path: root/guix/scripts/substitute.scm

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
;;; Copyright © 2014 Nikita Karetnikov <nikita@karetnikov.org>
;;; Copyright © 2018 Kyle Meyer <kyle@kyleam.com>
;;; Copyright © 2020 Christopher Baines <mail@cbaines.net>
;;;
;;; 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)
  #:use-module (guix ui)
  #:use-module (guix scripts)
  #:use-module (guix narinfo)
  #:use-module (guix store)
  #:use-module (guix substitutes)
  #:use-module (guix utils)
  #:use-module (guix combinators)
  #:use-module (guix config)
  #:use-module (guix records)
  #:use-module (guix diagnostics)
  #:use-module (guix i18n)
  #:use-module ((guix serialization) #:select (restore-file dump-file))
  #:autoload   (guix store deduplication) (dump-file/deduplicate)
  #:autoload   (guix scripts discover) (read-substitute-urls)
  #:use-module (gcrypt hash)
  #:use-module (guix base32)
  #:use-module (guix base64)
  #:use-module (guix cache)
  #:use-module (gcrypt pk-crypto)
  #:use-module (guix pki)
  #:use-module ((guix build utils) #:select (mkdir-p))
  #:use-module ((guix build download)
                #:select (uri-abbreviation nar-uri-abbreviation
                          (open-connection-for-uri
                           . guix:open-connection-for-uri)))
  #:autoload   (gnutls) (error/invalid-session)
  #:use-module (guix progress)
  #:use-module ((guix build syscalls)
                #:select (set-thread-name))
  #:use-module (ice-9 rdelim)
  #:use-module (ice-9 match)
  #:use-module (ice-9 format)
  #:use-module (ice-9 ftw)
  #:use-module (rnrs bytevectors)
  #:use-module (srfi srfi-1)
  #: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 (%allow-unauthenticated-substitutes?
            %error-to-file-descriptor-4?

            substitute-urls
            guix-substitute))

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

(define (warn-about-missing-authentication)
  (warning (G_ "authentication and authorization of substitutes \
disabled!~%"))
  #t)

(define %allow-unauthenticated-substitutes?
  ;; Whether to allow unchecked substitutes.  This is useful for testing
  ;; purposes, and should be avoided otherwise.
  (make-parameter
   (and=> (getenv "GUIX_ALLOW_UNAUTHENTICATED_SUBSTITUTES")
          (cut string-ci=? <> "yes"))))

(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 (at-most max-length lst)
  "If LST is shorter than MAX-LENGTH, return it and the empty list; otherwise
return its MAX-LENGTH first elements and its tail."
  (let loop ((len 0)
             (lst lst)
             (result '()))
    (match lst
      (()
       (values (reverse result) '()))
      ((head . tail)
       (if (>= len max-length)
           (values (reverse result) lst)
           (loop (+ 1 len) tail (cons head result)))))))

(define (narinfo-from-file file url)
  "Attempt to read a narinfo from FILE, using URL as the cache URL.  Return #f
if file doesn't exist, and the narinfo otherwise."
  (catch 'system-error
    (lambda ()
      (call-with-input-file file
        (cut read-narinfo <> url)))
    (lambda args
      (if (= ENOENT (system-error-errno args))
          #f
          (apply throw args)))))

(define (lookup-narinfo caches path authorized?)
  "Return the narinfo for PATH in CACHES, or #f when no substitute for PATH
was found."
  (match (lookup-narinfos/diverse caches (list path) authorized?)
    ((answer) answer)
    (_        #f)))

(define (cached-narinfo-expiration-time file)
  "Return the expiration time for FILE, which is a cached narinfo."
  (catch 'system-error
    (lambda ()
      (call-with-input-file file
        (lambda (port)
          (match (read port)
            (('narinfo ('version 2) ('cache-uri uri)
                       ('date date) ('ttl ttl) ('value #f))
             (+ date ttl))
            (('narinfo ('version 2) ('cache-uri uri)
                       ('date date) ('ttl ttl) ('value value))
             (+ date ttl))
            (x
             0)))))
    (lambda args
      ;; FILE may have been deleted.
      0)))

