sunet/scheme/httpd/core.scm

;;; http server in the Scheme Shell	-*- Scheme -*-
;;; Olin Shivers <shivers@lcs.mit.edu>

;;; Copyright (c) 1994 by Brian D. Carlstrom and Olin Shivers.

;;; This file implements the core of an HTTP server: code to establish
;;; net connections, read and parse requests, and handler errors. 
;;; It does not have the code to actually handle requests. That's up
;;; to other modules, and could vary from server to server. To build
;;; a complete server, you need to define path handlers (see below) --
;;; they determine how requests are to be handled.
;;;
;;; The RFC detailing the HTTP 1.0 protocol, RFC 1945, can be found at
;;; http://www.w3.org/Protocols/rfc1945/rfc1945

;;; Imports and non-R4RS'isms
;;;	\r \n in strings for cr and lf.
;;;	receive values				(MV return)
;;;	scsh system calls
;;; 	rfc822 header parsing
;;;	crlf-io					(read cr/lf terminated lines)
;;;	uri, url packages
;;;	ignore-errors				(HANDLE package)
;;;	char-set stuff
;;;     format					(Formatted output)
;;;	httpd error stuff
;;;	condition-stuff				(S48 error conditions)

;;; (httpd options)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; The server top-level. PATH-HANDLER is the top-level request path handler --
;;; the procedure that actually deals with the request.

(define (httpd options)
  (let ((port (httpd-options-port options))
	(root-dir (httpd-options-root-directory options))
	(rate-limiter
	 (cond
	  ((httpd-options-simultaneous-requests options)
	   => make-rate-limiter)
	  (else #f))))
    (init-http-log! options)
    (with-syslog-destination
     "httpd" #f #f #f
     (lambda ()
       (with-cwd
	root-dir
	(bind-listen-accept-loop
	 protocol-family/internet
	 ;; Why is the output socket unbuffered? So that if the client
	 ;; closes the connection, we won't lose when we try to close the
	 ;; socket by trying to flush the output buffer.
	 (lambda (sock addr)
	   (if rate-limiter
	       (begin
		 (rate-limit-block rate-limiter)
		 (rate-limit-open rate-limiter)))

	   (with-fatal-error-handler
	    (lambda (c decline)
	      (http-syslog (syslog-level notice) "error during connection negotiation~%")
	      (if rate-limiter
		  (rate-limit-close rate-limiter)))
	    (call-with-values
	     (lambda ()
	       (socket-address->internet-address (socket-remote-address sock)))
	     (lambda (host-address service-port)
	       (if (and rate-limiter *http-syslog?*)
		   (http-syslog (syslog-level info) "<~a>~a: concurrent request #~a~%"
				(pid)
				(format-internet-host-address host-address)
				(rate-limiter-current-requests rate-limiter)))

	       (set-port-buffering (socket:outport sock) bufpol/none) ; No buffering
	       (fork-thread 
		(lambda ()	
		  (with-current-input-port 
		   (socket:inport sock)
		   (with-current-output-port 
		    (socket:outport sock)
		    (set-port-buffering (current-input-port) bufpol/none)
		    (process-toplevel-request sock host-address options)))
		  (if *http-syslog?*
		      (http-syslog (syslog-level debug) "<~a>~a [closing]~%"
				   (pid)
				   (format-internet-host-address host-address)))
		  (with-fatal-error-handler
		   (lambda (c decline)
		     (if *http-syslog?*
			 (http-syslog (syslog-level notice) "<~a>~a [error closing (~a)]~%"
				      (pid)
				      (format-internet-host-address host-address)
				      c)))
		   (close-socket sock))
		  (if rate-limiter
		      (rate-limit-close rate-limiter))
		  (if *http-syslog?*
		      (http-syslog (syslog-level info) "<~a>~a [closed]~%"
				   (pid)
				   (format-internet-host-address host-address)))))))))
	 port))))))


;;; Top-level http request processor
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Read, parse, and handle a single http request. The only thing that makes
;;; this complicated is handling errors -- as a server, we can't just let the
;;; standard error handlers toss us into a breakpoint. We have to catch the
;;; error, send an error reply back to the client if we can, and then keep
;;; on trucking. This means using the S48's condition system to catch and
;;; handle the various errors, which introduces a major point of R4RS
;;; incompatibiliy -- R4RS has no exception system. So if you were to port
;;; this code to some other Scheme, you'd really have to sit down and think
;;; about this issue for a minute.

