#lang mzscheme

#| resolver.ss -- PLaneT client

1. Introduction

The PLaneT system is a method for automatically sharing code packages, both as
libraries and as full applications, that gives every user of a PLaneT client
the illusion of having a local copy of every code package on the server, but is
parsimonious in its transmission. It consists of a centralized server that
holds all packages and individual clients that hold some portion of the archive
locally. Maintenance of that archive should be transparent, and is the complete
responsibility of the PLaneT client.

2. Client behavior

The PLaneT client receives user requests (i.e., the "(require (planet ...))"
forms) and loads the appropriate modules in response. In the course of handling
these requests it may download new code packages from the PLaneT server.

2.1 User interface

The structure of user PLaneT invocations is listed below.

PLANET-REQUEST ::= (planet FILE-NAME PKG-SPEC [PATH ...]?)
FILE-NAME      ::= string
PKG-SPEC       ::= string | (FILE-PATH ... PKG-NAME)
                          | (FILE-PATH ... PKG-NAME VER-SPEC)
VER-SPEC       ::= Nat | (Nat MINOR)
MINOR          ::= Nat | (Nat Nat) | (= Nat) | (+ Nat) | (- Nat)
FILE-PATH      ::= string
PKG-NAME       ::= string
OWNER-NAME     ::= string
PATH           ::= string

All strings must be legal filename strings.

When encountered, a planet-request is interpreted as requiring the given file
name from the given logical package, specified by the package spec and the
collection specification, if given. If no VER-SPEC is provided, the most recent
version is assumed. If no owner-name/path ... clause is provided, the default
package is assumed.

2. PLaneT protocol

PLaneT clients support two protocols for communicating with the PLaneT server:
the standard HTTP GET/response system (currently the default) and a specialized
TCP-based protocol that may become more important if PLaneT becomes smarter
about downloading packages behind the scenes.

In the following sections we describe the specialized protocol only.

2.1 Overview

1. PLaneT client establishes TCP connection to PLaneT server.
2. Client transmits a version specifier.
3. Server either refuses that version and closes connection or accepts.
4. Client transmits a sequence of requests terminated by a special
   end-of-request marker.  Simultaneously, server transmits responses to those
   requests.
5. Once the server has handled every request, it closes the connection.


I am concerned about the overhead of opening and closing TCP connections for a
large program with many requires, so I want to allow many requests and
responses over the same connection. Unfortunately there's a wrinkle: the
standard client, implemented the obvious way, would be unable to send more than
one request at a time because it gets invoked purely as a response to a require
form and must load an appropriate file before it returns. This means I can't
batch up multiple requires, at least not with an obvious implementation.

A possible solution would be to implement an install program that walks over
the syntax tree of a program and gathers all requires, then communicates with
the server and learns what additional packages would be necessary due to those
requires, and then downloads all of them at once. We would have to implement
both methods simultaneously, though, to allow for REPL-based PLaneT use and
dynamic-require (unless we want it to be a runtime exception to use PLaneT from
the REPL or via dynamic-require, something I'd rather not do), so I want a
protocol that will allow both forms of access easily. This protocol does that,
and doesn't require too much additional overhead in the case that the client
only takes one package at a time.

2.2 Communication Details

After a TCP connection is established, the client transmits a
VERSION-SPECIFIER:

VERSION-SPECIFIER ::= "PLaneT/1.0\n"

The server responds with a VERSION-RESPONSE:

VERSION-RESPONSE ::=
| 'ok "\n"
| ('invalid string) "\n"

where the string in the invalid case is descriptive text intended for display
to the user that may indicate some specific message about the nature of the
error.

If the server sends 'invalid, the server closes the connection. Otherwise, the
client may send any number of requests, followed by an end-of-request marker:

REQUESTS ::= { REQUEST "\n"}* 'end "\n"
REQUEST  ::= (SEQ-NO 'get PKG-LANG PKG-NAME (Nat | #f) (Nat | #f) (Nat | #f)
                     [OWNER-NAME PATH ...]?)
PKG-LANG ::= String
SEQ-NO   ::= Nat

The fields in a request are a uniquely identifying sequence number, the literal
symbol 'get, the name of the package to receive, the required major version and
the lowest and highest acceptable version (with #f meaning that there is no
constraint for that field, and a #f in major-version field implying that both
other fields must also be #f), and the package path.

