thus, breaks are disabled by default during the
> > predicate and handler procedures (see Breaks

> > <https://docs.racket-lang.org/reference/breakhandler.html>), and the

> > exception handler is the one from the continuation of the with-handlers

> > <https://docs.racket-lang.org/reference/exns.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._with-handlers%29%29>
> > expression.
> >
> > with-handler*
> >
> > Like with-handlers
> > <https://docs.racket-lang.org/reference/exns.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._with-handlers%29%29>,

> > but if a handler-expr procedure is called, breaks are not explicitly
> > disabled, and the handler call is in tail position with respect to the
> > with-handlers*

> > <https://docs.racket-lang.org/reference/exns.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._with-handlers%2A%29%29>