(define (process-toplevel-request sock host-address options)
  ;; This top-level error-handler catches *all* uncaught errors and warnings.
  ;; If the error condition is a reportable HTTP error, we send a reply back 
  ;; to the client. In any event, we abort the transaction, and return from
  ;; PROCESS-TOPLEVEL-REQUEST.
  ;;
  ;; We *oughta* map non-http-errors into replies anyway.
  (with-fatal-error-handler (lambda (c decline)		; No call to decline
			      (http-syslog (syslog-level notice) "<~a>~a: error: ~s~%"
					(pid)
					(format-internet-host-address host-address)
					c)
			      (if (http-error? c)	; -- we handle all.
				  (apply (lambda (reply-code req . args)
					   (apply send-http-error-reply
						  reply-code req options
						  args))
					 (condition-stuff c))
				  (with-fatal-error-handler
				   (lambda (c decline)
				     (http-syslog (syslog-level notice) "<~a>~a [error shutting down: ~s]~%"
					       (pid)
					       (format-internet-host-address host-address)
					       c))
				   (shutdown-socket sock shutdown/sends+receives)
				   (http-syslog (syslog-level info) "<~a>~a [shut down]~%"
					     (pid)
					     (format-internet-host-address host-address)))))

    (let ((req (with-fatal-error-handler	; Map syntax errors
	           (lambda (c decline)		; to http errors.
		     (if (fatal-syntax-error? c)
			 (apply http-error http-reply/bad-request
				#f	; No request yet.	
				"Request parsing error -- report to client maintainer."
				(condition-stuff c))
			 (decline)))			   ; Actual work:
		 (parse-http-request sock options)))	   ; (1) Parse request.
	  (handler
	   (httpd-options-path-handler options)))
      (handler (http-url:path (request:url req)) req)   ; (2) Deal with it.
      (http-log req http-reply/ok))))



;;;; HTTP request parsing
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;; This code provides procedures to read requests from an input
;;;; port.

;;; Read and parse an http request from INPORT.
;;; 
;;; Note: this parser parses the URI into an http URL record. If the URI
;;; isn't an http URL, the parser fails. This may not be right. There's
;;; nothing in the http protocol to prevent you from passing a non-http
;;; URI -- what this would mean, however, is not clear. Like so much of
;;; the Web, the protocols are redundant, underconstrained, and ill-specified.

(define (parse-http-request sock options)
  (let ((line (read-crlf-line)))
;    (display line (current-error-port)) (newline (current-error-port))
    ;; Blat out some logging info.
    (if *http-syslog?*
	(call-with-values
	 (lambda ()
	   (socket-address->internet-address (socket-remote-address sock)))
	 (lambda (host-address service-port)
	   (http-syslog (syslog-level info) "<~a>~a: ~a~%"
		     (pid)
		     (format-internet-host-address host-address)
		     line))))
    
    (if (eof-object? line)
	(fatal-syntax-error "EOF while parsing request.")

	(let* ((elts (string->words line))	; Split at white-space.
	       (version (case (length elts)
			  ((2) '(0 . 9))
			  ((3) (parse-http-version (caddr elts)))
			  (else (fatal-syntax-error "Bad HTTP version.")))))

	  (let* ((meth (car elts))
		 (uri-string (cadr elts))
		 (url (parse-http-servers-url-fragment uri-string sock options))
		 (headers (if (equal? version '(0 . 9)) '()
			      (read-rfc822-headers))))
	    (make-request meth uri-string url version headers sock))))))

		

;;; Parse the URL, but if it begins without the "http://host:port" prefix,
;;; interpolate one from SOCKET. It would sleazier but faster if we just
;;; computed the default host and port at server-startup time, instead of
;;; on every request.