As the client is transmitting a REQUESTS sequence, the server begins responding
to it with RESPONSE structures, each with a sequence number indicating to which
request it is responding (except in the case of input too garbled to extract a
sequence number):

RESPONSE ::=
| ('error 'malformed-input string) "\n"
| (SEQ-NO 'error 'malformed-request string) "\n"
| (SEQ-NO 'bad-language string) "\n"
| (SEQ-NO 'get 'ok Nat Nat Nat) "\n" BYTE-DATA
| (SEQ-NO 'get 'error ERROR-CODE string) "\n"

ERROR-CODE ::= 'not-found

If the server receives a malformed request, it may close connection after
sending a malformed-request response without processing any other
requests. Otherwise it must process all requests even in the event of an
error. On a successful get, the three numbers the server returns are the
matched package's major version, the matched package's minor version, and the
number of bytes in the package.

3 Client Download Policies

Racket invokes the PLaneT client once for each instance of a require-planet
form in a program being run (i.e., the transitive closure of the "requires"
relation starting from some specified root module; this closure is calculable
statically). At each of these invocations, the client examines its internal
cache to see if an appropriate module exists that matches the specification
given by the user (for details see the next section). If one does, the client
loads that module and returns. If none does, it initiates a transaction with
the server using the PLaneT protocol described in the previous subsection and
sends a single request consisting of the user's request.  It installs the
resulting .plt file and then loads the appropriate file.

The client keeps a cache of downloaded packages locally. It does so in the
$PLTCOLLECTS/planet/cache/ directory and subdirectories, in an intuitive
manner: each item in the package's path in the PLaneT require line correspond
to a subdirectory in the cache directory, starting with the owner name. (They
should be unpacked relative to some user-specific rather than
installation-specific place, possibly, but that's difficult to do so we won't
do it yet).

To check whether a package is installed when attempting to satisfy a
requirement, the client checks its cache to see if an appropriate entry exists
in its link-table for that require line.  If one exists, it uses the named
package directly. If none exists, it checks to see if there is an appropriate
subdirectory.

||#


;; This `resolver' no longer fits the normal protocol for a 
;; module name resolver, because it accepts an extra argument in
;; the second and third cases. The extra argument is a parameterization
;; to use for installation actions.
(define resolver
  (case-lambda
    [(name) (void)]
    [(spec module-path stx orig-paramz) 
     (resolver spec module-path stx #t orig-paramz)]
    [(spec module-path stx load? orig-paramz)
     ;; ensure these directories exist
     (make-directory* (PLANET-DIR))
     (make-directory* (CACHE-DIR))
     (establish-diamond-property-monitor)
     (planet-resolve spec
                     (current-module-declare-name) ;; seems more reliable than module-path in v3.99
                     stx
                     load?
                     orig-paramz)]))

(require mzlib/match
         mzlib/file
         mzlib/port
         mzlib/list

         mzlib/date

         net/url
         net/head
         mzlib/struct

         "config.ss"
         "private/planet-shared.ss"
         "private/linkage.ss"
         "parsereq.ss"
         
         "terse-info.ss"
         compiler/cm) 

(provide (rename resolver planet-module-name-resolver)
         resolve-planet-path
         pkg-spec->full-pkg-spec
         get-package-from-cache
         get-package-from-server
         download-package
         pkg->download-url
         pkg-promise->pkg
         install-pkg
         get-planet-module-path/pkg
         download?
         install?
         exn:fail:planet?
         make-exn:fail:planet)

;; if #f, will not install packages and instead raise a exn:fail:install? error
(define install? (make-parameter #t))
;; if #f, will not download packages and instead raise a exn:fail:install? error
(define download? (make-parameter #t))
(define-struct (exn:fail:planet exn:fail) ())

;; update doc index only once for a set of installs:
(define planet-nested-install (make-parameter #f))

;; =============================================================================
;; DIAMOND PROPERTY STUFF
;; make sure a module isn't loaded twice with two different versions
;; =============================================================================
(define VER-CACHE-NAME #f)

(define (establish-diamond-property-monitor)
  (unless VER-CACHE-NAME (set! VER-CACHE-NAME (gensym)))
  (unless (namespace-variable-value VER-CACHE-NAME #t (lambda () #f))
    (namespace-set-variable-value! VER-CACHE-NAME (make-hash-table 'equal))))