(define (narinfo-cache-directories directory)
  "Return the list of narinfo cache directories (one per cache URL.)"
  (map (cut string-append directory "/" <>)
       (scandir %narinfo-cache-directory
                (lambda (item)
                  (and (not (member item '("." "..")))
                       (file-is-directory?
                        (string-append %narinfo-cache-directory
                                       "/" item)))))))

(define* (cached-narinfo-files #:optional
                               (directory %narinfo-cache-directory))
  "Return the list of cached narinfo files under DIRECTORY."
  (append-map (lambda (directory)
                (map (cut string-append directory "/" <>)
                     (scandir directory
                              (lambda (file)
                                (= (string-length file) 32)))))
              (narinfo-cache-directories directory)))

(define-syntax with-networking
  (syntax-rules ()
    "Catch DNS lookup errors and TLS 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 ...)
     ;; Use a pre-unwind handler so that re-throwing preserves useful
     ;; backtraces.  'with-throw-handler' works for Guile 2.2 and 3.0.
     (with-throw-handler #t
       (lambda () exp ...)
       (match-lambda*
         (('getaddrinfo-error error)
          (leave (G_ "host name lookup error: ~a~%")
                 (gai-strerror error)))
         (('gnutls-error error proc . rest)
          (let ((error->string (module-ref (resolve-interface '(gnutls))
                                           'error->string)))
            (leave (G_ "TLS error in procedure '~a': ~a~%")
                   proc (error->string error))))
         (args
          (apply throw args)))))))


;;;
;;; Help.
;;;

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



;;;
;;; Daemon/substituter protocol.
;;;

(define %prefer-fast-decompression?
  ;; Whether to prefer fast decompression over good compression ratios.  This
  ;; serves in particular to choose between lzip (high compression ratio but
  ;; low decompression throughput) and zstd (lower compression ratio but high
  ;; decompression throughput).
  #f)

(define (call-with-cpu-usage-monitoring proc)
  (let ((before (times)))
    (proc)
    (let ((after (times)))
      (if (= (tms:clock after) (tms:clock before))
          0
          (/ (- (tms:utime after) (tms:utime before))
             (- (tms:clock after) (tms:clock before))
             1.)))))

(define-syntax-rule (with-cpu-usage-monitoring exp ...)
  "Evaluate EXP...  Return its CPU usage as a fraction between 0 and 1."
  (call-with-cpu-usage-monitoring (lambda () exp ...)))

(define (display-narinfo-data narinfo)
  "Write to the current output port the contents of NARINFO in the format
expected by the daemon."
  (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))

  (let-values (((uri compression file-size)
                (narinfo-best-uri narinfo
                                  #:fast-decompression?
                                  %prefer-fast-decompression?)))
    (format #t "~a\n~a\n"
            (or file-size 0)
            (or (narinfo-size narinfo) 0))))

(define* (process-query command
                        #:key cache-urls acl)
  "Reply to COMMAND, a query as written by the daemon to this process's
standard input.  Use ACL as the access-control list against which to check
authorized substitutes."
  (define valid?
    (if (%allow-unauthenticated-substitutes?)
        (begin
          (warn-about-missing-authentication)

          (const #t))
        (lambda (obj)
          (valid-narinfo? obj acl))))

  (define* (make-progress-reporter total #:key url)
    (define done 0)

    (define (report-progress)
      (erase-current-line (current-error-port)) ;erase current line
      (force-output (current-error-port))
      (format (current-error-port)
              (G_ "updating substitutes from '~a'... ~5,1f%")
              url (* 100. (/ done total)))
      (set! done (+ 1 done)))

    (progress-reporter
     (start report-progress)
     (report report-progress)
     (stop (lambda ()
             (newline (current-error-port))))))

