path: root/guix/scripts/substitute-binary.scm

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
;;; Copyright © 2014 Nikita Karetnikov <nikita@karetnikov.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 scripts substitute-binary)
  #:use-module (guix ui)
  #:use-module (guix store)
  #:use-module (guix utils)
  #:use-module (guix config)
  #:use-module (guix records)
  #:use-module (guix nar)
  #:use-module (guix hash)
  #:use-module (guix base64)
  #:use-module (guix pk-crypto)
  #:use-module (guix pki)
  #:use-module ((guix build utils) #:select (mkdir-p))
  #:use-module ((guix build download)
                #:select (progress-proc uri-abbreviation))
  #:use-module (ice-9 rdelim)
  #:use-module (ice-9 regex)
  #:use-module (ice-9 match)
  #:use-module (ice-9 threads)
  #:use-module (ice-9 format)
  #:use-module (ice-9 ftw)
  #:use-module (ice-9 binary-ports)
  #:use-module (rnrs io ports)
  #:use-module (rnrs bytevectors)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-11)
  #:use-module (srfi srfi-19)
  #:use-module (srfi srfi-26)
  #:use-module (srfi srfi-34)
  #:use-module (srfi srfi-35)
  #:use-module (web uri)
  #:use-module (guix http-client)
  #:export (narinfo-signature->canonical-sexp
            read-narinfo
            write-narinfo
            guix-substitute-binary))

;;; Comment:
;;;
;;; This is the "binary substituter".  It is invoked by the daemon do check
;;; for the existence of available "substitutes" (pre-built binaries), and to
;;; actually use them as a substitute to building things locally.
;;;
;;; If possible, substitute a binary for the requested store path, using a Nix
;;; "binary cache".  This program implements the Nix "substituter" protocol.
;;;
;;; Code:

(define %narinfo-cache-directory
  ;; A local cache of narinfos, to avoid going to the network.
  (or (and=> (getenv "XDG_CACHE_HOME")
             (cut string-append <> "/guix/substitute-binary"))
      (string-append %state-directory "/substitute-binary/cache")))

(define %allow-unauthenticated-substitutes?
  ;; Whether to allow unchecked substitutes.  This is useful for testing
  ;; purposes, and should be avoided otherwise.
  (and (and=> (getenv "GUIX_ALLOW_UNAUTHENTICATED_SUBSTITUTES")
              (cut string-ci=? <> "yes"))
       (begin
         (warning (_ "authentication and authorization of substitutes \
disabled!~%"))
         #t)))

(define %narinfo-ttl
  ;; Number of seconds during which cached narinfo lookups are considered
  ;; valid.
  (* 24 3600))

(define %narinfo-negative-ttl
  ;; Likewise, but for negative lookups---i.e., cached lookup failures.
  (* 3 3600))

(define %narinfo-expired-cache-entry-removal-delay
  ;; How often we want to remove files corresponding to expired cache entries.
  (* 7 24 3600))

;; In Guile 2.0.9, `regexp-exec' is thread-unsafe, so work around it.
;; See <http://bugs.gnu.org/14404>.
(set! regexp-exec
      (let ((real regexp-exec)
            (lock (make-mutex)))
        (lambda (rx str . rest)
          (with-mutex lock
            (apply real rx str rest)))))

(define fields->alist
  ;; The narinfo format is really just like recutils.
  recutils->alist)

(define %fetch-timeout
  ;; Number of seconds after which networking is considered "slow".
  5)

(define %random-state
  (seed->random-state (+ (ash (cdr (gettimeofday)) 32) (getpid))))

(define-syntax-rule (with-timeout duration handler body ...)
  "Run BODY; when DURATION seconds have expired, call HANDLER, and run BODY
again."
  (begin
    (sigaction SIGALRM
      (lambda (signum)
        (sigaction SIGALRM SIG_DFL)
        handler))
    (alarm duration)
    (call-with-values
        (lambda ()
          (let try ()
            (catch 'system-error
              (lambda ()
                body ...)
              (lambda args
                ;; Before Guile v2.0.9-39-gfe51c7b, the SIGALRM triggers EINTR
                ;; because of the bug at
                ;; <http://lists.gnu.org/archive/html/guile-devel/2013-06/msg00050.html>.
                ;; When that happens, try again.  Note: SA_RESTART cannot be
                ;; used because of <http://bugs.gnu.org/14640>.
                (if (= EINTR (system-error-errno args))
                    (begin
                      ;; Wait a little to avoid bursts.
                      (usleep (random 3000000 %random-state))
                      (try))
                    (apply throw args))))))
      (lambda result
        (alarm 0)
        (sigaction SIGALRM SIG_DFL)
        (apply values result)))))

