guix-play/guix/inferior.scm

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2018, 2019, 2020, 2021 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 inferior)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-9 gnu)
  #:use-module (srfi srfi-34)
  #:use-module (srfi srfi-35)
  #:use-module ((guix diagnostics)
                #:select (source-properties->location))
  #:use-module ((guix utils)
                #:select (%current-system
                          call-with-temporary-directory
                          version>? version-prefix?
                          cache-directory))
  #:use-module ((guix store)
                #:select (store-connection-socket
                          store-connection-major-version
                          store-connection-minor-version
                          store-lift
                          &store-protocol-error))
  #:use-module ((guix derivations)
                #:select (read-derivation-from-file))
  #:use-module (guix gexp)
  #:use-module (guix search-paths)
  #:use-module (guix profiles)
  #:use-module (guix channels)
  #:use-module ((guix git) #:select (update-cached-checkout))
  #:use-module (guix monads)
  #:use-module (guix store)
  #:use-module (guix derivations)
  #:use-module (guix base32)
  #:use-module (gcrypt hash)
  #:autoload   (guix cache) (maybe-remove-expired-cache-entries
                             file-expiration-time)
  #:autoload   (guix ui) (build-notifier)
  #:autoload   (guix build utils) (mkdir-p)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-26)
  #:use-module (srfi srfi-71)
  #:autoload   (ice-9 ftw) (scandir)
  #:use-module (ice-9 match)
  #:use-module (ice-9 popen)
  #:use-module (ice-9 vlist)
  #:use-module (ice-9 binary-ports)
  #:use-module ((rnrs bytevectors) #:select (string->utf8))
  #:export (inferior?
            open-inferior
            port->inferior
            close-inferior
            inferior-eval
            inferior-eval-with-store
            inferior-object?
            inferior-exception?
            inferior-exception-arguments
            inferior-exception-inferior
            inferior-exception-stack
            read-repl-response

            inferior-packages
            inferior-available-packages
            lookup-inferior-packages

            inferior-package?
            inferior-package-name
            inferior-package-version
            inferior-package-synopsis
            inferior-package-description
            inferior-package-home-page
            inferior-package-location
            inferior-package-inputs
            inferior-package-native-inputs
            inferior-package-propagated-inputs
            inferior-package-transitive-propagated-inputs
            inferior-package-native-search-paths
            inferior-package-transitive-native-search-paths
            inferior-package-search-paths
            inferior-package-replacement
            inferior-package-provenance
            inferior-package-derivation

            inferior-package->manifest-entry

            gexp->derivation-in-inferior

            %inferior-cache-directory
            cached-channel-instance
            inferior-for-channels))

;;; Commentary:
;;;
;;; This module provides a way to spawn Guix "inferior" processes and to talk
;;; to them.  It allows us, from one instance of Guix, to interact with
;;; another instance of Guix coming from a different commit.
;;;
;;; Code:

;; Inferior Guix process.
(define-record-type <inferior>
  (inferior pid socket close version packages table)
  inferior?
  (pid      inferior-pid)
  (socket   inferior-socket)
  (close    inferior-close-socket)               ;procedure
  (version  inferior-version)                    ;REPL protocol version
  (packages inferior-package-promise)            ;promise of inferior packages
  (table    inferior-package-table))             ;promise of vhash

(define (write-inferior inferior port)
  (match inferior
    (($ <inferior> pid _ _ version)
     (format port "#<inferior ~a ~a ~a>"
             pid version
             (number->string (object-address inferior) 16)))))

(set-record-type-printer! <inferior> write-inferior)

(define* (inferior-pipe directory command error-port)
  "Return an input/output pipe on the Guix instance in DIRECTORY.  This runs
'DIRECTORY/COMMAND repl' if it exists, or falls back to some other method if
it's an old Guix."
  (let ((pipe (with-error-to-port error-port
                (lambda ()
                  (open-pipe* OPEN_BOTH
                              (string-append directory "/" command)
                              "repl" "-t" "machine")))))
    (if (eof-object? (peek-char pipe))
        (begin
          (close-pipe pipe)