  (match (string-tokenize command)
    (("have" paths ..1)
     ;; Return the subset of PATHS available in CACHE-URLS.
     (let ((substitutable (lookup-narinfos/diverse
                           cache-urls paths valid?
                           #:open-connection open-connection-for-uri/cached
                           #:make-progress-reporter make-progress-reporter)))
       (for-each (lambda (narinfo)
                   (format #t "~a~%" (narinfo-path narinfo)))
                 substitutable)
       (newline)))
    (("info" paths ..1)
     ;; Reply info about PATHS if it's in CACHE-URLS.
     (let ((substitutable (lookup-narinfos/diverse
                           cache-urls paths valid?
                           #:open-connection open-connection-for-uri/cached
                           #:make-progress-reporter make-progress-reporter)))
       (for-each display-narinfo-data substitutable)
       (newline)))
    (wtf
     (error "unknown `--query' command" wtf))))

(define %max-cached-connections
  ;; Maximum number of connections kept in cache by
  ;; 'open-connection-for-uri/cached'.
  16)

(define open-connection-for-uri/cached
  (let ((cache '()))
    (lambda* (uri #:key fresh? (timeout %fetch-timeout) verify-certificate?)
      "Return a connection for URI, possibly reusing a cached connection.
When FRESH? is true, delete any cached connections for URI and open a new one.
Return #f if URI's scheme is 'file' or #f.

When true, TIMEOUT is the maximum number of milliseconds to wait for
connection establishment.  When VERIFY-CERTIFICATE? is true, verify HTTPS
server certificates."
      (define host (uri-host uri))
      (define scheme (uri-scheme uri))
      (define key (list host scheme (uri-port uri)))

      (and (not (memq scheme '(file #f)))
           (match (assoc-ref cache key)
             (#f
              ;; Open a new connection to URI and evict old entries from
              ;; CACHE, if any.
              (let-values (((socket)
                            (guix:open-connection-for-uri
                             uri
                             #:verify-certificate? verify-certificate?
                             #:timeout timeout))
                           ((new-cache evicted)
                            (at-most (- %max-cached-connections 1) cache)))
                (for-each (match-lambda
                            ((_ . port)
                             (false-if-exception (close-port port))))
                          evicted)
                (set! cache (alist-cons key socket new-cache))
                socket))
             (socket
              (if (or fresh? (port-closed? socket))
                  (begin
                    (false-if-exception (close-port socket))
                    (set! cache (alist-delete key cache))
                    (open-connection-for-uri/cached uri #:timeout timeout
                                                    #:verify-certificate?
                                                    verify-certificate?))
                  (begin
                    ;; Drain input left from the previous use.
                    (drain-input socket)
                    socket))))))))

(define (call-with-cached-connection uri proc)
  (let ((port (open-connection-for-uri/cached uri
                                              #:verify-certificate? #f)))
    (catch #t
      (lambda ()
        (proc port))
      (lambda (key . args)
        ;; If PORT was cached and the server closed the connection in the
        ;; meantime, we get EPIPE.  In that case, open a fresh connection
        ;; and retry.  We might also get 'bad-response or a similar
        ;; exception from (web response) later on, once we've sent the
        ;; request, or a ERROR/INVALID-SESSION from GnuTLS.
        (if (or (and (eq? key 'system-error)
                     (= EPIPE (system-error-errno `(,key ,@args))))
                (and (eq? key 'gnutls-error)
                     (eq? (first args) error/invalid-session))
                (memq key '(bad-response bad-header bad-header-component)))
            (proc (open-connection-for-uri/cached uri
                                                  #:verify-certificate? #f
                                                  #:fresh? #t))
            (apply throw key args))))))

(define-syntax-rule (with-cached-connection uri port exp ...)
  "Bind PORT with EXP... to a socket connected to URI."
  (call-with-cached-connection uri (lambda (port) exp ...)))

(define* (process-substitution store-item destination
                               #:key cache-urls acl
                               deduplicate? print-build-trace?)
  "Substitute STORE-ITEM (a store file name) from CACHE-URLS, and write it to
DESTINATION as a nar file.  Verify the substitute against ACL, and verify its
hash against what appears in the narinfo.  When DEDUPLICATE? is true, and if
DESTINATION is in the store, deduplicate its files.  Print a status line on
the current output port."
  (define narinfo
    (lookup-narinfo cache-urls store-item
                    (if (%allow-unauthenticated-substitutes?)
                        (const #t)
                        (cut valid-narinfo? <> acl))))