(define (the-version-cache)    (namespace-variable-value VER-CACHE-NAME))
(define (pkg->diamond-key pkg) (cons (pkg-name pkg) (pkg-route pkg)))

(define (pkg-matches-bounds? pkg bound-info)
  (match-let ([(maj lo hi) bound-info])
    (and (= maj (pkg-maj pkg))
         (or (not lo) (>= (pkg-min pkg) lo))
         (or (not hi) (<= (pkg-min pkg) hi)))))

;; COMPAT ::= 'none | 'all | `(all-except ,VER-SPEC ...) | `(only ,VER-SPEC ...)
;; build-compatibility-fn : COMPAT -> PKG -> bool
(define (build-compatibility-fn compat-data)
  (define pre-fn
    (match compat-data
      [`none (lambda (_) #f)]
      [`all (lambda (_) #t)]
      [`(all-except ,vspec ...)
       (let ([bounders (map (λ (x) (version->bounds x (λ (_) #f))) vspec)])
         (if (andmap (lambda (x) x) bounders)
           (lambda (v)
             (not (ormap (lambda (bounder) (pkg-matches-bounds? v bounder))
                         bounders)))
           #f))]
      [`(only ,vspec ...)
       (let ([bounders (map (λ (x) (version->bounds x (λ (_) #f))) vspec)])
         (if (andmap (lambda (x) x) bounders)
           (lambda (v)
             (andmap (lambda (bounder) (pkg-matches-bounds? v bounder))
                     bounders)))
         #f)]
      [_ #f]))
  (or pre-fn (lambda (x) #f)))

;; can-be-loaded-together? : pkg pkg -> boolean
;; side constraint: pkg1 and pkg2 are versions of the same package assumption:
;; pkg1 and pkg2 are versions of the same package determines if the two
;; versions are side-by-side compatible
(define (can-be-loaded-together? pkg1 pkg2)
  (cond [(pkg> pkg1 pkg2) (can-be-loaded-together? pkg2 pkg1)]
        [(pkg= pkg1 pkg2) #t]
        [(pkg< pkg1 pkg2)
         (let* ([info (pkg->info pkg2)]
                [compat? (build-compatibility-fn
                          (info 'can-be-loaded-with (lambda () 'none)))])
           (compat? pkg1))]))

;; stx->origin-string : stx option -> string
;; returns a description [e.g. file name, line#] of the given syntax
(define (stx->origin-string stx)
  (if stx (format "~a" (syntax-source stx)) "[unknown]"))

(define (add-pkg-to-diamond-registry! pkg stx)
  (let ([loaded-packages
         (hash-table-get (the-version-cache) (pkg->diamond-key pkg) '())])
    (unless (list? loaded-packages)
      (error 'PLaneT "Inconsistent state: expected loaded-packages to be a list, received: ~s" loaded-packages))
    (let ([all-violations '()])
      (for-each
       (lambda (already-loaded-pkg-record)
         (let* ([already-loaded-pkg (car already-loaded-pkg-record)]
                [prior-stx (cadr already-loaded-pkg-record)]
                [prior-stx-origin-string (stx->origin-string prior-stx)])
           (unless (can-be-loaded-together? pkg already-loaded-pkg)
             (set!
              all-violations
              (cons
               (list
                stx
                (make-exn:fail
                 (format
                  "Package ~a loaded twice with multiple incompatible versions:
~a attempted to load version ~a.~a while version ~a.~a was already loaded by ~a"
                  (pkg-name pkg)
                  (stx->origin-string stx)
                  (pkg-maj pkg)
                  (pkg-min pkg)
                  (pkg-maj already-loaded-pkg)
                  (pkg-min already-loaded-pkg)
                  prior-stx-origin-string)
                 (current-continuation-marks)))
               all-violations)))))
       loaded-packages)
      (unless (null? all-violations)
        (let ([worst (or (assq values all-violations) (car all-violations))])
          (raise (cadr worst)))))
    (hash-table-put! (the-version-cache)
                     (pkg->diamond-key pkg)
                     (cons (list pkg stx) loaded-packages))))

;; =============================================================================
;; MAIN LOGIC
;; Handles the overall functioning of the resolver
;; =============================================================================

