sunet/scheme/httpd/handle-fatal-error.scm

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;;; Handle fatal errors in a sensible way. -*- Scheme -*-
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;;; This file is part of the Scheme Untergrund Networking package.
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;;; Copyright (c) 1995 by Olin Shivers.
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;;; For copyright information, see the file COPYING which comes with
;;; the distribution.
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;;; (with-fatal-error-handler* handler thunk)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Call THUNK, and return whatever it returns. If THUNK signals a condition,
;;; and that condition is an error condition (or a subtype of error), then
;;; HANDLER gets a chance to handle it.
;;; The HANDLER proc is applied to two values:
;;; (HANDLER condition decline)
;;; HANDLER's continuation is WITH-FATAL-ERROR-HANDLER*'s; whatever HANDLER
;;; returns is returned from WITH-FATAL-ERROR-HANDLER. HANDLER declines to
;;; handle the error by throwing to DECLINE, a nullary continuation.
;;;
;;; Why is it called with-FATAL-error-handler*? Because returning to the
;;; guy that signalled the error is not an option.
;;;
;;; Why the nested outer pair of CALL/CC's? Well, what happens if the user's
;;; error handler *itself* raises an error? This could potentially give
;;; rise to an infinite loop, because WITH-HANDLER runs its handler in
;;; the original condition-signaller's context, so you'd search back for a
;;; handler, and find yourself again. For example, here is an infinite loop:
;;;
;;; (with-handler (lambda (condition more)
;;; (display "Loop!")
;;; (error "ouch")) ; Get back, Loretta.
;;; (lambda () (error "start me up")))
;;;
;;; I could require W-F-E-H* users to code carefully, but instead I make sure
;;; the user's fatal-error handler runs in w-f-e-h*'s handler context, so
;;; if it signals a condition, we'll start the search from there. That's the
;;; point of continuation K. When the original thunk completes successfully,
;;; we dodge the K hackery by using ACCEPT to make a normal return.
(define (with-fatal-error-handler* handler thunk)
(call-with-current-continuation
(lambda (accept)
((call-with-current-continuation
(lambda (k)
(with-handler (lambda (condition more)
(if (error? condition)
(call-with-current-continuation
(lambda (decline)
(k (lambda () (handler condition decline))))))
(more)) ; Keep looking for a handler.
(lambda () (call-with-values thunk accept)))))))))
(define-syntax with-fatal-error-handler
(syntax-rules ()
((with-fatal-error-handler handler body ...)
(with-fatal-error-handler* handler
(lambda () body ...)))))
;This one ran HANDLER in the signaller's condition-handler context.
;It was therefore susceptible to infinite loops if you didn't code
;your handler's carefully.
;
;(define (with-fatal-error-handler* handler thunk)
; (call-with-current-continuation
; (lambda (accept)
; (with-handler (lambda (condition more)
; (if (error? condition)
; (call-with-current-continuation
; (lambda (decline)
; (accept (handler condition decline)))))
; (more)) ; Keep looking for a handler.
; thunk))))
;;; (%error-handler-cond kont eh-clauses cond-clauses)
;;; Transform error-handler clauses into COND clauses by wrapping continuation
;;; KONT around the body of each e-h clause, so that if it fires, the result
;;; is thrown to KONT, but if no clause fires, the cond returns to the default
;;; continuation.
;(define-syntax %error-handler-cond
; (syntax-rules (=> else)
;
; ((%error-handler-cond kont ((test => proc) clause ...) (ans ...))
; (%error-handler-cond kont
; (clause ...)
; ((test => (lambda (v) (kont (proc v)))) ans ...)))
;
; ((%error-handler-cond kont ((test body ...) clause ...) (ans ...))
; (%error-handler-cond kont
; (clause ...)
; ((test (kont (begin body ...))) ans ...)))
;
; ((%error-handler-cond kont ((else body ...)) (ans-clause ...))
; (cond (else body ...) ans-clause ...))
;
; ((%error-handler-cond kont () (ans-clause ...))
; (cond ans-clause ...))))