  (define destination-in-store?
    (string-prefix? (string-append (%store-prefix) "/")
                    destination))

  (define (dump-file/deduplicate* . args)
    ;; Make sure deduplication looks at the right store (necessary in test
    ;; environments).
    (apply dump-file/deduplicate
           (append args (list #:store (%store-prefix)))))

  (define (fetch uri)
    (case (uri-scheme uri)
      ((file)
       (let ((port (open-file (uri-path uri) "r0b")))
         (values port (stat:size (stat port)))))
      ((http https)
       (guard (c ((http-get-error? c)
                  (leave (G_ "download from '~a' failed: ~a, ~s~%")
                         (uri->string (http-get-error-uri c))
                         (http-get-error-code c)
                         (http-get-error-reason c))))
         ;; Test this with:
         ;;   sudo tc qdisc add dev eth0 root netem delay 1500ms
         ;; and then cancel with:
         ;;   sudo tc qdisc del dev eth0 root
         (with-timeout %fetch-timeout
           (begin
             (warning (G_ "while fetching ~a: server is somewhat slow~%")
                      (uri->string uri))
             (warning (G_ "try `--no-substitutes' if the problem persists~%")))
           (with-cached-connection uri port
             (http-fetch uri #:text? #f
                         #:port port
                         #:keep-alive? #t
                         #:buffered? #f)))))
      (else
       (leave (G_ "unsupported substitute URI scheme: ~a~%")
              (uri->string uri)))))

  (unless narinfo
    (leave (G_ "no valid substitute for '~a'~%")
           store-item))

  (let-values (((uri compression file-size)
                (narinfo-best-uri narinfo
                                  #:fast-decompression?
                                  %prefer-fast-decompression?)))
    (unless print-build-trace?
      (format (current-error-port)
              (G_ "Downloading ~a...~%") (uri->string uri)))

    (let*-values (((raw download-size)
                   ;; 'guix publish' without '--cache' doesn't specify a
                   ;; Content-Length, so DOWNLOAD-SIZE is #f in this case.
                   (fetch uri))
                  ((progress)
                   (let* ((dl-size  (or download-size
                                        (and (equal? compression "none")
                                             (narinfo-size narinfo))))
                          (reporter (if print-build-trace?
                                        (progress-reporter/trace
                                         destination
                                         (uri->string uri) dl-size
                                         (current-error-port))
                                        (progress-reporter/file
                                         (uri->string uri) dl-size
                                         (current-error-port)
                                         #:abbreviation nar-uri-abbreviation))))
                     ;; Keep RAW open upon completion so we can later reuse
                     ;; the underlying connection.
                     (progress-report-port reporter raw #:close? #f)))
                  ((input pids)
                   ;; NOTE: This 'progress' port of current process will be
                   ;; closed here, while the child process doing the
                   ;; reporting will close it upon exit.
                   (decompressed-port (string->symbol compression)
                                      progress))

                  ;; Compute the actual nar hash as we read it.
                  ((algorithm expected)
                   (narinfo-hash-algorithm+value narinfo))
                  ((hashed get-hash)
                   (open-hash-input-port algorithm input)))
      ;; Unpack the Nar at INPUT into DESTINATION.
      (define cpu-usage
        (with-cpu-usage-monitoring
         (restore-file hashed destination
                       #:dump-file (if (and destination-in-store?
                                            deduplicate?)
                                       dump-file/deduplicate*
                                       dump-file))))

      ;; Create a hysteresis: depending on CPU usage, favor compression
      ;; methods with faster decompression (like ztsd) or methods with better
      ;; compression ratios (like lzip).  This stems from the observation that
      ;; substitution can be CPU-bound when high-speed networks are used:
      ;; <https://lists.gnu.org/archive/html/guix-devel/2020-12/msg00177.html>.
      ;; To simulate "slow" networking or changing conditions, run:
      ;;   sudo tc qdisc add dev eno1 root tbf rate 512kbit latency 50ms burst 1540
      ;; and then cancel with:
      ;;   sudo tc qdisc del dev eno1 root
      (when (> cpu-usage .8)
        (set! %prefer-fast-decompression? #t))
      (when (< cpu-usage .2)
        (set! %prefer-fast-decompression? #f))

