;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
;;; Copyright © 2018 Ricardo Wurmus <rekado@elephly.net>
;;; Copyright © 2019 Jan (janneke) Nieuwenhuizen <janneke@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 channels)
  #:use-module (git)
  #:use-module (guix git)
  #:use-module (guix records)
  #:use-module (guix gexp)
  #:use-module (guix modules)
  #:use-module (guix discovery)
  #:use-module (guix monads)
  #:use-module (guix profiles)
  #:use-module (guix packages)
  #:use-module (guix derivations)
  #:use-module (guix combinators)
  #:use-module (guix diagnostics)
  #:use-module (guix sets)
  #:use-module (guix store)
  #:use-module (guix i18n)
  #:use-module ((guix utils)
                #:select (source-properties->location
                          &error-location
                          &fix-hint))
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-2)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-11)
  #:use-module (srfi srfi-26)
  #:use-module (srfi srfi-34)
  #:use-module (srfi srfi-35)
  #:autoload   (guix self) (whole-package make-config.scm)
  #:autoload   (guix inferior) (gexp->derivation-in-inferior) ;FIXME: circular dep
  #:use-module (ice-9 match)
  #:use-module (ice-9 vlist)
  #:use-module ((ice-9 rdelim) #:select (read-string))
  #:export (channel
            channel?
            channel-name
            channel-url
            channel-branch
            channel-commit
            channel-location

            %default-channels
            guix-channel?

            channel-instance?
            channel-instance-channel
            channel-instance-commit
            channel-instance-checkout

            latest-channel-instances
            checkout->channel-instance
            latest-channel-derivation
            channel-instances->manifest
            %channel-profile-hooks
            channel-instances->derivation

            profile-channels

            channel-news-entry?
            channel-news-entry-commit
            channel-news-entry-tag
            channel-news-entry-title
            channel-news-entry-body

            channel-news-for-commit))

;;; Commentary:
;;;
;;; This module implements "channels."  A channel is usually a source of
;;; package definitions.  There's a special channel, the 'guix' channel, that
;;; provides all of Guix, including its commands and its documentation.
;;; User-defined channels are expected to typically provide a bunch of .scm
;;; files meant to be added to the '%package-search-path'.
;;;
;;; This module provides tools to fetch and update channels from a Git
;;; repository and to build them.
;;;
;;; Code:

(define-record-type* <channel> channel make-channel
  channel?
  (name      channel-name)
  (url       channel-url)
  (branch    channel-branch (default "master"))
  (commit    channel-commit (default #f))
  (location  channel-location
             (default (current-source-location)) (innate)))

(define %default-channels
  ;; Default list of channels.
  (list (channel
         (name 'guix)
         (branch "master")
         (url "https://git.savannah.gnu.org/git/guix.git"))))

(define (guix-channel? channel)
  "Return true if CHANNEL is the 'guix' channel."
  (eq? 'guix (channel-name channel)))

(define-record-type <channel-instance>
  (channel-instance channel commit checkout)
  channel-instance?
  (channel   channel-instance-channel)
  (commit    channel-instance-commit)
  (checkout  channel-instance-checkout))

(define-record-type <channel-metadata>
  (channel-metadata directory dependencies news-file)
  channel-metadata?
  (directory     channel-metadata-directory)      ;string with leading slash
  (dependencies  channel-metadata-dependencies)   ;list of <channel>
  (news-file     channel-metadata-news-file))     ;string | #f

(define (channel-reference channel)
  "Return the \"reference\" for CHANNEL, an sexp suitable for
'latest-repository-commit'."
  (match (channel-commit channel)
    (#f      `(branch . ,(channel-branch channel)))
    (commit  `(commit . ,(channel-commit channel)))))

