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+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2017 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix build download-nar)
+  #:use-module (guix build download)
+  #:use-module (guix build utils)
+  #:use-module (guix serialization)
+  #:use-module (guix zlib)
+  #:use-module (guix progress)
+  #:use-module (web uri)
+  #:use-module (srfi srfi-11)
+  #:use-module (srfi srfi-26)
+  #:use-module (ice-9 format)
+  #:use-module (ice-9 match)
+  #:export (download-nar))
+
+;;; Commentary:
+;;;
+;;; Download a normalized archive or "nar", similar to what 'guix substitute'
+;;; does.  The intent here is to use substitute servers as content-addressed
+;;; mirrors of VCS checkouts.  This is mostly useful for users who have
+;;; disabled substitutes.
+;;;
+;;; Code:
+
+(define (urls-for-item item)
+  "Return the fallback nar URL for ITEM--e.g.,
+\"/gnu/store/cabbag3…-foo-1.2-checkout\"."
+  ;; Here we hard-code nar URLs without checking narinfos.  That's probably OK
+  ;; though.
+  ;; TODO: Use HTTPS?  The downside is the extra dependency.
+  (let ((bases '("http://mirror.hydra.gnu.org/guix"
+                 "http://berlin.guixsd.org"))
+        (item  (basename item)))
+    (append (map (cut string-append <> "/nar/gzip/" item) bases)
+            (map (cut string-append <> "/nar/" item) bases))))
+
+(define (restore-gzipped-nar port item size)
+  "Restore the gzipped nar read from PORT, of SIZE bytes (compressed), to
+ITEM."
+  ;; Since PORT is typically a non-file port (for instance because 'http-get'
+  ;; returns a delimited port), create a child process so we're back to a file
+  ;; port that can be passed to 'call-with-gzip-input-port'.
+  (match (pipe)
+    ((input . output)
+     (match (primitive-fork)
+       (0
+        (dynamic-wind
+          (const #t)
+          (lambda ()
+            (close-port output)
+            (close-port port)
+            (catch #t
+              (lambda ()
+                (call-with-gzip-input-port input
+                  (cut restore-file <> item)))
+              (lambda (key . args)
+                (print-exception (current-error-port)
+                                 (stack-ref (make-stack #t) 1)
+                                 key args)
+                (primitive-exit 1))))
+          (lambda ()
+            (primitive-exit 0))))
+       (child
+        (close-port input)
+        (dump-port* port output
+                    #:reporter (progress-reporter/file item size
+                                                       #:abbreviation
+                                                       store-path-abbreviation))
+        (close-port output)
+        (newline)
+        (match (waitpid child)
+          ((_ . status)
+           (unless (zero? status)
+             (error "nar decompression failed" status)))))))))
+
+(define (download-nar item)
+  "Download and extract the normalized archive for ITEM.  Return #t on
+success, #f otherwise."
+  ;; Let progress reports go through.
+  (setvbuf (current-error-port) _IONBF)
+  (setvbuf (current-output-port) _IONBF)
+
+  (let loop ((urls (urls-for-item item)))
+    (match urls
+      ((url rest ...)
+       (format #t "Trying content-addressed mirror at ~a...~%"
+               (uri-host (string->uri url)))
+       (let-values (((port size)
+                     (catch #t
+                       (lambda ()
+                         (http-fetch (string->uri url)))
+                       (lambda args
+                         (values #f #f)))))
+         (if (not port)
+             (loop rest)
+             (begin
+               (if size
+                   (format #t "Downloading from ~a (~,2h MiB)...~%" url
+                           (/ size (expt 2 20.)))
+                   (format #t "Downloading from ~a...~%" url))
+               (if (string-contains url "/gzip")
+                   (restore-gzipped-nar port item size)
+                   (begin
+                     ;; FIXME: Add progress report.
+                     (restore-file port item)
+                     (close-port port)))
+               #t))))
+      (()
+       #f))))