(define* (fetch uri #:key (buffered? #t) (timeout? #t) (quiet-404? #f))
  "Return a binary input port to URI and the number of bytes it's expected to
provide.  If QUIET-404? is true, HTTP 404 error conditions are passed through
to the caller without emitting an error message."
  (case (uri-scheme uri)
    ((file)
     (let ((port (open-file (uri-path uri)
                            (if buffered? "rb" "r0b"))))
       (values port (stat:size (stat port)))))
    ((http)
     (guard (c ((http-get-error? c)
                (let ((code (http-get-error-code c)))
                  (if (and (= code 404) quiet-404?)
                      (raise c)
                      (leave (_ "download from '~a' failed: ~a, ~s~%")
                             (uri->string (http-get-error-uri c))
                             code (http-get-error-reason c))))))
       ;; On Guile 2.0.5, `http-fetch' fetches the whole thing at once.  So
       ;; honor TIMEOUT? to disable the timeout when fetching a nar.
       ;;
       ;; Test this with:
       ;;   sudo tc qdisc add dev eth0 root netem delay 1500ms
       ;; and then cancel with:
       ;;   sudo tc qdisc del dev eth0 root
       (let ((port #f))
         (with-timeout (if (or timeout? (guile-version>? "2.0.5"))
                           %fetch-timeout
                           0)
           (begin
             (warning (_ "while fetching ~a: server is unresponsive~%")
                      (uri->string uri))
             (warning (_ "try `--no-substitutes' if the problem persists~%"))

             ;; Before Guile v2.0.9-39-gfe51c7b, EINTR was reported to the user,
             ;; and thus PORT had to be closed and re-opened.  This is not the
             ;; case afterward.
             (unless (or (guile-version>? "2.0.9")
                         (version>? (version) "2.0.9.39"))
               (when port
                 (close-port port))))
           (begin
             (when (or (not port) (port-closed? port))
               (set! port (open-socket-for-uri uri #:buffered? buffered?)))
             (http-fetch uri #:text? #f #:port port))))))))

(define-record-type <cache>
  (%make-cache url store-directory wants-mass-query?)
  cache?
  (url               cache-url)
  (store-directory   cache-store-directory)
  (wants-mass-query? cache-wants-mass-query?))

(define (open-cache url)
  "Open the binary cache at URL.  Return a <cache> object on success, or #f on
failure."
  (define (download-cache-info url)
    ;; Download the `nix-cache-info' from URL, and return its contents as an
    ;; list of key/value pairs.
    (and=> (false-if-exception (fetch (string->uri url)))
           fields->alist))

  (and=> (download-cache-info (string-append url "/nix-cache-info"))
         (lambda (properties)
           (alist->record properties
                          (cut %make-cache url <...>)
                          '("StoreDir" "WantMassQuery")))))

(define-record-type <narinfo>
  (%make-narinfo path uri uri-base compression file-hash file-size nar-hash nar-size
                 references deriver system signature contents)
  narinfo?
  (path         narinfo-path)
  (uri          narinfo-uri)
  (uri-base     narinfo-uri-base)        ; URI of the cache it originates from
  (compression  narinfo-compression)
  (file-hash    narinfo-file-hash)
  (file-size    narinfo-file-size)
  (nar-hash     narinfo-hash)
  (nar-size     narinfo-size)
  (references   narinfo-references)
  (deriver      narinfo-deriver)
  (system       narinfo-system)
  (signature    narinfo-signature)      ; canonical sexp
  ;; The original contents of a narinfo file.  This field is needed because we
  ;; want to preserve the exact textual representation for verification purposes.
  ;; See <https://lists.gnu.org/archive/html/guix-devel/2014-02/msg00340.html>
  ;; for more information.
  (contents     narinfo-contents))

(define (narinfo-signature->canonical-sexp str)
  "Return the value of a narinfo's 'Signature' field as a canonical sexp."
  (match (string-split str #\;)
    ((version _ sig)
     (let ((maybe-number (string->number version)))
       (cond ((not (number? maybe-number))
              (leave (_ "signature version must be a number: ~a~%")
                     version))
             ;; Currently, there are no other versions.
             ((not (= 1 maybe-number))
              (leave (_ "unsupported signature version: ~a~%")
                     maybe-number))
             (else
              (let ((signature (utf8->string (base64-decode sig))))
                (catch 'gcry-error
                  (lambda ()
                    (string->canonical-sexp signature))
                  (lambda (err . rest)
                    (leave (_ "signature is not a valid \
s-expression: ~s~%")
                           signature))))))))
    (x
     (leave (_ "invalid format of the signature field: ~a~%") x))))

