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+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
+;;;
+;;; 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 git)
+  #:use-module (git)
+  #:use-module (git object)
+  #:use-module (guix base32)
+  #:use-module (guix hash)
+  #:use-module (guix build utils)
+  #:use-module (guix store)
+  #:use-module (guix utils)
+  #:use-module (rnrs bytevectors)
+  #:use-module (ice-9 match)
+  #:use-module (srfi srfi-1)
+  #:export (%repository-cache-directory
+            latest-repository-commit))
+
+(define %repository-cache-directory
+  (make-parameter "/var/cache/guix/checkouts"))
+
+(define-syntax-rule (with-libgit2 thunk ...)
+  (dynamic-wind
+    (lambda ()
+      (libgit2-init!))
+    (lambda ()
+      thunk ...)
+    (lambda ()
+      (libgit2-shutdown))))
+
+(define* (url-cache-directory url
+                              #:optional (cache-directory
+                                          (%repository-cache-directory)))
+  "Return the directory associated to URL in %repository-cache-directory."
+  (string-append
+   cache-directory "/"
+   (bytevector->base32-string (sha256 (string->utf8 url)))))
+
+(define (clone* url directory)
+  "Clone git repository at URL into DIRECTORY.  Upon failure,
+make sure no empty directory is left behind."
+  (with-throw-handler #t
+    (lambda ()
+      (mkdir-p directory)
+      (clone url directory))
+    (lambda _
+      (false-if-exception (rmdir directory)))))
+
+(define (repository->head-sha1 repo)
+  "Return the sha1 of the HEAD commit in REPOSITORY as a string."
+  (let ((oid (reference-target (repository-head repo))))
+    (oid->string (commit-id (commit-lookup repo oid)))))
+
+(define (url+commit->name url sha1)
+  "Return the string \"<REPO-NAME>-<SHA1:7>\" where REPO-NAME is the name of
+the git repository, extracted from URL and SHA1:7 the seven first digits
+of SHA1 string."
+  (string-append
+   (string-replace-substring
+    (last (string-split url #\/)) ".git" "")
+   "-" (string-take sha1 7)))
+
+(define* (copy-to-store store cache-directory #:key url repository)
+  "Copy items in cache-directory to store.  URL and REPOSITORY are used
+to forge store directory name."
+  (let* ((commit (repository->head-sha1 repository))
+         (name   (url+commit->name url commit)))
+    (values (add-to-store store name #t "sha256" cache-directory) commit)))
+
+(define (switch-to-ref repository ref)
+  "Switch to REPOSITORY's branch, commit or tag specified by REF."
+  (let* ((oid (match ref
+                (('branch . branch)
+                 (reference-target
+                  (branch-lookup repository branch BRANCH-REMOTE)))
+                (('commit . commit)
+                 (string->oid commit))
+                (('tag    . tag)
+                 (reference-name->oid repository
+                                      (string-append "refs/tags/" tag)))))
+         (obj (object-lookup repository oid)))
+    (reset repository obj RESET_HARD)))
+
+(define* (latest-repository-commit store url
+                                   #:key
+                                   (cache-directory
+                                    (%repository-cache-directory))
+                                   (ref '(branch . "origin/master")))
+  "Return two values: the content of the git repository at URL copied into a
+store directory and the sha1 of the top level commit in this directory.  The
+reference to be checkout, once the repository is fetched, is specified by REF.
+REF is pair whose key is [branch | commit | tag] and value the associated
+data, respectively [<branch name> | <sha1> | <tag name>].
+
+Git repositories are kept in the cache directory specified by
+%repository-cache-directory parameter."
+  (with-libgit2
+   (let* ((cache-dir     (url-cache-directory url cache-directory))
+          (cache-exists? (openable-repository? cache-dir))
+          (repository    (if cache-exists?
+                             (repository-open cache-dir)
+                             (clone* url cache-dir))))
+     ;; Only fetch remote if it has not been cloned just before.
+     (when cache-exists?
+       (remote-fetch (remote-lookup repository "origin")))
+     (switch-to-ref repository ref)
+     (copy-to-store store cache-dir
+                    #:url url
+                    #:repository repository))))