(define (read-channel-metadata port)
  "Read from PORT channel metadata in the format expected for the
'.guix-channel' file.  Return a <channel-metadata> record, or raise an error
if valid metadata could not be read from PORT."
  (match (read port)
    (('channel ('version 0) properties ...)
     (let ((directory    (and=> (assoc-ref properties 'directory) first))
           (dependencies (or (assoc-ref properties 'dependencies) '()))
           (news-file    (and=> (assoc-ref properties 'news-file) first)))
       (channel-metadata
        (cond ((not directory) "/")               ;directory
              ((string-prefix? "/" directory) directory)
              (else (string-append "/" directory)))
        (map (lambda (item)                       ;dependencies
               (let ((get (lambda* (key #:optional default)
                            (or (and=> (assoc-ref item key) first) default))))
                 (and-let* ((name (get 'name))
                            (url (get 'url))
                            (branch (get 'branch "master")))
                   (channel
                    (name name)
                    (branch branch)
                    (url url)
                    (commit (get 'commit))))))
             dependencies)
        news-file)))                              ;news-file
    ((and ('channel ('version version) _ ...) sexp)
     (raise (condition
             (&message (message "unsupported '.guix-channel' version"))
             (&error-location
              (location (source-properties->location
                         (source-properties sexp)))))))
    (sexp
     (raise (condition
             (&message (message "invalid '.guix-channel' file"))
             (&error-location
              (location (source-properties->location
                         (source-properties sexp)))))))))

(define (read-channel-metadata-from-source source)
  "Return a channel-metadata record read from channel's SOURCE/.guix-channel
description file, or return the default channel-metadata record if that file
doesn't exist."
  (catch 'system-error
    (lambda ()
      (call-with-input-file (string-append source "/.guix-channel")
        read-channel-metadata))
    (lambda args
      (if (= ENOENT (system-error-errno args))
          (channel-metadata "/" '() #f)
          (apply throw args)))))

(define (channel-instance-metadata instance)
  "Return a channel-metadata record read from the channel INSTANCE's
description file or its default value."
  (read-channel-metadata-from-source (channel-instance-checkout instance)))

(define (channel-instance-dependencies instance)
  "Return the list of channels that are declared as dependencies for the given
channel INSTANCE."
  (channel-metadata-dependencies (channel-instance-metadata instance)))

(define* (latest-channel-instances store channels #:optional (previous-channels '()))
  "Return a list of channel instances corresponding to the latest checkouts of
CHANNELS and the channels on which they depend.  PREVIOUS-CHANNELS is a list
of previously processed channels."
  ;; Only process channels that are unique, or that are more specific than a
  ;; previous channel specification.
  (define (ignore? channel others)
    (member channel others
            (lambda (a b)
              (and (eq? (channel-name a) (channel-name b))
                   (or (channel-commit b)
                       (not (or (channel-commit a)
                                (channel-commit b))))))))

  ;; Accumulate a list of instances.  A list of processed channels is also
  ;; accumulated to decide on duplicate channel specifications.
  (define-values (resulting-channels instances)
    (fold2 (lambda (channel previous-channels instances)
             (if (ignore? channel previous-channels)
                 (values previous-channels instances)
                 (begin
                   (format (current-error-port)
                           (G_ "Updating channel '~a' from Git repository at '~a'...~%")
                           (channel-name channel)
                           (channel-url channel))
                   (let-values (((checkout commit)
                                 (latest-repository-commit store (channel-url channel)
                                                           #:ref (channel-reference
                                                                  channel))))
                     (let ((instance (channel-instance channel commit checkout)))
                       (let-values (((new-instances new-channels)
                                     (latest-channel-instances
                                      store
                                      (channel-instance-dependencies instance)
                                      previous-channels)))
                         (values (append (cons channel new-channels)
                                         previous-channels)
                                 (append (cons instance new-instances)
                                         instances))))))))
           previous-channels
           '()                                    ;instances
           channels))

  (let ((instance-name (compose channel-name channel-instance-channel)))
    ;; Remove all earlier channel specifications if they are followed by a
    ;; more specific one.
    (values (delete-duplicates instances
                               (lambda (a b)
                                 (eq? (instance-name a) (instance-name b))))
            resulting-channels)))

(define* (checkout->channel-instance checkout
                                     #:key commit
                                     (url checkout) (name 'guix))
  "Return a channel instance for CHECKOUT, which is assumed to be a checkout
of COMMIT at URL.  Use NAME as the channel name."
  (let* ((commit  (or commit (make-string 40 #\0)))
         (channel (channel (name name)
                           (commit commit)
                           (url url))))
    (channel-instance channel commit checkout)))

(define %self-build-file
  ;; The file containing code to build Guix.  This serves the same purpose as
  ;; a makefile, and, similarly, is intended to always keep this name.
  "build-aux/build-self.scm")

(define %pull-version
  ;; This is the version of the 'guix pull' protocol.  It specifies what's
  ;; expected from %SELF-BUILD-FILE.  The initial version ("0") was when we'd
  ;; place a set of compiled Guile modules in ~/.config/guix/latest.
  1)

(define (standard-module-derivation name source core dependencies)
  "Return a derivation that builds with CORE, a Guix instance, the Scheme
modules in SOURCE and that depend on DEPENDENCIES, a list of lowerable
objects.  The assumption is that SOURCE contains package modules to be added
to '%package-module-path'."