(define (narinfo-maker str cache-url)
  "Return a narinfo constructor for narinfos originating from CACHE-URL.  STR
must contain the original contents of a narinfo file."
  (lambda (path url compression file-hash file-size nar-hash nar-size
                references deriver system signature)
    "Return a new <narinfo> object."
    (%make-narinfo path
                   ;; Handle the case where URL is a relative URL.
                   (or (string->uri url)
                       (string->uri (string-append cache-url "/" url)))
                   cache-url

                   compression file-hash
                   (and=> file-size string->number)
                   nar-hash
                   (and=> nar-size string->number)
                   (string-tokenize references)
                   (match deriver
                     ((or #f "") #f)
                     (_ deriver))
                   system
                   (false-if-exception
                    (and=> signature narinfo-signature->canonical-sexp))
                   str)))

(define* (assert-valid-signature narinfo signature hash
                                 #:optional (acl (current-acl)))
  "Bail out if SIGNATURE, a canonical sexp representing the signature of
NARINFO, doesn't match HASH, a bytevector containing the hash of NARINFO."
  (let ((uri (uri->string (narinfo-uri narinfo))))
    (signature-case (signature hash acl)
      (valid-signature #t)
      (invalid-signature
       (leave (_ "invalid signature for '~a'~%") uri))
      (hash-mismatch
       (leave (_ "hash mismatch for '~a'~%") uri))
      (unauthorized-key
       (leave (_ "'~a' is signed with an unauthorized key~%") uri))
      (corrupt-signature
       (leave (_ "signature on '~a' is corrupt~%") uri)))))

(define* (read-narinfo port #:optional url)
  "Read a narinfo from PORT.  If URL is true, it must be a string used to
build full URIs from relative URIs found while reading PORT.

No authentication and authorization checks are performed here!"
  (let ((str (utf8->string (get-bytevector-all port))))
    (alist->record (call-with-input-string str fields->alist)
                   (narinfo-maker str url)
                   '("StorePath" "URL" "Compression"
                     "FileHash" "FileSize" "NarHash" "NarSize"
                     "References" "Deriver" "System"
                     "Signature"))))

(define %signature-line-rx
  ;; Regexp matching a signature line in a narinfo.
  (make-regexp "(.+)^[[:blank:]]*Signature:[[:blank:]].+$"))

(define (narinfo-sha256 narinfo)
  "Return the sha256 hash of NARINFO as a bytevector, or #f if NARINFO lacks a
'Signature' field."
  (let ((contents (narinfo-contents narinfo)))
    (match (regexp-exec %signature-line-rx contents)
      (#f #f)
      ((= (cut match:substring <> 1) above-signature)
       (sha256 (string->utf8 above-signature))))))

(define* (assert-valid-narinfo narinfo
                               #:optional (acl (current-acl))
                               #:key (verbose? #t))
  "Raise an exception if NARINFO lacks a signature, has an invalid signature,
or is signed by an unauthorized key."
  (let ((hash (narinfo-sha256 narinfo)))
    (if (not hash)
        (if %allow-unauthenticated-substitutes?
            narinfo
            (leave (_ "narinfo for '~a' lacks a signature~%")
                   (uri->string (narinfo-uri narinfo))))
        (let ((signature (narinfo-signature narinfo)))
          (unless %allow-unauthenticated-substitutes?
            (assert-valid-signature narinfo signature hash acl)
            (when verbose?
              (format (current-error-port)
                      "found valid signature for '~a', from '~a'~%"
                      (narinfo-path narinfo)
                      (uri->string (narinfo-uri narinfo)))))
          narinfo))))

(define* (valid-narinfo? narinfo #:optional (acl (current-acl)))
  "Return #t if NARINFO's signature is not valid."
  (or %allow-unauthenticated-substitutes?
      (let ((hash      (narinfo-sha256 narinfo))
            (signature (narinfo-signature narinfo)))
        (and hash signature
             (signature-case (signature hash acl)
               (valid-signature #t)
               (else #f))))))

(define (write-narinfo narinfo port)
  "Write NARINFO to PORT."
  (put-bytevector port (string->utf8 (narinfo-contents narinfo))))

