Add (guix git-download).

* guix/git-download.scm, guix/build/git.scm: New files.
* Makefile.am (MODULES): Add them.
* guix/packages.scm (<origin>): Fix comment for 'method' field.

Makefile.am

@@ -34,6 +34,7 @@ MODULES =					\
   guix/pki.scm					\
   guix/utils.scm				\
   guix/download.scm				\
+  guix/git-download.scm				\
   guix/monads.scm				\
   guix/profiles.scm				\
   guix/serialization.scm			\
@@ -54,6 +55,7 @@ MODULES =					\
   guix/ui.scm					\
   guix/build/download.scm			\
   guix/build/cmake-build-system.scm		\
+  guix/build/git.scm				\
   guix/build/gnome.scm				\
   guix/build/gnu-build-system.scm		\
   guix/build/gnu-dist.scm			\

guix/build/git.scm

@@ -0,0 +1,45 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2014 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 build git)
+  #:use-module (guix build utils)
+  #:export (git-fetch))
+
+;;; Commentary:
+;;;
+;;; This is the build-side support code of (guix git-download).  It allows a
+;;; Git repository to be cloned and checked out at a specific commit.
+;;;
+;;; Code:
+
+(define* (git-fetch url commit directory
+                    #:key (git-command "git"))
+  "Fetch COMMIT from URL into DIRECTORY.  COMMIT must be a valid Git commit
+identifier.  Return #t on success, #f otherwise."
+  (and (zero? (system* git-command "clone" url directory))
+       (with-directory-excursion directory
+         (system* git-command "tag" "-l")
+         (and (zero? (system* git-command "checkout" commit))
+              (begin
+                ;; The contents of '.git' vary as a function of the current
+                ;; status of the Git repo.  Since we want a fixed output, this
+                ;; directory needs to be taken out.
+                (delete-file-recursively ".git")
+                #t)))))
+
+;;; git.scm ends here

guix/git-download.scm

@@ -0,0 +1,89 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2014 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 git-download)
+  #:use-module (guix records)
+  #:use-module (guix derivations)
+  #:use-module (guix packages)
+  #:use-module (ice-9 match)
+  #:export (git-reference
+            git-reference?
+            git-reference-url
+            git-reference-commit
+
+            git-fetch))
+
+;;; Commentary:
+;;;
+;;; An <origin> method that fetches a specific commit from a Git repository.
+;;; The repository URL and commit hash are specified with a <git-reference>
+;;; object.
+;;;
+;;; Code:
+
+(define-record-type* <git-reference>
+  git-reference make-git-reference
+  git-reference?
+  (url    git-reference-url)
+  (commit git-reference-commit))
+
+(define* (git-fetch store ref hash-algo hash
+                    #:optional name
+                    #:key (system (%current-system)) guile git)
+  "Return a fixed-output derivation in STORE that fetches REF, a
+<git-reference> object.  The output is expected to have recursive hash HASH of
+type HASH-ALGO (a symbol).  Use NAME as the file name, or a generic name if
+#f."
+  (define guile-for-build
+    (match guile
+      ((? package?)
+       (package-derivation store guile system))
+      (#f                                         ; the default
+       (let* ((distro (resolve-interface '(gnu packages base)))
+              (guile  (module-ref distro 'guile-final)))
+         (package-derivation store guile system)))))
+
+  (define git-for-build
+    (match git
+      ((? package?)
+       (package-derivation store git system))
+      (#f                                         ; the default
+       (let* ((distro (resolve-interface '(gnu packages version-control)))
+              (git    (module-ref distro 'git)))
+         (package-derivation store git system)))))
+
+  (let* ((command (string-append (derivation->output-path git-for-build)
+                                 "/bin/git"))
+         (builder `(begin
+                     (use-modules (guix build git))
+                     (git-fetch ',(git-reference-url ref)
+                                ',(git-reference-commit ref)
+                                %output
+                                #:git-command ',command))))
+    (build-expression->derivation store (or name "git-checkout") builder
+                                  #:system system
+                                  #:local-build? #t
+                                  #:inputs `(("git" ,git-for-build))
+                                  #:hash-algo hash-algo
+                                  #:hash hash
+                                  #:recursive? #t
+                                  #:modules '((guix build git)
+                                              (guix build utils))
+                                  #:guile-for-build guile-for-build)))
+
+;;; git-download.scm ends here

guix/packages.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -106,7 +106,7 @@ (define-record-type* <origin>
   origin make-origin
   origin?
   (uri       origin-uri)                          ; string
-  (method    origin-method)                       ; symbol
+  (method    origin-method)                       ; procedure
   (sha256    origin-sha256)                       ; bytevector
   (file-name origin-file-name (default #f))       ; optional file name
   (patches   origin-patches (default '()))        ; list of file names