      (close-port hashed)
      (close-port input)

      ;; Wait for the reporter to finish.
      (every (compose zero? cdr waitpid) pids)

      ;; Skip a line after what 'progress-reporter/file' printed, and another
      ;; one to visually separate substitutions.  When PRINT-BUILD-TRACE? is
      ;; true, leave it up to (guix status) to prettify things.
      (newline (current-error-port))
      (unless print-build-trace?
        (newline (current-error-port)))

      ;; Check whether we got the data announced in NARINFO.
      (let ((actual (get-hash)))
        (if (bytevector=? actual expected)
            ;; Tell the daemon that we're done.
            (format (current-output-port) "success ~a ~a~%"
                    (narinfo-hash narinfo) (narinfo-size narinfo))
            ;; The actual data has a different hash than that in NARINFO.
            (format (current-output-port) "hash-mismatch ~a ~a ~a~%"
                    (hash-algorithm-name algorithm)
                    (bytevector->nix-base32-string expected)
                    (bytevector->nix-base32-string actual)))))))

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

(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
                               read-string))))
           (match acl
             ((thing)
              (equal? (canonical-sexp->string thing)
                      (canonical-sexp->string key)))
             (_
              #f)))))

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

(define (daemon-options)
  "Return a list of name/value pairs denoting build daemon options."
  (define %not-newline
    (char-set-complement (char-set #\newline)))

  (match (getenv "_NIX_OPTIONS")
    (#f                           ;should not happen when called by the daemon
     '())
    (newline-separated
     ;; Here we get something of the form "OPTION1=VALUE1\nOPTION2=VALUE2\n".
     (filter-map (lambda (option=value)
                   (match (string-index option=value #\=)
                     (#f                          ;invalid option setting
                      #f)
                     (equal-sign
                      (cons (string-take option=value equal-sign)
                            (string-drop option=value (+ 1 equal-sign))))))
                 (string-tokenize newline-separated %not-newline)))))

(define (find-daemon-option option)
  "Return the value of build daemon option OPTION, or #f if it could not be
found."
  (assoc-ref (daemon-options) option))

(define %default-substitute-urls
  (match (and=> (or (find-daemon-option "untrusted-substitute-urls") ;client
                    (find-daemon-option "substitute-urls"))          ;admin
                string-tokenize)
    ((urls ...)
     urls)
    (#f
     ;; This can only happen when this script is not invoked by the
     ;; daemon.
     '("http://ci.guix.gnu.org"))))

;; In order to prevent using large number of discovered local substitute
;; servers, limit the local substitute urls list size.
(define %max-substitute-urls 50)

(define* (randomize-substitute-urls urls
                                    #:key
                                    (max %max-substitute-urls))
  "Return a list containing MAX urls from URLS, picked randomly. If URLS list
is shorter than MAX elements, then it is directly returned."
  (define (random-item list)
    (list-ref list (random (length list))))

  (if (<= (length urls) max)
      urls
      (let loop ((res '())
                 (urls urls))
        (if (eq? (length res) max)
            res
            (let ((url (random-item urls)))
              (loop (cons url res) (delete url urls)))))))

(define %local-substitute-urls
  ;; If the following option is passed to the daemon, use the substitutes list
  ;; provided by "guix discover" process.
  (let* ((option (find-daemon-option "discover"))
         (discover? (and option (string=? option "true"))))
    (if discover?
     (randomize-substitute-urls (read-substitute-urls))
     '())))

(define substitute-urls
  ;; List of substitute URLs.
  (make-parameter (append %local-substitute-urls
                          %default-substitute-urls)))

(define (client-terminal-columns)
  "Return the number of columns in the client's terminal, if it is known, or a
default value."
  (or (and=> (or (find-daemon-option "untrusted-terminal-columns")
                 (find-daemon-option "terminal-columns"))
             (lambda (str)
               (let ((number (string->number str)))
                 (and number (max 20 (- number 1))))))
      80))

(define (validate-uri uri)
  (unless (string->uri uri)
    (leave (G_ "~a: invalid URI~%") uri)))