(define (narinfo->string narinfo)
  "Return the external representation of NARINFO."
  (call-with-output-string (cut write-narinfo narinfo <>)))

(define (string->narinfo str cache-uri)
  "Return the narinfo represented by STR.  Assume CACHE-URI as the base URI of
the cache STR originates form."
  (call-with-input-string str (cut read-narinfo <> cache-uri)))

(define (fetch-narinfo cache path)
  "Return the <narinfo> record for PATH, or #f if CACHE does not hold PATH."
  (define (download url)
    ;; Download the .narinfo from URL, and return its contents as a list of
    ;; key/value pairs.  Don't emit an error message upon 404.
    (false-if-exception (fetch (string->uri url)
                               #:quiet-404? #t)))

  (and (string=? (cache-store-directory cache) (%store-prefix))
       (and=> (download (string-append (cache-url cache) "/"
                                       (store-path-hash-part path)
                                       ".narinfo"))
              (cute read-narinfo <> (cache-url cache)))))

(define (obsolete? date now ttl)
  "Return #t if DATE is obsolete compared to NOW + TTL seconds."
  (time>? (subtract-duration now (make-time time-duration 0 ttl))
          (make-time time-monotonic 0 date)))

(define %lookup-threads
  ;; Number of threads spawned to perform lookup operations.  This means we
  ;; can have this many simultaneous HTTP GET requests to the server, which
  ;; limits the impact of connection latency.
  20)

(define (lookup-narinfo cache path)
  "Check locally if we have valid info about PATH, otherwise go to CACHE and
check what it has."
  (define now
    (current-time time-monotonic))

  (define cache-file
    (string-append %narinfo-cache-directory "/"
                   (store-path-hash-part path)))

  (define (cache-entry cache-uri narinfo)
    `(narinfo (version 1)
              (cache-uri ,cache-uri)
              (date ,(time-second now))
              (value ,(and=> narinfo narinfo->string))))

  (let*-values (((valid? cached)
                 (catch 'system-error
                   (lambda ()
                     (call-with-input-file cache-file
                       (lambda (p)
                         (match (read p)
                           (('narinfo ('version 1)
                                      ('cache-uri cache-uri)
                                      ('date date) ('value #f))
                            ;; A cached negative lookup.
                            (if (obsolete? date now %narinfo-negative-ttl)
                                (values #f #f)
                                (values #t #f)))
                           (('narinfo ('version 1)
                                      ('cache-uri cache-uri)
                                      ('date date) ('value value))
                            ;; A cached positive lookup
                            (if (obsolete? date now %narinfo-ttl)
                                (values #f #f)
                                (values #t (string->narinfo value
                                                            cache-uri))))
                           (('narinfo ('version v) _ ...)
                            (values #f #f))))))
                   (lambda _
                     (values #f #f)))))
    (if valid?
        cached                                    ; including negative caches
        (let* ((cache   (force cache))
               (narinfo (and cache (fetch-narinfo cache path))))
          ;; Cache NARINFO only when CACHE was actually accessible.  This
          ;; avoids caching negative hits when in fact we just lacked network
          ;; access.
          (when cache
            (with-atomic-file-output cache-file
              (lambda (out)
                (write (cache-entry (cache-url cache) narinfo) out))))
          narinfo))))

(define (remove-expired-cached-narinfos)
  "Remove expired narinfo entries from the cache.  The sole purpose of this
function is to make sure `%narinfo-cache-directory' doesn't grow
indefinitely."
  (define now
    (current-time time-monotonic))

  (define (expired? file)
    (catch 'system-error
      (lambda ()
        (call-with-input-file file
          (lambda (port)
            (match (read port)
              (('narinfo ('version 1) ('cache-uri _) ('date date)
                         ('value #f))
               (obsolete? date now %narinfo-negative-ttl))
              (('narinfo ('version 1) ('cache-uri _) ('date date)
                         ('value _))
               (obsolete? date now %narinfo-ttl))
              (_ #t)))))
      (lambda args
        ;; FILE may have been deleted.
        #t)))

  (for-each (lambda (file)
              (let ((file (string-append %narinfo-cache-directory
                                         "/" file)))
                (when (expired? file)
                  ;; Wrap in `false-if-exception' because FILE might have been
                  ;; deleted in the meantime (TOCTTOU).
                  (false-if-exception (delete-file file)))))
            (scandir %narinfo-cache-directory
                     (lambda (file)
                       (= (string-length file) 32)))))

