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Fix exn dispatch for exns within pre-unwind handlers

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* libguile/exceptions.c (exception_epoch_fluid): Rename from
active_exception_handlers_fluid.
(scm_dynwind_throw_handler): Increment exception epoch instead of
resetting active exception handlers.
(scm_init_exceptions): Update.
* module/ice-9/boot-9.scm  (with-exception-handler): Rework to associate
an "epoch" fluid with each exception handler.
(with-throw-handler): Establish a new epoch, during the execution of a
throw handler.
(raise-exception): Rework to avoid capturing a list of exception
handlers, and to use epochs as a way to know which handlers have already
been examined and which are on the dispatch stack.
* test-suite/tests/exceptions.test ("throwing within exception
handlers"): New test.
This commit is contained in:
Andy Wingo 2023-06-07 22:26:05 +02:00
parent ee18ca9a35
commit 02dfb6e776
3 changed files with 81 additions and 46 deletions
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11
libguile/exceptions.c
View file

@ -1,4 +1,4 @@
/* Copyright 1995-1998,2000-2001,2003-2004,2006,2008,2009-2014,2017-2019
/* Copyright 1995-1998,2000-2001,2003-2004,2006,2008,2009-2014,2017-2019,2023
Free Software Foundation, Inc.
This file is part of Guile.
@ -109,7 +109,7 @@ call_exception_handler (SCM clo, SCM exn)
SCM_KEYWORD (kw_unwind_p, "unwind?");
SCM_KEYWORD (kw_unwind_for_type, "unwind-for-type");
static SCM exception_handler_fluid;
static SCM active_exception_handlers_fluid;
static SCM exception_epoch_fluid;
static SCM with_exception_handler_var;
static SCM raise_exception_var;
@ -257,7 +257,8 @@ exception_has_type (SCM exn, SCM type)
void
scm_dynwind_throw_handler (void)
{
scm_dynwind_fluid (active_exception_handlers_fluid, SCM_BOOL_F);
SCM depth = scm_oneplus (scm_fluid_ref (exception_epoch_fluid));
scm_dynwind_fluid (exception_epoch_fluid, depth);
}
@ -499,11 +500,11 @@ scm_init_exceptions ()
scm_set_smob_apply (tc16_exception_handler, call_exception_handler, 1, 0, 0);
exception_handler_fluid = scm_make_thread_local_fluid (SCM_BOOL_F);
active_exception_handlers_fluid = scm_make_thread_local_fluid (SCM_BOOL_F);
exception_epoch_fluid = scm_make_fluid_with_default (SCM_INUM1);
/* These binding are later removed when the Scheme definitions of
raise and with-exception-handler are created in boot-9.scm. */
scm_c_define ("%exception-handler", exception_handler_fluid);
scm_c_define ("%active-exception-handlers", active_exception_handlers_fluid);
scm_c_define ("%exception-epoch", exception_epoch_fluid);
with_exception_handler_var =
scm_c_define ("with-exception-handler", SCM_BOOL_F);