          ;; Older versions of Guix didn't have a 'guix repl' command, so
          ;; emulate it.
          (with-error-to-port error-port
            (lambda ()
              (open-pipe* OPEN_BOTH "guile"
                          "-L" (string-append directory "/share/guile/site/"
                                              (effective-version))
                          "-C" (string-append directory "/share/guile/site/"
                                              (effective-version))
                          "-C" (string-append directory "/lib/guile/"
                                              (effective-version) "/site-ccache")
                          "-c"
                          (object->string
                           `(begin
                              (primitive-load ,(search-path %load-path
                                                            "guix/repl.scm"))
                              ((@ (guix repl) machine-repl))))))))
        pipe)))

(define* (port->inferior pipe #:optional (close close-port))
  "Given PIPE, an input/output port, return an inferior that talks over PIPE.
PIPE is closed with CLOSE when 'close-inferior' is called on the returned
inferior."
  (setvbuf pipe 'line)

  (match (read pipe)
    (('repl-version 0 rest ...)
     (letrec ((result (inferior 'pipe pipe close (cons 0 rest)
                                (delay (%inferior-packages result))
                                (delay (%inferior-package-table result)))))

       ;; For protocol (0 1) and later, send the protocol version we support.
       (match rest
         ((n _ ...)
          (when (>= n 1)
            (send-inferior-request '(() repl-version 0 1 1) result)))
         (_
          #t))

       (inferior-eval '(use-modules (guix)) result)
       (inferior-eval '(use-modules (gnu)) result)
       (inferior-eval '(use-modules (ice-9 match)) result)
       (inferior-eval '(use-modules (srfi srfi-34)) result)
       (inferior-eval '(define %package-table (make-hash-table))
                      result)
       result))
    (_
     #f)))

(define* (open-inferior directory
                        #:key (command "bin/guix")
                        (error-port (%make-void-port "w")))
  "Open the inferior Guix in DIRECTORY, running 'DIRECTORY/COMMAND repl' or
equivalent.  Return #f if the inferior could not be launched."
  (define pipe
    (inferior-pipe directory command error-port))

  (port->inferior pipe close-pipe))

(define (close-inferior inferior)
  "Close INFERIOR."
  (let ((close (inferior-close-socket inferior)))
    (close (inferior-socket inferior))))

;; Non-self-quoting object of the inferior.
(define-record-type <inferior-object>
  (inferior-object address appearance)
  inferior-object?
  (address     inferior-object-address)
  (appearance  inferior-object-appearance))

(define (write-inferior-object object port)
  (match object
    (($ <inferior-object> _ appearance)
     (format port "#<inferior-object ~a>" appearance))))

(set-record-type-printer! <inferior-object> write-inferior-object)

;; Reified exception thrown by an inferior.
(define-condition-type &inferior-exception &error
  inferior-exception?
  (arguments  inferior-exception-arguments)       ;key + arguments
  (inferior   inferior-exception-inferior)        ;<inferior> | #f
  (stack      inferior-exception-stack))          ;list of (FILE COLUMN LINE)

(define* (read-repl-response port #:optional inferior)
  "Read a (guix repl) response from PORT and return it as a Scheme object.
Raise '&inferior-exception' when an exception is read from PORT."
  (define sexp->object
    (match-lambda
      (('value value)
       value)
      (('non-self-quoting address string)
       (inferior-object address string))))

  (match (read port)
    (('values objects ...)
     (apply values (map sexp->object objects)))
    (('exception ('arguments key objects ...)
                 ('stack frames ...))
     ;; Protocol (0 1 1) and later.
     (raise (condition (&inferior-exception
                        (arguments (cons key (map sexp->object objects)))
                        (inferior inferior)
                        (stack frames)))))
    (('exception key objects ...)
     ;; Protocol (0 0).
     (raise (condition (&inferior-exception
                        (arguments (cons key (map sexp->object objects)))
                        (inferior inferior)
                        (stack '())))))))