(define (parse-http-servers-url-fragment uri-string socket options)
  (receive (scheme path search frag-id) (parse-uri uri-string)
    (if frag-id				; Can't have a #frag part.
	(fatal-syntax-error "HTTP URL contains illegal #<fragment> suffix."
			    uri-string)

	(if scheme
	    (if (string-ci=? scheme "http") ; Better be an http url.
		(parse-http-url path search #f)
		(fatal-syntax-error "Non-HTTP URL" uri-string))

	    ;; Interpolate the userhost struct from our net connection.
	    (if (and (pair? path) (string=? (car path) ""))
		(let* ((addr (socket-local-address socket))
		       (local-name (my-reported-fqdn addr options))
		       (portnum (my-reported-port addr options)))
		  (make-http-url (make-userhost #f #f
						local-name
						(number->string portnum))
				 (map unescape-uri (cdr path)) ; Skip initial /.
				 search
				 #f))

		(fatal-syntax-error "Path fragment must begin with slash"
				    uri-string))))))


(define parse-http-version
  (let ((re (make-regexp "^HTTP/([0-9]+)\\.([0-9]+)$"))
	(lose (lambda (s) (fatal-syntax-error "Bad HTTP version" s))))
    (lambda (vstring)
      (let ((m (regexp-exec re vstring)))
	(if m
	    (cons (or (string->number (match:substring m 1) 10) (lose vstring))
		  (or (string->number (match:substring m 2) 10) (lose vstring)))
	    (lose vstring))))))
				      

;;; Split string into a list of whitespace-separated strings.
;;; This could have been trivially defined in scsh as (field-splitter " \t\n")
;;; but I hand-coded it because it's short, and I didn't want invoke the
;;; regexp machinery for something so simple.

(define non-whitespace (char-set-complement char-set:whitespace))

(define (string->words s)
  (let recur ((start 0))
    (cond ((string-index s non-whitespace start) =>
        (lambda (start)
	  (cond ((string-index s char-set:whitespace start) => 
	      (lambda (end)
		(cons (substring s start end)
		      (recur end))))
	     (else (list (substring s start (string-length s)))))))
       (else '()))))


;;; (send-http-error-reply reply-code req options [message . extras])
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Take an http-error condition, and format it into a reply to the client.
;;;
;;; As a special case, request REQ is allowed to be #f, meaning we haven't
;;; even had a chance to parse and construct the request. This is only used
;;; for 400 BAD-REQUEST error report, and we make minimal assumptions in this 
;;; case (0.9 protocol for the reply, for example). I might be better off
;;; writing a special-case procedure for that case...

;;; SEND-HTTP-ERROR-REPLY is called from error handlers, so to avoid
;;; infinite looping, if an error occurs while it is running, we just
;;; silently return. (We no longer need to do this; I have changed 
;;; WITH-FATAL-ERROR-HANDLER* so that this is not necessary, but I'll
;;; leave it in to play it safe.)

(define (send-http-error-reply reply-code req options . args)
  (ignore-errors (lambda ()	; Ignore errors -- see note above.
		   (apply really-send-http-error-reply reply-code req options args))))

(define (really-send-http-error-reply reply-code req options . args)
  (http-log req reply-code)

  (let* ((message (if (pair? args) (car args)))
	 (extras  (if (pair? args) (cdr args) '()))

	 (new-protocol? (and req (not (v0.9-request? req)))) ; 1.0 or better?

	 ;; Is it OK to send back an HTML body explaining things?
	 (html-ok? (or (not req)
		       (not (string=? (request:method req) "HEAD"))))

	 (out (current-output-port))

	 (generic-title (lambda ()
			  (title-html out
				      (reply-code->text reply-code)
				      new-protocol?)))

