;;; POP3.scm --- implement the POP3 maildrop protocol in the Scheme Shell

;;; This file is part of the Scheme Untergrund Networking package.

;;; Copyright (c) 1998 by Eric Marsden
;;; For copyright information, see the file COPYING which comes with
;;; the distribution.

;;; Related work =====================================================
;;
;; * Emacs is distributed with a C program called movemail which can
;;   be compiled with support for the POP protocol. There is also an
;;   Emacs Lisp library called pop3.el by Richard Pieri which includes
;;   APOP support.
;;
;; * Shriram Krishnamurth has written a POP3 library for MzScheme (as
;;   well as support for the NNTP protocol, for SMTP, ...).
;;
;; * Siod (a small-footprint Scheme implementation by George Carette)
;;   includes support for the POP3 protocol.
;;
;; * rfc1939 describes the POP3 protocol.
;;   http://www.ietf.org/rfc/rfc1939.txt

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Communication is initiated by the client. The server responds to
;; each request with a status indicator and an explanatory message.
;; The client starts off by opening a connection to a well known port
;; on the server machine (typically TCP 110, or 109 on some broken
;; systems). Messages sent to the server are of the form
;; 
;;            CMD [ <space> arg ] <CR> <LF>
;;
;; Replies from the server are of the form
;;
;;            status [ <space> Informative message ] <CR> <LF>
;;
;; where status is either "+OK" or "-ERR". If the server is sending
;; data (the contents of a message for example), it marks the end of
;; the data by a line consisting only of a decimal point (thus the
;; bytes to look out for are <CR><LF>.<CR><LF>. Any lines in the data
;; starting with a . have an additional . added to the beginning, to
;; avoid the client thinking that the line marks the end of the
;; message. The client should therefore replace double decimal points
;; at the beginning of a line by a single decimal point.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (pop3-connect . args)
  (let-optionals args ((host-arg #f)
		       (login #f)
		       (password #f)
		       (log #f))
    (let* ((host (or host-arg
		     (getenv "MAILHOST")))
	   (hst-info (host-info host))
	   (hostname (host-info:name hst-info))
	   (srvc-info (service-info "pop3" "tcp"))
	   (sock (socket-connect protocol-family/internet
				 socket-type/stream
				 hostname
				 (service-info:port srvc-info)))
	   (connection (make-pop3-connection hostname
					     sock
					     log "" "" #f #f)))
      (pop3-log connection
		(string-append "-- "
			       (date->string (date))
			       ": opened POP3 connection to "
			       hostname))

      ;; read the challenge the server sends in its welcome banner
      (let* ((banner (pop3-read-response connection))
	     (match (regexp-search (rx (: "+OK " (* (~ #\<))
					  #\< (submatch (+ (~ #\>))) #\>))
				   banner))
	     (challenge (and match (match:substring match 1))))
	(set-pop3-connection-challenge! connection challenge))
      
      (pop3-login connection login password)
      
      connection)))

;; first try standard USER/PASS authentication, and switch to APOP
;; authentication if the server prefers.

(define (pop3-login connection login password)
  (let* ((netrc-record #f)
	 (get-netrc-record
	  (lambda ()
	    (cond
	     (netrc-record)
	     (else
	      (set! netrc-record (netrc-parse))
	      netrc-record)))))
    (let ((login (or login
		     (netrc-lookup-login (get-netrc-record)
					 (pop3-connection-host-name connection)
					 #f)))
	  (password (or password
			(netrc-lookup-password (get-netrc-record)
					       (pop3-connection-host-name connection)
					       #f))))
      (with-fatal-error-handler*
       (lambda (result punt)
	 (cond
	  ((not (pop3-error? result)) (punt))
	  ((pop3-connection-challenge connection)
	   (pop3-apop-login connection login password))))
       (lambda ()
	 (pop3-send-command connection (format #f "USER ~a" login))
	 (pop3-send-command connection (format #f "PASS ~a" password))
	 (set-pop3-connection-login! connection login)
	 (set-pop3-connection-password! connection password)
	 (set-pop3-connection-state! connection 'connected))))))

;; Login to the server using APOP authentication (no cleartext
;; passwords are sent over the network). The server appends a token to
;; its welcome message, which is built from the server's fully
;; qualified domain name and a unique serial number. The client
;; concatenates this token and the pass phrase and applies the MD5
;; digest algorithm (a one-way hash) to produce a digest. The user
;; name and the digest are sent to the server to authenticate the
;; user. The following example comes from the RFC:
;;
;;      S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
;;      C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
;;      S: +OK maildrop has 1 message (369 octets)
;; 
;;      In this example, the shared  secret  is  the  string  `tan-
;;      staaf'.  Hence, the MD5 algorithm is applied to the string
;; 
;;         <1896.697170952@dbc.mtview.ca.us>tanstaaf
;; 
;;      which produces a digest value of
;; 
;;         c4c9334bac560ecc979e58001b3e22fb
;;         
;;: connection x string x string -> status
(define (pop3-apop-login connection login password)
  (let* ((key (string-append (pop3-connection-challenge connection)
                             password))
         (digest (number->string
		  (md5-digest->number (md5-digest-for-string key))
		  16))
         (status (pop3-send-command connection
                                    (format #f "APOP ~a ~a" login digest))))
  (set-pop3-connection-login! connection login)
  (set-pop3-connection-password! connection password)
  (set-pop3-connection-state! connection 'connected)
  status))