(define (maybe-remove-expired-cached-narinfo)
  "Remove expired narinfo entries from the cache if deemed necessary."
  (define now
    (current-time time-monotonic))

  (define expiry-file
    (string-append %narinfo-cache-directory "/last-expiry-cleanup"))

  (define last-expiry-date
    (or (false-if-exception
         (call-with-input-file expiry-file read))
        0))

  (when (obsolete? last-expiry-date now %narinfo-expired-cache-entry-removal-delay)
    (remove-expired-cached-narinfos)
    (call-with-output-file expiry-file
      (cute write (time-second now) <>))))

(define (progress-report-port report-progress port)
  "Return a port that calls REPORT-PROGRESS every time something is read from
PORT.  REPORT-PROGRESS is a two-argument procedure such as that returned by
`progress-proc'."
  (define total 0)
  (define (read! bv start count)
    (let ((n (match (get-bytevector-n! port bv start count)
               ((? eof-object?) 0)
               (x x))))
      (set! total (+ total n))
      (report-progress total (const n))
      ;; XXX: We're not in control, so we always return anyway.
      n))

  ;; Since `http-fetch' in Guile 2.0.5 returns all the data once it's done,
  ;; don't pretend to report any progress in that case.
  (if (guile-version>? "2.0.5")
      (make-custom-binary-input-port "progress-port-proc"
                                     read! #f #f
                                     (cut close-port port))
      (begin
        (format (current-error-port) (_ "Downloading, please wait...~%"))
        (format (current-error-port)
                (_ "(Please consider upgrading Guile to get proper progress report.)~%"))
        port)))

(define %cache-url
  (or (getenv "GUIX_BINARY_SUBSTITUTE_URL")
      "http://hydra.gnu.org"))

(define-syntax with-networking
  (syntax-rules ()
    "Catch DNS lookup errors and gracefully exit."
    ;; Note: no attempt is made to catch other networking errors, because DNS
    ;; lookup errors are typically the first one, and because other errors are
    ;; a subset of `system-error', which is harder to filter.
    ((_ exp ...)
     (catch 'getaddrinfo-error
       (lambda () exp ...)
       (lambda (key error)
         (leave (_ "host name lookup error: ~a~%")
                (gai-strerror error)))))))


;;;
;;; Help.
;;;

(define (show-help)
  (display (_ "Usage: guix substitute-binary [OPTION]...
Internal tool to substitute a pre-built binary to a local build.\n"))
  (display (_ "
      --query            report on the availability of substitutes for the
                         store file names passed on the standard input"))
  (display (_ "
      --substitute STORE-FILE DESTINATION
                         download STORE-FILE and store it as a Nar in file
                         DESTINATION"))
  (newline)
  (display (_ "
  -h, --help             display this help and exit"))
  (display (_ "
  -V, --version          display version information and exit"))
  (newline)
  (show-bug-report-information))



;;;
;;; Entry point.
;;;

(define n-par-map*
  ;; We want the ability to run many threads in parallel, regardless of the
  ;; number of cores.  However, Guile 2.0.5 has a bug whereby 'n-par-map' ends
  ;; up consuming a lot of memory, possibly leading to death.  Thus, resort to
  ;; 'par-map' on 2.0.5.
  (if (guile-version>? "2.0.5")
      n-par-map
      (lambda (n proc lst)
        (par-map proc lst))))

(define (check-acl-initialized)
  "Warn if the ACL is uninitialized."
  (define (singleton? acl)
    ;; True if ACL contains just the user's public key.
    (and (file-exists? %public-key-file)
         (let ((key (call-with-input-file %public-key-file
                      (compose string->canonical-sexp
                               get-string-all))))
           (equal? (acl->public-keys acl) (list key)))))

  (let ((acl (current-acl)))
    (when (or (null? acl) (singleton? acl))
      (warning (_ "ACL for archive imports seems to be uninitialized, \
substitutes may be unavailable\n")))))

(define (guix-substitute-binary . args)
  "Implement the build daemon's substituter protocol."
  (mkdir-p %narinfo-cache-directory)
  (maybe-remove-expired-cached-narinfo)