(define (read-inferior-response inferior)
  (read-repl-response (inferior-socket inferior)
                      inferior))

(define (send-inferior-request exp inferior)
  (write exp (inferior-socket inferior))
  (newline (inferior-socket inferior)))

(define (inferior-eval exp inferior)
  "Evaluate EXP in INFERIOR."
  (send-inferior-request exp inferior)
  (read-inferior-response inferior))


;;;
;;; Inferior packages.
;;;

(define-record-type <inferior-package>
  (inferior-package inferior name version id)
  inferior-package?
  (inferior   inferior-package-inferior)
  (name       inferior-package-name)
  (version    inferior-package-version)
  (id         inferior-package-id))

(define (write-inferior-package package port)
  (match package
    (($ <inferior-package> _ name version)
     (format port "#<inferior-package ~a@~a ~a>"
             name version
             (number->string (object-address package) 16)))))

(set-record-type-printer! <inferior-package> write-inferior-package)

(define (%inferior-packages inferior)
  "Compute the list of inferior packages from INFERIOR."
  (let ((result (inferior-eval
                 '(fold-packages (lambda (package result)
                                   (let ((id (object-address package)))
                                     (hashv-set! %package-table id package)
                                     (cons (list (package-name package)
                                                 (package-version package)
                                                 id)
                                           result)))
                                 '())
                 inferior)))
    (map (match-lambda
           ((name version id)
            (inferior-package inferior name version id)))
         result)))

(define (inferior-packages inferior)
  "Return the list of packages known to INFERIOR."
  (force (inferior-package-promise inferior)))

(define (%inferior-package-table inferior)
  "Compute a package lookup table for INFERIOR."
  (fold (lambda (package table)
          (vhash-cons (inferior-package-name package) package
                      table))
        vlist-null
        (inferior-packages inferior)))

(define (inferior-available-packages inferior)
  "Return the list of name/version pairs corresponding to the set of packages
available in INFERIOR.

This is faster and less resource-intensive than calling 'inferior-packages'."
  (if (inferior-eval '(defined? 'fold-available-packages)
                     inferior)
      (inferior-eval '(fold-available-packages
                       (lambda* (name version result
                                      #:key supported? deprecated?
                                      #:allow-other-keys)
                         (if (and supported? (not deprecated?))
                             (acons name version result)
                             result))
                       '())
                     inferior)

      ;; As a last resort, if INFERIOR is old and lacks
      ;; 'fold-available-packages', fall back to 'inferior-packages'.
      (map (lambda (package)
             (cons (inferior-package-name package)
                   (inferior-package-version package)))
           (inferior-packages inferior))))

(define* (lookup-inferior-packages inferior name #:optional version)
  "Return the sorted list of inferior packages matching NAME in INFERIOR, with
highest version numbers first.  If VERSION is true, return only packages with
a version number prefixed by VERSION."
  ;; This is the counterpart of 'find-packages-by-name'.
  (sort (filter (lambda (package)
                  (or (not version)
                      (version-prefix? version
                                       (inferior-package-version package))))
                (vhash-fold* cons '() name
                             (force (inferior-package-table inferior))))
        (lambda (p1 p2)
          (version>? (inferior-package-version p1)
                     (inferior-package-version p2)))))

(define (inferior-package-field package getter)
  "Return the field of PACKAGE, an inferior package, accessed with GETTER."
  (let ((inferior (inferior-package-inferior package))
        (id       (inferior-package-id package)))
    (inferior-eval `(,getter (hashv-ref %package-table ,id))
                   inferior)))

(define* (inferior-package-synopsis package #:key (translate? #t))
  "Return the Texinfo synopsis of PACKAGE, an inferior package.  When
TRANSLATE? is true, translate it to the current locale's language."
  (inferior-package-field package
                          (if translate?
                              '(compose (@ (guix ui) P_) package-synopsis)
                              'package-synopsis)))