;; planet-resolve : PLANET-REQUEST (resolved-module-path | #f) syntax[PLANET-REQUEST] -> symbol
;; resolves the given request. Returns a name corresponding to the module in
;; the correct environment
(define (planet-resolve spec rmp stx load? orig-paramz)
  ;; install various parameters that can affect the compilation of a planet package back to their original state
  (parameterize ([current-compile (call-with-parameterization orig-paramz current-compile)]
                 [current-eval (call-with-parameterization orig-paramz current-eval)]
                 [use-compiled-file-paths (call-with-parameterization orig-paramz use-compiled-file-paths)]
                 [current-library-collection-paths (call-with-parameterization orig-paramz current-library-collection-paths)])
    (let-values ([(path pkg) (get-planet-module-path/pkg spec rmp stx)])
      (when load? (add-pkg-to-diamond-registry! pkg stx))
      (do-require path (pkg-path pkg) rmp stx load?))))

;; resolve-planet-path : planet-require-spec -> path
;; retrieves the path to the given file in the planet package. downloads and
;; installs the package if necessary
(define (resolve-planet-path spec)
  (let-values ([(path pkg) (get-planet-module-path/pkg spec #f #f)])
    path))

;; get-planet-module-path/pkg :PLANET-REQUEST (resolved-module-path | #f) syntax[PLANET-REQUEST] -> (values path PKG)
;; returns the matching package and the file path to the specific request
(define (get-planet-module-path/pkg spec rmp stx)
  (request->pkg (spec->req spec stx) rmp stx))

;; request->pkg : request (resolved-module-path | #f) syntax[PLANET-REQUEST] -> (values path PKG)
(define (request->pkg req rmp stx)
  (let* ([result (get-package rmp (request-full-pkg-spec req))])
    (cond [(string? result)
           (raise-syntax-error 'require result stx)]
          [(pkg? result)
           (values (apply build-path (pkg-path result)
                          (append (request-path req) (list (request-file req))))
                   result)])))

;; PKG-GETTER ::= module-path pspec
;;                (pkg -> A)
;;                ((uninstalled-pkg -> void)
;;                 (pkg -> void)
;;                 ((string | #f) -> string | #f) -> A)
;;                -> A
;;
;; a pkg-getter is a function that tries to fetch a package; it is written in a
;; quasi-cps style; the first argument is what it calls to succeed, and the
;; second argument is what it calls when it fails. In the second case, it must
;; provide two things: a function to take action if a match is found
;; eventually, and a function that gets to mess with the error message if the
;; entire message eventually fails.

;; get-package : (resolved-module-path | #f) FULL-PKG-SPEC -> (PKG | string)
;; gets the package specified by pspec requested by the module in the given
;; module path, or returns a descriptive error message string if that's not
;; possible
(define (get-package rmp pspec)
  (let loop ([getters (*package-search-chain*)]
             [pre-install-updaters '()]
             [post-install-updaters '()]
             [error-reporters '()])
    (if (null? getters)
      ;; we have failed to fetch the package, generate an appropriate error
      ;; message and bail
      (let ([msg (foldl (λ (f str) (f str)) #f error-reporters)])
        (or msg (format "Could not find package matching ~s"
                        (list (pkg-spec-name pspec)
                              (pkg-spec-maj pspec)
                              (list (pkg-spec-minor-lo pspec)
                                    (pkg-spec-minor-hi pspec))
                              (pkg-spec-path pspec)))))
      ;; try the next error reporter. recursive call is in the failure
      ;; continuation
      ((car getters)
       rmp
       pspec
       (λ (pkg)
         (when (uninstalled-pkg? pkg)
           (for-each (λ (u) (u pkg)) pre-install-updaters))
         (let ([installed-pkg (pkg-promise->pkg pkg)])
           (for-each (λ (u) (u installed-pkg)) post-install-updaters)
           installed-pkg))
       (λ (pre-updater post-updater error-reporter)
         (loop (cdr getters)
               (cons pre-updater pre-install-updaters)
               (cons post-updater post-install-updaters)
               (cons error-reporter error-reporters)))))))

;; =============================================================================
;; PHASE 2: CACHE SEARCH
;; If there's no linkage, there might still be an appropriate cached module
;; (either installed or uninstalled)
;; =============================================================================