  (let* ((metadata (read-channel-metadata-from-source source))
         (directory (channel-metadata-directory metadata)))

    (define build
      ;; This is code that we'll run in CORE, a Guix instance, with its own
      ;; modules and so on.  That way, we make sure these modules are built for
      ;; the right Guile version, with the right dependencies, and that they get
      ;; to see the right (gnu packages …) modules.
      (with-extensions dependencies
        #~(begin
            (use-modules (guix build compile)
                         (guix build utils)
                         (srfi srfi-26))

            (define go
              (string-append #$output "/lib/guile/" (effective-version)
                             "/site-ccache"))
            (define scm
              (string-append #$output "/share/guile/site/"
                             (effective-version)))

            (let* ((subdir #$directory)
                   (source (string-append #$source subdir)))
              (compile-files source go (find-files source "\\.scm$"))
              (mkdir-p (dirname scm))
              (symlink (string-append #$source subdir) scm))

            scm)))

    (gexp->derivation-in-inferior name build core)))

(define (syscalls-reexports-local-variables? source)
  "Return true if (guix build syscalls) contains the bug described at
<https://bugs.gnu.org/36723>."
  (catch 'system-error
    (lambda ()
      (define content
        (call-with-input-file (string-append source
                                             "/guix/build/syscalls.scm")
          read-string))

      ;; The faulty code would use the 're-export' macro, causing the
      ;; 'AT_SYMLINK_NOFOLLOW' local variable to be re-exported when using
      ;; Guile > 2.2.4.
      (string-contains content "(re-export variable)"))
    (lambda args
      (if (= ENOENT (system-error-errno args))
          #f
          (apply throw args)))))

(define (guile-2.2.4)
  (module-ref (resolve-interface '(gnu packages guile))
              'guile-2.2.4))

(define %quirks
  ;; List of predicate/package pairs.  This allows us provide information
  ;; about specific Guile versions that old Guix revisions might need to use
  ;; just to be able to build and run the trampoline in %SELF-BUILD-FILE.  See
  ;; <https://bugs.gnu.org/37506>
  `((,syscalls-reexports-local-variables? . ,guile-2.2.4)))

(define* (guile-for-source source #:optional (quirks %quirks))
  "Return the Guile package to use when building SOURCE or #f if the default
'%guile-for-build' should be good enough."
  (let loop ((quirks quirks))
    (match quirks
      (()
       #f)
      (((predicate . guile) rest ...)
       (if (predicate source) (guile) (loop rest))))))

(define (with-trivial-build-handler mvalue)
  "Run MVALUE, a monadic value, with a \"trivial\" build handler installed
that unconditionally resumes the continuation."
  (lambda (store)
    (with-build-handler (lambda (continue . _)
                          (continue #t))
      (values (run-with-store store mvalue)
              store))))

(define* (build-from-source name source
                            #:key core verbose? commit
                            (dependencies '()))
  "Return a derivation to build Guix from SOURCE, using the self-build script
contained therein; use COMMIT as the version string.  When CORE is true, build
package modules under SOURCE using CORE, an instance of Guix."
  ;; Running the self-build script makes it easier to update the build
  ;; procedure: the self-build script of the Guix-to-be-installed contains the
  ;; right dependencies, build procedure, etc., which the Guix-in-use may not
  ;; be know.
  (define script
    (string-append source "/" %self-build-file))

  (if (file-exists? script)
      (let ((build (save-module-excursion
                    (lambda ()
                      ;; Disable deprecation warnings; it's OK for SCRIPT to
                      ;; use deprecated APIs and the user doesn't have to know
                      ;; about it.
                      (parameterize ((guix-warning-port
                                      (%make-void-port "w")))
                        (primitive-load script)))))
            (guile (guile-for-source source)))
        ;; BUILD must be a monadic procedure of at least one argument: the
        ;; source tree.
        ;;
        ;; Note: BUILD can return #f if it does not support %PULL-VERSION.  In
        ;; the future we'll fall back to a previous version of the protocol
        ;; when that happens.
        (mbegin %store-monad
          (mwhen guile
            (set-guile-for-build guile))