(define* (inferior-package-description package #:key (translate? #t))
  "Return the Texinfo description of PACKAGE, an inferior package.  When
TRANSLATE? is true, translate it to the current locale's language."
  (inferior-package-field package
                          (if translate?
                              '(compose (@ (guix ui) P_) package-description)
                              'package-description)))

(define (inferior-package-home-page package)
  "Return the home page of PACKAGE."
  (inferior-package-field package 'package-home-page))

(define (inferior-package-location package)
  "Return the source code location of PACKAGE, either #f or a <location>
record."
  (source-properties->location
   (inferior-package-field package
                           '(compose (lambda (loc)
                                       (and loc
                                            (location->source-properties
                                             loc)))
                                     package-location))))

(define (inferior-package-input-field package field)
  "Return the input field FIELD (e.g., 'native-inputs') of PACKAGE, an
inferior package."
  (define field*
    `(compose (lambda (inputs)
                (map (match-lambda
                       ;; XXX: Origins are not handled.
                       ((label (? package? package) rest ...)
                        (let ((id (object-address package)))
                          (hashv-set! %package-table id package)
                          `(,label (package ,id
                                            ,(package-name package)
                                            ,(package-version package))
                                   ,@rest)))
                       (x
                        x))
                     inputs))
              ,field))

  (define inputs
    (inferior-package-field package field*))

  (define inferior
    (inferior-package-inferior package))

  (map (match-lambda
         ((label ('package id name version) . rest)
          ;; XXX: eq?-ness of inferior packages is not preserved here.
          `(,label ,(inferior-package inferior name version id)
                   ,@rest))
         (x x))
       inputs))

(define inferior-package-inputs
  (cut inferior-package-input-field <> 'package-inputs))

(define inferior-package-native-inputs
  (cut inferior-package-input-field <> 'package-native-inputs))

(define inferior-package-propagated-inputs
  (cut inferior-package-input-field <> 'package-propagated-inputs))

(define inferior-package-transitive-propagated-inputs
  (cut inferior-package-input-field <> 'package-transitive-propagated-inputs))

(define (%inferior-package-search-paths package field)
  "Return the list of search path specifications of PACKAGE, an inferior
package."
  (define paths
    (inferior-package-field package
                            `(compose (lambda (paths)
                                        (map (@ (guix search-paths)
                                                search-path-specification->sexp)
                                             paths))
                                      ,field)))

  (map sexp->search-path-specification paths))

(define inferior-package-native-search-paths
  (cut %inferior-package-search-paths <> 'package-native-search-paths))

(define inferior-package-search-paths
  (cut %inferior-package-search-paths <> 'package-search-paths))

(define inferior-package-transitive-native-search-paths
  (cut %inferior-package-search-paths <> 'package-transitive-native-search-paths))

(define (inferior-package-replacement package)
  "Return the replacement for PACKAGE.  This will either be an inferior
package, or #f."
  (match (inferior-package-field
          package
          '(compose (match-lambda
                      ((? package? package)
                       (let ((id (object-address package)))
                         (hashv-set! %package-table id package)
                         (list id
                               (package-name package)
                               (package-version package))))
                      (#f #f))
                    package-replacement))
    (#f #f)
    ((id name version)
     (inferior-package (inferior-package-inferior package)
                       name
                       version
                       id))))

(define (inferior-package-provenance package)
  "Return a \"provenance sexp\" for PACKAGE, an inferior package.  The result
is similar to the sexp returned by 'package-provenance' for regular packages."
  (inferior-package-field package
                          '(let* ((describe
                                   (false-if-exception
                                    (resolve-interface '(guix describe))))
                                  (provenance
                                   (false-if-exception
                                    (module-ref describe
                                                'package-provenance))))
                             (or provenance (const #f)))))