;; get/installed-cache : pkg-getter
(define (get/installed-cache _ pkg-spec success-k failure-k)
  (let ([p (lookup-package pkg-spec)])
    (if p (success-k p) (failure-k void void (λ (x) x)))))

;; get-package-from-cache : FULL-PKG-SPEC -> PKG | #f
(define (get-package-from-cache pkg-spec)
  (lookup-package pkg-spec))

;; get/uninstalled-cache-dummy : pkg-getter
;; always fails, but records the package to the uninstalled package cache upon
;; the success of some other getter later in the chain.
(define (get/uninstalled-cache-dummy _ pkg-spec success-k failure-k)
  (failure-k save-to-uninstalled-pkg-cache! void (λ (x) x)))

;; get/uninstalled-cache : pkg-getter
;; note: this does not yet work with minimum-required-version specifiers if you
;; install a package and then use an older mzscheme
(define (get/uninstalled-cache _ pkg-spec success-k failure-k)
  (let ([p (lookup-package pkg-spec (UNINSTALLED-PACKAGE-CACHE))])
    (if (and p (file-exists? (build-path (pkg-path p)
                                         (pkg-spec-name pkg-spec))))
        (begin
          (planet-log "found local, uninstalled copy of package at ~a"
                      (build-path (pkg-path p)
                                  (pkg-spec-name pkg-spec)))
          (success-k
           ;; note: it's a little sloppy that lookup-pkg returns PKG structures,
           ;; since it doesn't actually know whether or not the package is
           ;; installed. hence I have to convert what appears to be an installed
           ;; package into an uninstalled package
           (make-uninstalled-pkg (build-path (pkg-path p) (pkg-spec-name pkg-spec))
                                 pkg-spec
                                 (pkg-maj p)
                                 (pkg-min p))))
      (failure-k void void (λ (x) x)))))

;; save-to-uninstalled-pkg-cache! : uninstalled-pkg -> path[file]
;; copies the given uninstalled package into the uninstalled-package cache,
;; replacing any old file that might be there. Returns the path it copied the
;; file into.
(define (save-to-uninstalled-pkg-cache! uninst-p)
  (let* ([pspec (uninstalled-pkg-spec uninst-p)]
         [owner (car (pkg-spec-path pspec))]
         [name (pkg-spec-name pspec)]
         [maj (uninstalled-pkg-maj uninst-p)]
         [min (uninstalled-pkg-min uninst-p)]
         [dir (build-path (UNINSTALLED-PACKAGE-CACHE)
                          owner
                          name
                          (number->string maj)
                          (number->string min))]
         [full-pkg-path (build-path dir name)])
    (make-directory* dir)
    (unless (equal? (normalize-path (uninstalled-pkg-path uninst-p))
                    (normalize-path full-pkg-path))
      (when (file-exists? full-pkg-path) (delete-file full-pkg-path))
      (copy-file (uninstalled-pkg-path uninst-p) full-pkg-path))
    full-pkg-path))

;; =============================================================================
;; PHASE 3: SERVER RETRIEVAL
;; Ask the PLaneT server for an appropriate package if we don't have one
;; locally.
;; =============================================================================

(define (get/server _ pkg-spec success-k failure-k)
  (let ([p (get-package-from-server pkg-spec)])
    (cond
      [(pkg-promise? p) (success-k p)]
      [(string? p)
       ;; replace any existing error message with the server download error
       ;; message
       (planet-log p)
       (failure-k void void (λ (_) p))])))

;; get-package-from-server : FULL-PKG-SPEC -> PKG-PROMISE | #f | string[error message]
;; downloads the given package file from the PLaneT server and installs it in
;; the uninstalled-packages cache, then returns a promise for it
(define (get-package-from-server pkg)
  (match (download-package pkg)
    [(#t tmpfile-path maj min)
     (let* ([upkg (make-uninstalled-pkg tmpfile-path pkg maj min)]
            [cached-path (save-to-uninstalled-pkg-cache! upkg)]
            [final (make-uninstalled-pkg cached-path pkg maj min)])
       (unless (equal? (normalize-path tmpfile-path)
                       (normalize-path cached-path))
         (delete-file tmpfile-path)) ;; remove the tmp file, we're done with it
       final)]
    [(#f str)
     (string-append "PLaneT could not find the requested package: " str)]
    [(? string? s)
     (string-append "PLaneT could not download the requested package: " s)]))