          ;; BUILD is usually quite costly.  Install a "trivial" build handler
          ;; so we don't bounce an outer build-accumulator handler that could
          ;; cause us to redo half of the BUILD computation several times just
          ;; to realize it gives the same result.
          (with-trivial-build-handler
           (build source #:verbose? verbose? #:version commit
                  #:pull-version %pull-version))))

      ;; Build a set of modules that extend Guix using the standard method.
      (standard-module-derivation name source core dependencies)))

(define* (build-channel-instance instance
                                 #:optional core (dependencies '()))
  "Return, as a monadic value, the derivation for INSTANCE, a channel
instance.  DEPENDENCIES is a list of extensions providing Guile modules that
INSTANCE depends on."
  (build-from-source (symbol->string
                      (channel-name (channel-instance-channel instance)))
                     (channel-instance-checkout instance)
                     #:commit (channel-instance-commit instance)
                     #:core core
                     #:dependencies dependencies))

(define (resolve-dependencies instances)
  "Return a procedure that, given one of the elements of INSTANCES, returns
list of instances it depends on."
  (define channel-instance-name
    (compose channel-name channel-instance-channel))

  (define table                                   ;map a name to an instance
    (fold (lambda (instance table)
            (vhash-consq (channel-instance-name instance)
                         instance table))
          vlist-null
          instances))

  (define edges
    (fold (lambda (instance edges)
            (fold (lambda (channel edges)
                    (let ((name (channel-name channel)))
                      (match (vhash-assq name table)
                        ((_ . target)
                         (vhash-consq instance target edges)))))
                  edges
                  (channel-instance-dependencies instance)))
          vlist-null
          instances))

  (lambda (instance)
    (vhash-foldq* cons '() instance edges)))

(define (channel-instance-derivations instances)
  "Return the list of derivations to build INSTANCES, in the same order as
INSTANCES."
  (define core-instance
    ;; The 'guix' channel is treated specially: it's an implicit dependency of
    ;; all the other channels.
    (find (lambda (instance)
            (guix-channel? (channel-instance-channel instance)))
          instances))

  (define edges
    (resolve-dependencies instances))

  (define (instance->derivation instance)
    (mlet %store-monad ((system (current-system)))
      (mcached (if (eq? instance core-instance)
                   (build-channel-instance instance)
                   (mlet %store-monad ((core (instance->derivation core-instance))
                                       (deps (mapm %store-monad instance->derivation
                                                   (edges instance))))
                     (build-channel-instance instance core deps)))
               instance
               system)))

  (unless core-instance
    (let ((loc (and=> (any (compose channel-location channel-instance-channel)
                           instances)
                      source-properties->location)))
      (raise (apply make-compound-condition
                    (condition
                     (&message (message "'guix' channel is lacking")))
                    (condition
                     (&fix-hint (hint (G_ "Make sure your list of channels
contains one channel named @code{guix} providing the core of Guix."))))
                    (if loc
                        (list (condition (&error-location (location loc))))
                        '())))))

  (mapm %store-monad instance->derivation instances))

(define (whole-package-for-legacy name modules)
  "Return a full-blown Guix package for MODULES, a derivation that builds Guix
modules in the old ~/.config/guix/latest style."
  (define packages
    (resolve-interface '(gnu packages guile)))

  (define modules+compiled
    ;; Since MODULES contains both .scm and .go files at its root, re-bundle
    ;; it so that it has share/guile/site and lib/guile, which is what
    ;; 'whole-package' expects.
    (computed-file (derivation-name modules)
                   (with-imported-modules '((guix build utils))
                     #~(begin
                         (use-modules (guix build utils))

                         (define version
                           (effective-version))
                         (define share
                           (string-append #$output "/share/guile/site"))
                         (define lib
                           (string-append #$output "/lib/guile/" version))

                         (mkdir-p share) (mkdir-p lib)
                         (symlink #$modules (string-append share "/" version))
                         (symlink #$modules (string-append lib "/site-ccache"))))))

  (letrec-syntax ((list (syntax-rules (->)
                          ((_)
                           '())
                          ((_ (module -> variable) rest ...)
                           (cons (module-ref (resolve-interface
                                              '(gnu packages module))
                                             'variable)
                                 (list rest ...)))
                          ((_ variable rest ...)
                           (cons (module-ref packages 'variable)
                                 (list rest ...))))))
    (whole-package name modules+compiled