(define (proxy client backend)                    ;adapted from (guix ssh)
  "Proxy communication between CLIENT and BACKEND until CLIENT closes the
connection, at which point CLIENT is closed (both CLIENT and BACKEND must be
input/output ports.)"
  ;; Use buffered ports so that 'get-bytevector-some' returns up to the
  ;; whole buffer like read(2) would--see <https://bugs.gnu.org/30066>.
  (setvbuf client 'block 65536)
  (setvbuf backend 'block 65536)

  (let loop ()
    (match (select (list client backend) '() '())
      ((reads () ())
       (when (memq client reads)
         (match (get-bytevector-some client)
           ((? eof-object?)
            (close-port client))
           (bv
            (put-bytevector backend bv)
            (force-output backend))))
       (when (memq backend reads)
         (match (get-bytevector-some backend)
           (bv
            (put-bytevector client bv)
            (force-output client))))
       (unless (port-closed? client)
         (loop))))))

(define (inferior-eval-with-store inferior store code)
  "Evaluate CODE in INFERIOR, passing it STORE as its argument.  CODE must
thus be the code of a one-argument procedure that accepts a store."
  ;; Create a named socket in /tmp and let INFERIOR connect to it and use it
  ;; as its store.  This ensures the inferior uses the same store, with the
  ;; same options, the same per-session GC roots, etc.
  ;; FIXME: This strategy doesn't work for remote inferiors (SSH).
  (call-with-temporary-directory
   (lambda (directory)
     (chmod directory #o700)
     (let* ((name     (string-append directory "/inferior"))
            (socket   (socket AF_UNIX SOCK_STREAM 0))
            (major    (store-connection-major-version store))
            (minor    (store-connection-minor-version store))
            (proto    (logior major minor)))
       (bind socket AF_UNIX name)
       (listen socket 1024)
       (send-inferior-request
        `(let ((proc   ,code)
               (socket (socket AF_UNIX SOCK_STREAM 0))
               (error? (if (defined? 'store-protocol-error?)
                           store-protocol-error?
                           nix-protocol-error?))
               (error-message (if (defined? 'store-protocol-error-message)
                                  store-protocol-error-message
                                  nix-protocol-error-message)))
           (connect socket AF_UNIX ,name)

           ;; 'port->connection' appeared in June 2018 and we can hardly
           ;; emulate it on older versions.  Thus fall back to
           ;; 'open-connection', at the risk of talking to the wrong daemon or
           ;; having our build result reclaimed (XXX).
           (let ((store (if (defined? 'port->connection)
                            (port->connection socket #:version ,proto)
                            (open-connection))))
             (dynamic-wind
               (const #t)
               (lambda ()
                 ;; Serialize '&store-protocol-error' conditions.  The
                 ;; exception serialization mechanism that
                 ;; 'read-repl-response' expects is unsuitable for SRFI-35
                 ;; error conditions, hence this special case.
                 (guard (c ((error? c)
                            `(store-protocol-error ,(error-message c))))
                   `(result ,(proc store))))
               (lambda ()
                 (close-connection store)
                 (close-port socket)))))
        inferior)
       (match (accept socket)
         ((client . address)
          (proxy client (store-connection-socket store))))
       (close-port socket)

       (match (read-inferior-response inferior)
         (('store-protocol-error message)
          (raise (condition
                  (&store-protocol-error (message message)
                                         (status 1)))))
         (('result result)
          result))))))

(define* (inferior-package-derivation store package
                                      #:optional
                                      (system (%current-system))
                                      #:key target)
  "Return the derivation for PACKAGE, an inferior package, built for SYSTEM
and cross-built for TARGET if TARGET is true.  The inferior corresponding to
PACKAGE must be live."
  (define proc
    `(lambda (store)
       (let* ((package (hashv-ref %package-table
                                  ,(inferior-package-id package)))
              (drv     ,(if target
                            `(package-cross-derivation store package
                                                       ,target
                                                       ,system)
                            `(package-derivation store package
                                                 ,system))))
         (derivation-file-name drv))))