(define (download-package pkg)
  (unless (download?)
    (raise (make-exn:fail:planet
            (format
             "PLaneT error: cannot download package ~s since the download? parameter is set to #f"
             (list (car (pkg-spec-path pkg)) (pkg-spec-name pkg)))
            (current-continuation-marks))))
  ((if (USE-HTTP-DOWNLOADS?) download-package/http download-package/planet)
   pkg))

(define (current-time)
  (let ([date (seconds->date (current-seconds))])
    (parameterize ([date-display-format 'rfc2822])
      (format "~a ~a:~a:~a"
              (date->string date)
              (date-hour date)
              (date-minute date)
              (date-second date)))))

;; pkg-promise->pkg : pkg-promise -> pkg
;; "forces" the given pkg-promise (i.e., installs the package if it isn't
;; installed yet)
(define (pkg-promise->pkg p)
  (cond [(pkg? p) p]
        [(uninstalled-pkg? p)
         (install-pkg (uninstalled-pkg-spec p)
                      (uninstalled-pkg-path p)
                      (uninstalled-pkg-maj p)
                      (uninstalled-pkg-min p))]))

;; install-pkg : FULL-PKG-SPEC path[file] Nat Nat -> PKG
;; install the given pkg to the planet cache and return a PKG representing the
;; installed file
(define (install-pkg pkg path maj min)
  (unless (install?)
    (raise (make-exn:fail:planet
            (format
             "PLaneT error: cannot install package ~s since the install? parameter is set to #f"
             (list (car (pkg-spec-path pkg)) (pkg-spec-name pkg) maj min))
            (current-continuation-marks))))
  (let* ([owner (car (pkg-spec-path pkg))]
         [extra-path (cdr (pkg-spec-path pkg))]
         [the-dir
          (apply build-path (CACHE-DIR)
                 (append (pkg-spec-path pkg) (list (pkg-spec-name pkg)
                                                   (number->string maj)
                                                   (number->string min))))]
         [was-nested? (planet-nested-install)])
    (if (directory-exists? the-dir)
      (raise (make-exn:fail
              "Internal PLaneT error: trying to install already-installed package"
              (current-continuation-marks)))
      (parameterize ([planet-nested-install #t])
        (planet-terse-log 'install (pkg-spec->string pkg))
        (with-logging
         (LOG-FILE)
         (lambda ()
           (printf "\n============= Installing ~a on ~a =============\n"
                   (pkg-spec-name pkg)
                   (current-time))
           ;; oh man is this a bad hack!
           (parameterize ([current-namespace (make-namespace)])
             (printf "new namespace\n")
             (let ([ipp (dynamic-require 'setup/plt-single-installer
                                         'install-planet-package)]
                   [rud (dynamic-require 'setup/plt-single-installer
                                         'reindex-user-documentation)]
                   [msfh (dynamic-require 'compiler/cm 'manager-skip-file-handler)])
               (printf "starting setup-plt ~s ~s\n" (manager-skip-file-handler) (msfh))
               (parameterize ([msfh (manager-skip-file-handler)])
                 (printf "starting setup-plt.2 ~s ~s\n" (manager-skip-file-handler) (msfh))
                 (ipp path the-dir (list owner (pkg-spec-name pkg)
                                         extra-path maj min))
                 (unless was-nested?
                   (printf "------------- Rebuilding documentation index -------------\n")
                   (rud)))))))
        (planet-terse-log 'finish (pkg-spec->string pkg))
        (make-pkg (pkg-spec-name pkg) (pkg-spec-path pkg)
                  maj min the-dir 'normal)))))

;; download-package : FULL-PKG-SPEC -> RESPONSE
;; RESPONSE ::= (list #f string) | (list #t path[file] Nat Nat)

;; downloads the given package and returns (list bool string): if bool is #t,
;; the path is to a file that contains the package. If bool is #f, the package
;; didn't exist and the string is the server's informative message.
;; raises an exception if some protocol failure occurs in the download process
(define (download-package/planet pkg)

  (define stupid-internal-define-syntax 
    (let ([msg (format "downloading ~a from ~a via planet protocol" 
                       (pkg-spec->string pkg)
                       (PLANET-SERVER-NAME))])
      (planet-terse-log 'download (pkg-spec->string pkg))
      (planet-log msg)))
  