                   ;; In the "old style", %SELF-BUILD-FILE would simply return a
                   ;; derivation that builds modules.  We have to infer what the
                   ;; dependencies of these modules were.
                   (list guile-json-3 guile-git guile-bytestructures
                         (ssh -> guile-ssh) (tls -> gnutls)))))

(define (old-style-guix? drv)
  "Return true if DRV corresponds to a ~/.config/guix/latest style of
derivation."
  ;; Here we rely on a gross historical fact: that derivations produced by the
  ;; "old style" (before commit 8a0d9bc8a3f153159d9e239a151c0fa98f1e12d8,
  ;; dated May 30, 2018) did not depend on "guix-command.drv".
  (not (find (lambda (input)
               (string=? "guix-command"
                         (derivation-name
                          (derivation-input-derivation input))))
             (derivation-inputs drv))))

(define (channel-instances->manifest instances)
  "Return a profile manifest with entries for all of INSTANCES, a list of
channel instances."
  (define (instance->entry instance drv)
    (let ((commit  (channel-instance-commit instance))
          (channel (channel-instance-channel instance)))
      (manifest-entry
        (name (symbol->string (channel-name channel)))
        (version (string-take commit 7))
        (item (if (guix-channel? channel)
                  (if (old-style-guix? drv)
                      (whole-package-for-legacy (string-append name "-" version)
                                                drv)
                      drv)
                  drv))
        (properties
         `((source (repository
                    (version 0)
                    (url ,(channel-url channel))
                    (branch ,(channel-branch channel))
                    (commit ,commit))))))))

  (mlet* %store-monad ((derivations (channel-instance-derivations instances))
                       (entries ->  (map instance->entry instances derivations)))
    (return (manifest entries))))

(define (package-cache-file manifest)
  "Build a package cache file for the instance in MANIFEST.  This is meant to
be used as a profile hook."
  (let ((profile (profile (content manifest) (hooks '()))))
    (define build
      #~(begin
          (use-modules (gnu packages))

          (if (defined? 'generate-package-cache)
              (begin
                ;; Delegate package cache generation to the inferior.
                (format (current-error-port)
                        "Generating package cache for '~a'...~%"
                        #$profile)
                (generate-package-cache #$output))
              (mkdir #$output))))

    (gexp->derivation-in-inferior "guix-package-cache" build
                                  profile

                                  ;; If the Guix in PROFILE is too old and
                                  ;; lacks 'guix repl', don't build the cache
                                  ;; instead of failing.
                                  #:silent-failure? #t

                                  #:properties '((type . profile-hook)
                                                 (hook . package-cache))
                                  #:local-build? #t)))

(define %channel-profile-hooks
  ;; The default channel profile hooks.
  (cons package-cache-file %default-profile-hooks))

(define (channel-instances->derivation instances)
  "Return the derivation of the profile containing INSTANCES, a list of
channel instances."
  (mlet %store-monad ((manifest (channel-instances->manifest instances)))
    (profile-derivation manifest
                        #:hooks %channel-profile-hooks)))

(define latest-channel-instances*
  (store-lift latest-channel-instances))

(define* (latest-channel-derivation #:optional (channels %default-channels))
  "Return as a monadic value the derivation that builds the profile for the
latest instances of CHANNELS."
  (mlet %store-monad ((instances (latest-channel-instances* channels)))
    (channel-instances->derivation instances)))

(define (profile-channels profile)
  "Return the list of channels corresponding to entries in PROFILE.  If
PROFILE is not a profile created by 'guix pull', return the empty list."
  (filter-map (lambda (entry)
                (match (assq 'source (manifest-entry-properties entry))
                  (('source ('repository ('version 0)
                                         ('url url)
                                         ('branch branch)
                                         ('commit commit)
                                         _ ...))
                   (channel (name (string->symbol
                                   (manifest-entry-name entry)))
                            (url url)
                            (commit commit)))

                  ;; No channel information for this manifest entry.
                  ;; XXX: Pre-0.15.0 Guix did not provide that information,
                  ;; but there's not much we can do in that case.
                  (_ #f)))

              ;; Show most recently installed packages last.
              (reverse
               (manifest-entries (profile-manifest profile)))))

\f
;;;
;;; News.
;;;