	 (do-msg (lambda () (cond (message (display message out) (newline out))))))

    (if new-protocol? (begin-http-header out reply-code))

    ;; Don't output the blank line, as individual clauses might
    ;; want to add more headers.
    (if html-ok? (write-string "Content-type: text/html\r\n" out))

    ;; If html-ok?, we must send back some html, with the <body> tag unclosed.
    (cond

      ;; This error reply requires two args: message is the new URI: field,
      ;; and the first EXTRA is the older Location: field.
      ((or (= reply-code http-reply/moved-temp)
	   (= reply-code http-reply/moved-perm))
       (if new-protocol?
	   (begin
	     (format out "URI: ~A\r~%" message)
	     (format out "Location: ~A\r~%" (car extras))))
       (if html-ok?
	   (begin
	     (title-html out "Document moved" new-protocol?)
	     (format out
		     "This document has ~A moved to a <A HREF=\"~A\">new location</A>.~%"
		     (if (= reply-code http-reply/moved-temp) "temporarily" "permanently")
		     message))))

      ((= reply-code http-reply/bad-request)
       (if html-ok?
	   (begin
	     (generic-title)
	     (write-string "<P>Client sent a query that this server could not understand.\n"
			   out)
	     (if message (format out "<BR>~%Reason: ~A~%" message)))))

      ((= reply-code http-reply/unauthorized)
       (if new-protocol?
	   (format out "WWW-Authenticate: ~A\r~%\r~%" message)) ; Vas is das?
       (if html-ok?
	   (begin
	     (title-html out "Authorization Required" new-protocol?)
	     (write-string "<P>Browser not authentication-capable or\n" out)
	     (write-string "authentication failed.\n" out)
	     (if message (format out "~a~%" message)))))

      ((= reply-code http-reply/forbidden)
       (if (not html-ok?)
	   (begin
	     (title-html out "Request not allowed." new-protocol?)
	     (format out 
		     "Your client does not have permission to perform a ~A~%"
		     (request:method req))
	     (format out "operation on url ~a.~%" (request:uri req))
	     (if message (format out "<P>~%~a~%" message)))))
       
      ((= reply-code http-reply/not-found)
       (if html-ok?
	   (begin
	     (title-html out "URL not found" new-protocol?)
	     (write-string 
	      "<P>The requested URL was not found on this server.\n"
			   out)
	     (if message (format out "<P>~%~a~%" message)))))

      ((= reply-code http-reply/internal-error)
       (http-syslog (syslog-level error) "internal-error: ~A" message)
       (if html-ok? 
	   (begin
	     (generic-title)
	     (format out "The server encountered an internal error or
misconfiguration and was unable to complete your request.
<P>
Please inform the server administrator, ~A, of the circumstances leading to
the error, and time it occured.~%"
		     (httpd-options-server-admin options))
	     (if message (format out "<P>~%~a~%" message)))))

      ((= reply-code http-reply/not-implemented)
       (if html-ok?
	   (begin
	     (generic-title)
	     (format out "This server does not currently implement 
the requested method (~A).~%"
		     (request:method req))
	     (if message (format out "<P>~a~%" message)))))

      (else 
       (http-syslog (syslog-level info) "Skipping unhandled reply code ~A.~%" reply-code)
       (if html-ok? (generic-title))))

    (cond 
     (html-ok?
      ;; Output extra stuff and close the <body> tag.
      (for-each (lambda (x) (format out "<BR>~s~%" x)) extras)
      (write-string "</BODY>\n" out)))
    ; (force-output out)  ;;; TODO check this
   ; (flush-all-ports)
    (force-output out)
;   (if bkp? (breakpoint "http error"))
    ))


;;; Return my Internet host name (my fully-qualified domain name).
;;; This works only if an actual resolver is behind host-info.
;;;
;;; On systems that do DNS via NIS/Yellow Pages, you only get an
;;; unqualified hostname.  Also, in case of aliased names, you just
;;; might get the wrong one.  Furthermore, you may get screwed in the
;;; presence of a server accelerator such as Squid.


(define *fqdn-cache* #f)

(define (my-reported-fqdn addr options)
  (or *fqdn-cache*
      (begin
      (set! *fqdn-cache* (or (httpd-options-fqdn options)
			     (dns-lookup-ip addr)
			     (host-info:name (host-info addr))))
      *fqdn-cache*)))

(define (my-reported-port addr options)
  (or (httpd-options-reported-port options)
      (receive (ip-addr portnum) (socket-address->internet-address addr)
	portnum)))