  (define-values (ip op) (tcp-connect (PLANET-SERVER-NAME) (PLANET-SERVER-PORT)))

  (define (close-ports) (close-input-port ip) (close-output-port op))

  (define (request-pkg-list pkgs)
    (for-each/n (lambda (pkg seqno)
                  (write-line (list* seqno 'get
                                     (DEFAULT-PACKAGE-LANGUAGE)
                                     (pkg-spec-name pkg)
                                     (pkg-spec-maj pkg)
                                     (pkg-spec-minor-lo pkg)
                                     (pkg-spec-minor-hi pkg)
                                     (pkg-spec-path pkg))
                              op))
                pkgs)
    (write-line 'end op)
    (flush-output op))

  (define (state:initialize)
    (fprintf op "PLaneT/1.0\n")
    (flush-output op)
    (match (read ip)
      ['ok                        (state:send-pkg-request)]
      [('invalid (? string? msg)) (state:abort (string-append "protocol version error: " msg))]
      [bad-msg                    (state:abort (format "server protocol error (received invalid response): ~a" bad-msg))]))

  (define (state:send-pkg-request)
    (request-pkg-list (list pkg))
    (state:receive-package))

  (define (state:receive-package)
    (match (read ip)
      [(_ 'get 'ok (? nat? maj) (? nat? min) (? nat? bytes))
       (let ([filename (make-temporary-file "planettmp~a.plt")])
         (read-char ip) ; throw away newline that must be present
         (read-n-chars-to-file bytes ip filename)
         (list #t filename maj min))]
      [(_ 'error 'malformed-request (? string? msg))
       (state:abort (format "Internal error (malformed request): ~a" msg))]
      [(_ 'get 'error 'not-found (? string? msg))
       (state:failure (format "Server had no matching package: ~a" msg))]
      [(_ 'get 'error (? symbol? code) (? string? msg))
       (state:abort (format "Unknown error ~a receiving package: ~a" code msg))]
      [bad-response  (state:abort (format "Server returned malformed message: ~e" bad-response))]))

  (define (state:abort msg)
    (raise (make-exn:i/o:protocol msg (current-continuation-marks))))
  (define (state:failure msg) (list #f msg))

  (with-handlers ([void (lambda (e) (close-ports) (raise e))])
    (begin0 
      (state:initialize)
      (close-ports))))

;; ------------------------------------------------------------
;; HTTP VERSION OF THE PROTOCOL

;; pkg->servlet-args : FULL-PKG-SPEC -> environment[from net/url]
;; gets the appropriate query arguments to request the given package from the
;; PLaneT HTTP download servlet
(define (pkg->servlet-args pkg)
  (let ([get (lambda (access) (format "~s" (access pkg)))])
    `((lang   . ,(format "~s" (DEFAULT-PACKAGE-LANGUAGE)))
      (name   . ,(get pkg-spec-name))
      (maj    . ,(get pkg-spec-maj))
      (min-lo . ,(get pkg-spec-minor-lo))
      (min-hi . ,(get pkg-spec-minor-hi))
      (path   . ,(get pkg-spec-path)))))

;; get-http-response-code : header[from net/head] -> string or #f
;; gets the HTTP response code in the given header
(define (get-http-response-code header)
  (let ([parsed (regexp-match #rx"^HTTP/[^ ]* ([^ ]*)" header)])
    (and parsed (cadr parsed))))

;; pkg->download-url : FULL-PKG-SPEC -> url
;; gets the download url for the given package
(define (pkg->download-url pkg)
  (copy-struct url (string->url (HTTP-DOWNLOAD-SERVLET-URL))
               (url-query (pkg->servlet-args pkg))))

;; download-package/http : FULL-PKG-SPEC -> RESPONSE
;; a drop-in replacement for download-package that uses HTTP rather than the
;; planet protocol.  The HTTP protocol does not allow any kind of complicated
;; negotiation, but it appears that many more users can make HTTP requests than
;; requests from nonstandard protocols.
(define (download-package/http pkg)
  (let/ec return
    (let loop ([attempts 1])
      (when (> attempts 5)
        (return "Download failed too many times (possibly due to an unreliable network connection)"))