;; return number of messages and number of bytes waiting at the maildrop
;;: connection -> integer x integer
(define (pop3-stat connection)
  (pop3-check-transaction-state connection 'pop3-stat)
  (let* ((response (pop3-send-command connection "STAT"))
         (match (regexp-search (rx (posix-string "([0-9]+) ([0-9]+)")) response)))
    (values (string->number (match:substring match 1))
            (string->number (match:substring match 2)))))

;; dump the message number MSGID to (current-output-port)
;;: connection x integer -> status
(define (pop3-get connection msgid)
  (pop3-check-transaction-state connection 'pop3-get)
  (let ((status (pop3-send-command connection (format #f "RETR ~a" msgid))))
    (pop3-dump (socket:inport (pop3-connection-command-socket connection)))
    status))

;;: connection x integer -> status
(define (pop3-headers connection msgid)
  (pop3-check-transaction-state connection 'pop3-headers)
  (let ((status (pop3-send-command connection (format #f "TOP ~a 0" msgid))))
    (pop3-dump (socket:inport (pop3-connection-command-socket connection)))
    status))

;; Return highest accessed message-id number for the session. This
;; ain't in the RFC, but seems to be supported by several servers.
;;: connection -> integer
(define (pop3-last connection)
  (pop3-check-transaction-state connection 'pop3-last)
  (let ((response (pop3-send-command connection "LAST")))
    (string->number (car ((infix-splitter) response)))))

;; mark the message number MSGID for deletion. Note that the messages
;; are not truly deleted until the QUIT command is sent, and messages
;; can be undeleted using the RSET command.
;;: connection x integer -> status
(define (pop3-delete connection msgid)
  (pop3-check-transaction-state connection 'pop3-delete)
  (pop3-send-command connection (format #f "DELE ~a" msgid)))


;; any messages which have been marked for deletion are unmarked
;;: connection -> status
(define (pop3-reset connection)
  (pop3-check-transaction-state connection 'pop3-reset)
  (pop3-send-command connection "RSET"))

;;: connection -> status
(define (pop3-quit connection)
  (pop3-check-transaction-state connection 'pop3-quit)
  (let ((status (pop3-send-command connection "QUIT")))
    (close-socket (pop3-connection-command-socket connection))
    status))


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Nothing exported below.

(define-record-type pop3-connection :pop3-connection
  (make-pop3-connection host-name command-socket log-port login password challenge state)
  pop3-connection
  (host-name pop3-connection-host-name)
  (command-socket pop3-connection-command-socket)
  (log-port pop3-connection-log-port)
  (login pop3-connection-login set-pop3-connection-login!)
  (password pop3-connection-password set-pop3-connection-password!)
  (challenge pop3-connection-challenge set-pop3-connection-challenge!)
  (state pop3-connection-state set-pop3-connection-state!))

(define-condition-type 'pop3-error '(error))
(define pop3-error? (condition-predicate 'pop3-error))

(define (pop3-check-transaction-state connection caller)
  (if (not (eq? (pop3-connection-state connection) 'connected))
      (call-error "not in transaction state" caller)))

(define (pop3-read-response connection)
  (let* ((sock (pop3-connection-command-socket connection))
         (in (socket:inport sock))
         (line (read-crlf-line in)))
    (pop3-log connection (format #f "-> ~a" line))
    line))

;; this could perhaps be improved
(define (pop3-handle-response response command)
  (let ((match (regexp-search (rx (posix-string "^\\+OK(.*)")) response)))
    (if match 
	(match:substring match 1)
        (let ((match2 (regexp-search (rx (posix-string "^-ERR(.*)")) response)))
	  (if match2
	      (signal '-ERR (match:substring match2 1) command)
	      (signal '-ERR response command))))))


(define (pop3-log connection line)
  (let ((log (pop3-connection-log-port connection)))
    (if log
	(begin
	  (write-string line log)
	  (write-string "\n" log)
	  (force-output log)))))

(define (pop3-send-command connection command)
  (let* ((sock (pop3-connection-command-socket connection))
         (out (socket:outport sock)))
    (write-string command out)
    (write-crlf out)
    (pop3-log connection (format #f "<- ~a" command))
    (pop3-handle-response (pop3-read-response connection) command)))

(define (pop3-dump fd)
  (let loop ()
    (let ((line (read-crlf-line fd)))
      (if (and (not (eof-object? line))
	       (not (string=? line ".")))
	  (let ((line (if (string-prefix? ".." line)
			  (substring line 1 (string-length line))
			  line)))
	    (write-string line)
	    (newline)
	    (loop))))))

;; EOF