76
module/ice-9/boot-9.scm
View file

@ -1586,8 +1586,7 @@ exception that is an instance of @var{rtd}."
val))
(define %exception-handler (steal-binding! '%exception-handler))
(define %active-exception-handlers
(steal-binding! '%active-exception-handlers))
(define %exception-epoch (steal-binding! '%exception-epoch))
(define %init-exceptions! (steal-binding! '%init-exceptions!))
(%init-exceptions! &compound-exception
@ -1639,13 +1638,6 @@ If @var{continuable?} is true, the handler is invoked in tail position
relative to the @code{raise-exception} call. Otherwise if the handler
returns, a non-continuable exception of type @code{&non-continuable} is
raised in the same dynamic environment as the handler."
(define (capture-current-exception-handlers)
;; FIXME: This is quadratic.
(let lp ((depth 0))
(let ((h (fluid-ref* %exception-handler depth)))
(if h
(cons h (lp (1+ depth)))
(list fallback-exception-handler)))))
(define (exception-has-type? exn type)
(cond
((eq? type #t)
@ -1656,10 +1648,11 @@ raised in the same dynamic environment as the handler."
(and (exception? exn)
((exception-predicate type) exn)))
(else #f)))
(let lp ((handlers (or (fluid-ref %active-exception-handlers)
(capture-current-exception-handlers))))
(let ((handler (car handlers))
(handlers (cdr handlers)))
(let ((current-epoch (fluid-ref %exception-epoch)))
(let lp ((depth 0))
;; FIXME: fluid-ref* takes time proportional to depth, which
;; makes this loop quadratic.
(let ((val (fluid-ref* %exception-handler depth)))
;; There are two types of exception handlers: unwinding handlers
;; and pre-unwind handlers. Although you can implement unwinding
;; handlers with pre-unwind handlers, it's better to separate them
@ -1667,24 +1660,33 @@ raised in the same dynamic environment as the handler."
;; overflow" or "out of memory" to unwind the stack before calling
;; a handler.
(cond
((pair? handler)
(let ((prompt-tag (car handler))
(type (cdr handler)))
((not val)
;; No exception handlers bound; use fallback.
(fallback-exception-handler exn))
((fluid? (car val))
(let ((epoch (car val))
(handler (cdr val)))
(cond
((exception-has-type? exn type)
(abort-to-prompt prompt-tag exn)
(error "unreachable"))
(else
(lp handlers)))))
(else
(with-fluids ((%active-exception-handlers handlers))
((< (fluid-ref epoch) current-epoch)
(with-fluids ((epoch current-epoch))
(cond
(continuable?
(handler exn))
(else
(handler exn)
(raise-exception
((record-constructor &non-continuable)))))))))))
((record-constructor &non-continuable)))))))
(else
(lp (1+ depth))))))
(else
(let ((prompt-tag (car val))
(type (cdr val)))
(cond
((exception-has-type? exn type)
(abort-to-prompt prompt-tag exn)
(error "unreachable"))
(else
(lp (1+ depth)))))))))))
(define* (with-exception-handler handler thunk #:key (unwind? #f)
(unwind-for-type #t))
@ -1748,8 +1750,9 @@ exceptions with the given @code{exception-kind} will be handled."
(lambda (k exn)
(handler exn)))))
(else
(with-fluids ((%exception-handler handler))
(thunk)))))
(let ((epoch (make-fluid 0)))
(with-fluids ((%exception-handler (cons epoch handler)))
(thunk))))))
(define (throw key . args)
"Invoke the catch form matching @var{key}, passing @var{args} to the
@ -1771,11 +1774,30 @@ for key @var{k}, then invoke @var{thunk}."
"Wrong type argument in position ~a: ~a"
(list 1 k) (list k)))
(define running? (make-fluid))
;; Throw handlers have two semantic oddities.
;;
;; One is that throw handlers are not re-entrant: if one is
;; already active in the current continuation, it won't handle
;; exceptions thrown within that continuation. It's a restrictive
;; choice, but it does ensure progress. We ensure this property
;; by having a running? fluid associated with each
;; with-throw-handler.
;;
;; The other oddity is that any exception thrown within a throw
;; handler starts the whole raise-exception dispatch procedure
;; again from the top. This can have its uses if you want to have
;; handlers for multiple specific keys active at the same time,
;; without specifying an order between them. But, it's weird. We
;; ensure this property by having a %exception-epoch fluid and
;; also associating an epoch with each pre-unwind handler; a
;; handler is active if its epoch is less than the current
;; %exception-epoch. We increment the epoch with the extent of
;; the throw handler.
(with-exception-handler
(lambda (exn)
(when (and (or (eq? k #t) (eq? k (exception-kind exn)))
(not (fluid-ref running?)))
(with-fluids ((%active-exception-handlers #f)
(with-fluids ((%exception-epoch (1+ (fluid-ref %exception-epoch)))
(running? #t))
(apply pre-unwind-handler (exception-kind exn)
(exception-args exn))))

12
test-suite/tests/exceptions.test
View file

@ -17,6 +17,7 @@
(define-module (test-suite exceptions)
#:use-module (ice-9 control)
#:use-module (test-suite lib))
(define-syntax-parameter push
@ -392,3 +393,14 @@
(let* ((thunk1 (catch* 'foo (lambda () (throw 'bar))))
(thunk2 (catch* 'bar (lambda () (thunk1)))))
(thunk2))))
(with-test-prefix "throwing within exception handlers"
(pass-if "https://github.com/wingo/fibers/issues/76"
(let/ec return
(with-exception-handler
(lambda (e)
(catch #t
(lambda () (error "bar"))
(lambda args #f))
(return #t))
(lambda () (error "foo"))))))