sunet/scheme/httpd/handlers.scm

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;;; http server in the Scheme Shell -*- Scheme -*-
;;; This file is part of the Scheme Untergrund Networking package.
;;; Copyright (c) 1995 by Olin Shivers.
;;; Copyright (c) 1996-2002 by Mike Sperber.
;;; Copyright (c) 2002 by Andreas Bernauer.
;;; For copyright information, see the file COPYING which comes with
;;; the distribution.
;;; Path handlers
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Path handlers are the guys that actually perform the requested operation
;;; on the URL. The handler interface is
;;; (handler path-list request)
;;; The path-list is a URL path list that is a suffix of REQUEST's url's
;;; path-list. Path handlers can decide how to handle an operation by
;;; recursively keying off of the elements in path-list.
;;;
;;; The object-oriented view:
;;; One way to look at this is to think of the request's METHOD as a
;;; generic operation on the URL. Recursive request handlers do method
;;; lookup to determine how to implement a given operation on a particular
;;; path.
;;;
;;; The REQUEST is a request record, as defined in httpd-core.scm, containing
;;; the details of the client request.
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;; general request handler combinator:
;; predicate: path x request --> boolean
;; if #t, handler is called
;; if #f, default-handler is called
(define (make-predicate-handler predicate handler default-handler)
(lambda (path req)
(if (predicate path req)
(handler path req)
(default-handler path req))))
;; same as MAKE-PREDICATE-HANDLER except that the predicate is only
;; called with the path:
;; predicate: path --> boolean
(define (make-path-predicate-handler predicate handler default-handler)
(make-predicate-handler
(lambda (path req) (predicate path)) handler default-handler))
;; selects handler according to host-field of http-request
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(define (make-host-name-handler hostname handler default-handler)
(make-predicate-handler
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(lambda (path req)
;; we expect only one host-header-field
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(string=? hostname (string-trim (get-header (request-headers req) 'host))))
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handler default-handler))
(define (get-header headers tag)
(cond
((assq tag headers) => cdr)
(else
(http-error (status-code bad-request) #f
(string-append "Request did not contain "
(symbol->string tag)
" header")))))
;; selects handler according to path-prefix
;; if path-prefix matches, handler is called without the path-prefix
(define (make-path-prefix-handler path-prefix handler default-handler)
(lambda (path req)
(if (and (pair? path) (string=? path-prefix (car path)))
(handler (cdr path) req)
(default-handler path req))))
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;;; (alist-path-dispatcher handler-alist default-handler) -> handler
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; This function creates a table-driven request handler that dispatches off
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;;; of the car of the request path. The handler uses the car to index into
;;; a request handler alist. If it finds a hit, it recurses using the table's
;;; request handler. If no hits, it handles the path with a default handler.
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;;; An alist handler is passed the tail of the original path; the
;;; default handler gets the entire original path.
;;;
;;; This procedure is how you say: "If the first element of the URL's
;;; path is 'foo', do X; if it's 'bar', do Y; otherwise, do Z."
(define (alist-path-dispatcher handler-alist default-handler)
(fold-right
(lambda (handler-pair default-handler)
(make-path-prefix-handler
(car handler-pair)
(cdr handler-pair)
default-handler))
default-handler
handler-alist))
;;; The null request handler -- handles nothing, sends back an error response.
;;; Can be useful as the default in table-driven request handlers.
(define (null-request-handler path req)
(make-error-response (status-code not-found) req))