      (let ([msg (format "downloading ~a from ~a via HTTP~a" 
                         (pkg-spec->string pkg)
                         (PLANET-SERVER-NAME)
                         (if (= attempts 1)
                             ""
                             (format ", attempt #~a" attempts)))])
        (planet-terse-log 'download (pkg-spec->string pkg))
        (planet-log "~a" msg))

      (with-handlers ([exn:fail:network? (λ (e) (return (exn-message e)))])
        (let* ([target            (pkg->download-url pkg)]
               [ip                (get-impure-port target)]
               [head              (purify-port ip)]
               [response-code/str (get-http-response-code head)]
               [response-code     (and response-code/str 
                                       (string->number response-code/str))])

          (define (abort msg)
            (close-input-port ip)
            (return msg))

          (case response-code
            [(#f)
             (abort (format "Server returned invalid HTTP response code ~s"
                            response-code/str))]
            [(200)
             (let ([maj/str (extract-field "Package-Major-Version" head)]
                   [min/str (extract-field "Package-Minor-Version" head)]
                   [content-length/str (extract-field "Content-Length" head)])
               (unless (and maj/str min/str content-length/str
                            (nat? (string->number maj/str))
                            (nat? (string->number min/str))
                            (nat? (string->number content-length/str)))
                 (abort "Server did not include valid major and minor version information"))
               (let* ([filename (make-temporary-file "planettmp~a.plt")]
                      [maj      (string->number maj/str)]
                      [min      (string->number min/str)]
                      [content-length (string->number content-length/str)]
                      [op (open-output-file filename 'truncate/replace)])
                 (copy-port ip op)
                 (close-input-port ip)
                 (close-output-port op)
                 (if (= (file-size filename) content-length)
                   (list #t filename maj min)
                   (loop (add1 attempts)))))]
            [(404)
             (begin0 (list #f (format "Server had no matching package: ~a"
                                      (read-line ip)))
               (close-input-port ip))]
            [(400)
             (abort (format "Internal error (malformed request): ~a"
                            (read-line ip)))]
            [(500)
             (abort (format "Server internal error: ~a"
                            (apply string-append
                                   (let loop ()
                                     (let ([line (read-line ip)])
                                       (if (eof-object? line)
                                         '()
                                         (list* line "\n" (loop))))))))]
            [else
             (abort (format "Internal error (unknown HTTP response code ~a)"
                            response-code))]))))))

;; formats the pkg-spec back into a string the way the user typed it in.
;; assumes that the pkg-spec comes from the command-line
(define (pkg-spec->string pkg)
  (format "~a/~a~a~a"
          (if (pair? (pkg-spec-path pkg))
              (car (pkg-spec-path pkg))
              "<<unknown>>") ;; this shouldn't happen
          (regexp-replace #rx"\\.plt$" (pkg-spec-name pkg) "")
          (if (pkg-spec-maj pkg) 
              (format ":~a" (pkg-spec-maj pkg))
              "")
          (cond
            [(and (pkg-spec-maj pkg)
                  (pkg-spec-minor-lo pkg))
             (format ".~a" (pkg-spec-minor-lo pkg))]
            [else ""])))
  
;; =============================================================================
;; MODULE MANAGEMENT
;; Handles interaction with the module system
;; =============================================================================

;; do-require : path path symbol syntax -> symbol
;; requires the given filename, which must be a module, in the given path.
(define (do-require file-path package-path module-path stx load?)
  (parameterize ([current-load-relative-directory package-path])
    ((current-module-name-resolver) file-path module-path stx load?)))

(define *package-search-chain*
  (make-parameter
   (list get/linkage
         get/installed-cache
         get/uninstalled-cache-dummy
         get/server
         get/uninstalled-cache)))

;; ============================================================
;; UTILITY
;; A few small utility functions

(define (last l) (car (last-pair l)))

;; make-directory*/paths : path -> (listof path)
;; like make-directory*, but returns what directories it actually created
(define (make-directory*/paths dir)
  (let ([dir (if (string? dir) (string->path dir) dir)])
    (let-values ([(base name dir?) (split-path dir)])
      (cond [(directory-exists? dir) '()]
            [(directory-exists? base) (make-directory dir) (list dir)]
            [else (let ([dirs (make-directory*/paths base)])
                    (make-directory dir)
                    (cons dir dirs))]))))