  (and=> (inferior-eval-with-store (inferior-package-inferior package) store
                                   proc)
         read-derivation-from-file))

(define inferior-package->derivation
  (store-lift inferior-package-derivation))

(define-gexp-compiler (package-compiler (package <inferior-package>) system
                                        target)
  ;; Compile PACKAGE for SYSTEM, optionally cross-building for TARGET.
  (inferior-package->derivation package system #:target target))

(define* (gexp->derivation-in-inferior name exp guix
                                       #:key silent-failure?
                                       #:allow-other-keys
                                       #:rest rest)
  "Return a derivation that evaluates EXP with GUIX, an instance of Guix as
returned for example by 'channel-instances->derivation'.  Other arguments are
passed as-is to 'gexp->derivation'.

When SILENT-FAILURE? is true, create an empty output directory instead of
failing when GUIX is too old and lacks the 'guix repl' command."
  (define script
    ;; EXP wrapped with a proper (set! %load-path …) prologue.
    (scheme-file "inferior-script.scm" exp))

  (define trampoline
    ;; This is a crude way to run EXP on GUIX.  TODO: use 'raw-derivation' and
    ;; make 'guix repl' the "builder"; this will require "opening up" the
    ;; mechanisms behind 'gexp->derivation', and adding '-l' to 'guix repl'.
    #~(begin
        (use-modules (ice-9 popen))

        (let ((pipe (open-pipe* OPEN_WRITE
                                #+(file-append guix "/bin/guix")
                                "repl" "-t" "machine")))

          ;; XXX: EXP presumably refers to #$output but that reference is lost
          ;; so explicitly reference it here.
          #$output

          (write `(primitive-load #$script) pipe)

          (unless (zero? (close-pipe pipe))
            (if #$silent-failure?
                (mkdir #$output)
                (error "inferior failed" #+guix))))))

  (define (drop-extra-keyword lst)
    (let loop ((lst lst)
               (result '()))
      (match lst
        (()
         (reverse result))
        ((#:silent-failure? _ . rest)
         (loop rest result))
        ((kw value . tail)
         (loop tail (cons* value kw result))))))

  (apply gexp->derivation name trampoline
         (drop-extra-keyword rest)))


;;;
;;; Manifest entries.
;;;

(define* (inferior-package->manifest-entry package
                                           #:optional (output "out")
                                           #:key (properties '()))
  "Return a manifest entry for the OUTPUT of package PACKAGE."
  (define cache
    (make-hash-table))

  (define-syntax-rule (memoized package output exp)
    ;; Memoize the entry returned by EXP for PACKAGE/OUTPUT.  This is
    ;; important as the same package may be traversed many times through
    ;; propagated inputs, and querying the inferior is costly.  Use
    ;; 'hash'/'equal?', which is okay since <inferior-package> is simple.
    (let ((compute (lambda () exp))
          (key     (cons package output)))
      (or (hash-ref cache key)
          (let ((result (compute)))
            (hash-set! cache key result)
            result))))

  (let loop ((package package)
             (output  output)
             (parent  (delay #f)))
    (memoized package output
      ;; For each dependency, keep a promise pointing to its "parent" entry.
      (letrec* ((deps  (map (match-lambda
                              ((label package)
                               (loop package "out" (delay entry)))
                              ((label package output)
                               (loop package output (delay entry))))
                            (inferior-package-propagated-inputs package)))
                (entry (manifest-entry
                         (name (inferior-package-name package))
                         (version (inferior-package-version package))
                         (output output)
                         (item package)
                         (dependencies (delete-duplicates deps))
                         (search-paths
                          (inferior-package-transitive-native-search-paths package))
                         (parent parent)
                         (properties properties))))
        entry))))


;;;
;;; Cached inferiors.
;;;

(define %inferior-cache-directory
  ;; Directory for cached inferiors (GC roots).
  (make-parameter (string-append (cache-directory #:ensure? #f)
                                 "/inferiors")))