  ;; Starting from commit 22144afa in Nix, we are allowed to bail out directly
  ;; when we know we cannot substitute, but we must emit a newline on stdout
  ;; when everything is alright.
  (let ((uri (string->uri %cache-url)))
    (case (uri-scheme uri)
      ((http)
       ;; Exit gracefully if there's no network access.
       (let ((host (uri-host uri)))
         (catch 'getaddrinfo-error
           (lambda ()
             (getaddrinfo host))
           (lambda (key error)
             (warning (_ "failed to look up host '~a' (~a), \
substituter disabled~%")
                      host (gai-strerror error))
             (exit 0)))))
      (else #t)))

  ;; Say hello (see above.)
  (newline)
  (force-output (current-output-port))

  (with-networking
   (with-error-handling                           ; for signature errors
     (match args
       (("--query")
        (let ((cache (delay (open-cache %cache-url)))
              (acl   (current-acl)))
          (define (valid? obj)
            (and (narinfo? obj) (valid-narinfo? obj acl)))

          (let loop ((command (read-line)))
            (or (eof-object? command)
                (begin
                  (match (string-tokenize command)
                    (("have" paths ..1)
                     ;; Return the subset of PATHS available in CACHE.
                     (let ((substitutable
                            (if cache
                                (n-par-map* %lookup-threads
                                            (cut lookup-narinfo cache <>)
                                            paths)
                                '())))
                       (for-each (lambda (narinfo)
                                   (format #t "~a~%" (narinfo-path narinfo)))
                                 (filter valid? substitutable))
                       (newline)))
                    (("info" paths ..1)
                     ;; Reply info about PATHS if it's in CACHE.
                     (let ((substitutable
                            (if cache
                                (n-par-map* %lookup-threads
                                            (cut lookup-narinfo cache <>)
                                            paths)
                                '())))
                       (for-each (lambda (narinfo)
                                   (format #t "~a\n~a\n~a\n"
                                           (narinfo-path narinfo)
                                           (or (and=> (narinfo-deriver narinfo)
                                                      (cute string-append
                                                            (%store-prefix) "/"
                                                            <>))
                                               "")
                                           (length (narinfo-references narinfo)))
                                   (for-each (cute format #t "~a/~a~%"
                                                   (%store-prefix) <>)
                                             (narinfo-references narinfo))
                                   (format #t "~a\n~a\n"
                                           (or (narinfo-file-size narinfo) 0)
                                           (or (narinfo-size narinfo) 0)))
                                 (filter valid? substitutable))
                       (newline)))
                    (wtf
                     (error "unknown `--query' command" wtf)))
                  (loop (read-line)))))))
       (("--substitute" store-path destination)
        ;; Download STORE-PATH and add store it as a Nar in file DESTINATION.
        (let* ((cache   (delay (open-cache %cache-url)))
               (narinfo (lookup-narinfo cache store-path))
               (uri     (narinfo-uri narinfo)))
          ;; Make sure it is signed and everything.
          (assert-valid-narinfo narinfo)

          ;; Tell the daemon what the expected hash of the Nar itself is.
          (format #t "~a~%" (narinfo-hash narinfo))

          (format (current-error-port) "downloading `~a' from `~a'~:[~*~; (~,1f MiB installed)~]...~%"
                  store-path (uri->string uri)

                  ;; Use the Nar size as an estimate of the installed size.
                  (narinfo-size narinfo)
                  (and=> (narinfo-size narinfo)
                         (cute / <> (expt 2. 20))))
          (let*-values (((raw download-size)
                         ;; Note that Hydra currently generates Nars on the fly
                         ;; and doesn't specify a Content-Length, so
                         ;; DOWNLOAD-SIZE is #f in practice.
                         (fetch uri #:buffered? #f #:timeout? #f))
                        ((progress)
                         (let* ((comp     (narinfo-compression narinfo))
                                (dl-size  (or download-size
                                              (and (equal? comp "none")
                                                   (narinfo-size narinfo))))
                                (progress (progress-proc (uri-abbreviation uri)
                                                         dl-size
                                                         (current-error-port))))
                           (progress-report-port progress raw)))
                        ((input pids)
                         (decompressed-port (and=> (narinfo-compression narinfo)
                                                   string->symbol)
                                            progress)))
            ;; Unpack the Nar at INPUT into DESTINATION.
            (restore-file input destination)
            (every (compose zero? cdr waitpid) pids))))
       (("--version")
        (show-version-and-exit "guix substitute-binary"))
       (("--help")
        (show-help))
       (opts
        (leave (_ "~a: unrecognized options~%") opts))))))


;;; Local Variables:
;;; eval: (put 'with-timeout 'scheme-indent-function 1)
;;; End:

;;; substitute-binary.scm ends here