;; Channel news.
(define-record-type <channel-news>
  (channel-news entries)
  channel-news?
  (entries channel-news-entries))                 ;list of <channel-news-entry>

;; News entry, associated with a specific commit of the channel.
(define-record-type <channel-news-entry>
  (channel-news-entry commit tag title body)
  channel-news-entry?
  (commit  channel-news-entry-commit)             ;hex string | #f
  (tag     channel-news-entry-tag)                ;#f | string
  (title   channel-news-entry-title)              ;list of language tag/string pairs
  (body    channel-news-entry-body))              ;list of language tag/string pairs

(define (sexp->channel-news-entry entry)
  "Return the <channel-news-entry> record corresponding to ENTRY, an sexp."
  (define (pair language message)
    (cons (symbol->string language) message))

  (match entry
    (('entry ((and (or 'commit 'tag) type) commit-or-tag)
             ('title ((? symbol? title-tags) (? string? titles)) ...)
             ('body ((? symbol? body-tags) (? string? bodies)) ...)
             _ ...)
     (channel-news-entry (and (eq? type 'commit) commit-or-tag)
                         (and (eq? type 'tag) commit-or-tag)
                         (map pair title-tags titles)
                         (map pair body-tags bodies)))
    (_
     (raise (condition
             (&message (message "invalid channel news entry"))
             (&error-location
              (location (source-properties->location
                         (source-properties entry)))))))))

(define (read-channel-news port)
  "Read a channel news feed from PORT and return it as a <channel-news>
record."
  (match (false-if-exception (read port))
    (('channel-news ('version 0) entries ...)
     (channel-news (map sexp->channel-news-entry entries)))
    (('channel-news ('version version) _ ...)
     ;; This is an unsupported version from the future.  There's nothing wrong
     ;; with that (the user may simply need to upgrade the 'guix' channel to
     ;; be able to read it), so silently ignore it.
     (channel-news '()))
    (#f
     (raise (condition
             (&message (message "syntactically invalid channel news file")))))
    (sexp
     (raise (condition
             (&message (message "invalid channel news file"))
             (&error-location
              (location (source-properties->location
                         (source-properties sexp)))))))))

(define (resolve-channel-news-entry-tag repository entry)
  "If ENTRY has its 'commit' field set, return ENTRY.  Otherwise, lookup
ENTRY's 'tag' in REPOSITORY and return ENTRY with its 'commit' field set to
the field its 'tag' refers to.  A 'git-error' exception is raised if the tag
cannot be found."
  (if (channel-news-entry-commit entry)
      entry
      (let* ((tag       (channel-news-entry-tag entry))
             (reference (string-append "refs/tags/" tag))
             (oid       (reference-name->oid repository reference)))
        (channel-news-entry (oid->string oid) tag
                            (channel-news-entry-title entry)
                            (channel-news-entry-body entry)))))

(define* (channel-news-for-commit channel new #:optional old)
  "Return a list of <channel-news-entry> for CHANNEL between commits OLD and
NEW.  When OLD is omitted or is #f, return all the news entries of CHANNEL."
  (catch 'git-error
    (lambda ()
      (let* ((checkout  (update-cached-checkout (channel-url channel)
                                                #:ref `(commit . ,new)))
             (metadata  (read-channel-metadata-from-source checkout))
             (news-file (channel-metadata-news-file metadata))
             (news-file (and news-file
                             (string-append checkout "/" news-file))))
        (if (and news-file (file-exists? news-file))
            (with-repository checkout repository
              (let* ((news    (call-with-input-file news-file
                                read-channel-news))
                     (entries (map (lambda (entry)
                                     (resolve-channel-news-entry-tag repository
                                                                     entry))
                                   (channel-news-entries news))))
                (if old
                    (let* ((new     (commit-lookup repository (string->oid new)))
                           (old     (commit-lookup repository (string->oid old)))
                           (commits (list->set
                                     (map (compose oid->string commit-id)
                                          (commit-difference new old)))))
                      (filter (lambda (entry)
                                (set-contains? commits
                                               (channel-news-entry-commit entry)))
                              entries))
                    entries)))
            '())))
    (lambda (key error . rest)
      ;; If commit NEW or commit OLD cannot be found, then something must be
      ;; wrong (for example, the history of CHANNEL was rewritten and these
      ;; commits no longer exist upstream), so quietly return the empty list.
      (if (= GIT_ENOTFOUND (git-error-code error))
          '()
          (apply throw key error rest)))))