(define (channel-full-commit channel)
  "Return the commit designated by CHANNEL as quickly as possible.  If
CHANNEL's 'commit' field is a full SHA1, return it as-is; if it's a SHA1
prefix, resolve it; and if 'commit' is unset, fetch CHANNEL's branch tip."
  (let ((commit (channel-commit channel))
        (branch (channel-branch channel)))
    (if (and commit (= (string-length commit) 40))
        commit
        (let* ((ref (if commit `(commit . ,commit) `(branch . ,branch)))
               (cache commit relation
                     (update-cached-checkout (channel-url channel)
                                             #:ref ref
                                             #:check-out? #f)))
          commit))))

(define* (cached-channel-instance store
                                  channels
                                  #:key
                                  (authenticate? #t)
                                  (cache-directory (%inferior-cache-directory))
                                  (ttl (* 3600 24 30)))
  "Return a directory containing a guix filetree defined by CHANNELS, a list of channels.
The directory is a subdirectory of CACHE-DIRECTORY, where entries can be reclaimed after TTL seconds.
This procedure opens a new connection to the build daemon.  AUTHENTICATE?
determines whether CHANNELS are authenticated."
  (define commits
    ;; Since computing the instances of CHANNELS is I/O-intensive, use a
    ;; cheaper way to get the commit list of CHANNELS.  This limits overhead
    ;; to the minimum in case of a cache hit.
    (map channel-full-commit channels))

  (define key
    (bytevector->base32-string
     (sha256
      (string->utf8 (string-concatenate commits)))))

  (define cached
    (string-append cache-directory "/" key))

  (define (base32-encoded-sha256? str)
    (= (string-length str) 52))

  (define (cache-entries directory)
    (map (lambda (file)
           (string-append directory "/" file))
         (scandir directory base32-encoded-sha256?)))

  (define (symlink/safe old new)
    (catch 'system-error
      (lambda ()
        (symlink old new))
      (lambda args
        (unless (= EEXIST (system-error-errno args))
          (apply throw args)))))

  (define symlink*
    (lift2 symlink/safe %store-monad))

  (define add-indirect-root*
    (store-lift add-indirect-root))

  (define add-temp-root*
    (store-lift add-temp-root))

  (mkdir-p cache-directory)
  (maybe-remove-expired-cache-entries cache-directory
                                      cache-entries
                                      #:entry-expiration
                                      (file-expiration-time ttl))

  (if (file-exists? cached)
      cached
      (run-with-store store
        (mlet* %store-monad ((instances
                              -> (latest-channel-instances store channels
                                                           #:authenticate?
                                                           authenticate?))
                             (profile
                              (channel-instances->derivation instances)))
          (mbegin %store-monad
            ;; It's up to the caller to install a build handler to report
            ;; what's going to be built.
            (built-derivations (list profile))

            ;; Cache if and only if AUTHENTICATE? is true.
            (if authenticate?
                (mbegin %store-monad
                  (symlink* (derivation->output-path profile) cached)
                  (add-indirect-root* cached)
                  (return cached))
                (mbegin %store-monad
                  (add-temp-root* profile)
                  (return profile))))))))

(define* (inferior-for-channels channels
                                #:key
                                (cache-directory (%inferior-cache-directory))
                                (ttl (* 3600 24 30)))
  "Return an inferior for CHANNELS, a list of channels.  Use the cache at
CACHE-DIRECTORY, where entries can be reclaimed after TTL seconds.  This
procedure opens a new connection to the build daemon.

This is a convenience procedure that people may use in manifests passed to
'guix package -m', for instance."
  (define cached
    (with-store store
      ;; XXX: Install a build notifier out of convenience, so users know
      ;; what's going on.  However, we cannot be sure that its options, such
      ;; as #:use-substitutes?, correspond to the daemon's default settings.
      (with-build-handler (build-notifier)
        (cached-channel-instance store
                                 channels
                                 #:cache-directory cache-directory
                                 #:ttl ttl))))
  (open-inferior cached))

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