(define %error-to-file-descriptor-4?
  ;; Whether to direct 'current-error-port' to file descriptor 4 like
  ;; 'guix-daemon' expects.
  (make-parameter #t))

;; The daemon's agent code opens file descriptor 4 for us and this is where
;; stderr should go.
(define-syntax-rule (with-redirected-error-port exp ...)
  "Evaluate EXP... with the current error port redirected to file descriptor 4
if needed, as expected by the daemon's agent."
  (let ((thunk (lambda () exp ...)))
    (if (%error-to-file-descriptor-4?)
        (parameterize ((current-error-port (fdopen 4 "wl")))
          ;; Redirect diagnostics to file descriptor 4 as well.
          (guix-warning-port (current-error-port))

          ;; 'with-continuation-barrier' captures the initial value of
          ;; 'current-error-port' to report backtraces in case of uncaught
          ;; exceptions.  Without it, backtraces would be printed to FD 2,
          ;; thereby confusing the daemon.
          (with-continuation-barrier thunk))
        (thunk))))

(define-command (guix-substitute . args)
  (category internal)
  (synopsis "implement the build daemon's substituter protocol")

  (define print-build-trace?
    (match (or (find-daemon-option "untrusted-print-extended-build-trace")
               (find-daemon-option "print-extended-build-trace"))
      (#f #f)
      ((= string->number number) (> number 0))
      (_ #f)))

  (define deduplicate?
    (find-daemon-option "deduplicate"))

  (with-redirected-error-port
    (mkdir-p %narinfo-cache-directory)
    (maybe-remove-expired-cache-entries %narinfo-cache-directory
                                        cached-narinfo-files
                                        #:entry-expiration
                                        cached-narinfo-expiration-time
                                        #:cleanup-period
                                        %narinfo-expired-cache-entry-removal-delay)
    (check-acl-initialized)

    ;; Sanity-check SUBSTITUTE-URLS so we can provide a meaningful error
    ;; message.
    (for-each validate-uri (substitute-urls))

    ;; Attempt to install the client's locale so that messages are suitably
    ;; translated.  LC_CTYPE must be a UTF-8 locale; it's the case by default
    ;; so don't change it.
    (match (or (find-daemon-option "untrusted-locale")
               (find-daemon-option "locale"))
      (#f     #f)
      (locale (false-if-exception (setlocale LC_MESSAGES locale))))

    (catch 'system-error
      (lambda ()
        (set-thread-name "guix substitute"))
      (const #t))                                 ;GNU/Hurd lacks 'prctl'

    (with-networking
     (with-error-handling                         ; for signature errors
       (match args
         (("--query")
          (let ((acl (current-acl)))
            (let loop ((command (read-line)))
              (or (eof-object? command)
                  (begin
                    (process-query command
                                   #:cache-urls (substitute-urls)
                                   #:acl acl)
                    (loop (read-line)))))))
         (("--substitute")
          ;; Download STORE-PATH and store it as a Nar in file DESTINATION.
          ;; Specify the number of columns of the terminal so the progress
          ;; report displays nicely.
          (parameterize ((current-terminal-columns (client-terminal-columns)))
            (let loop ()
              (match (read-line)
                ((? eof-object?)
                 #t)
                ((= string-tokenize ("substitute" store-path destination))
                 (process-substitution store-path destination
                                       #:cache-urls (substitute-urls)
                                       #:acl (current-acl)
                                       #:deduplicate? deduplicate?
                                       #:print-build-trace?
                                       print-build-trace?)
                 (loop))))))
         ((or ("-V") ("--version"))
          (show-version-and-exit "guix substitute"))
         (("--help")
          (show-help))
         (opts
          (leave (G_ "~a: unrecognized options~%") opts)))))))

;;; Local Variables:
;;; eval: (put 'with-timeout 'scheme-indent-function 1)
;;; eval: (put 'with-redirected-error-port 'scheme-indent-function 0)
;;; eval: (put 'with-cached-connection 'scheme-indent-function 2)
;;; eval: (put 'call-with-cached-connection 'scheme-indent-function 1)
;;; End:

;;; substitute.scm ends here