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+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2019 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 tests git)
+  #:use-module (git)
+  #:use-module ((guix git) #:select (with-repository))
+  #:use-module (guix utils)
+  #:use-module (guix build utils)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 control)
+  #:export (git-command
+            with-temporary-git-repository
+            find-commit))
+
+(define git-command
+  (make-parameter "git"))
+
+(define (populate-git-repository directory directives)
+  "Initialize a new Git checkout and repository in DIRECTORY and apply
+DIRECTIVES.  Each element of DIRECTIVES is an sexp like:
+
+  (add \"foo.txt\" \"hi!\")
+
+Return DIRECTORY on success."
+
+  ;; Note: As of version 0.2.0, Guile-Git lacks the necessary bindings to do
+  ;; all this, so resort to the "git" command.
+  (define (git command . args)
+    (apply invoke (git-command) "-C" directory
+           command args))
+
+  (mkdir-p directory)
+  (git "init")
+
+  (let loop ((directives directives))
+    (match directives
+      (()
+       directory)
+      ((('add file contents) rest ...)
+       (let ((file (string-append directory "/" file)))
+         (mkdir-p (dirname file))
+         (call-with-output-file file
+           (lambda (port)
+             (display (if (string? contents)
+                          contents
+                          (with-repository directory repository
+                            (contents repository)))
+                      port)))
+         (git "add" file)
+         (loop rest)))
+      ((('commit text) rest ...)
+       (git "commit" "-m" text)
+       (loop rest))
+      ((('tag name) rest ...)
+       (git "tag" name)
+       (loop rest))
+      ((('branch name) rest ...)
+       (git "branch" name)
+       (loop rest))
+      ((('checkout branch) rest ...)
+       (git "checkout" branch)
+       (loop rest))
+      ((('merge branch message) rest ...)
+       (git "merge" branch "-m" message)
+       (loop rest)))))
+
+(define (call-with-temporary-git-repository directives proc)
+  (call-with-temporary-directory
+   (lambda (directory)
+     (populate-git-repository directory directives)
+     (proc directory))))
+
+(define-syntax-rule (with-temporary-git-repository directory
+                                                   directives exp ...)
+  "Evaluate EXP in a context where DIRECTORY contains a checkout populated as
+per DIRECTIVES."
+  (call-with-temporary-git-repository directives
+                                      (lambda (directory)
+                                        exp ...)))
+
+(define (find-commit repository message)
+  "Return the commit in REPOSITORY whose message includes MESSAGE, a string."
+  (let/ec return
+    (fold-commits (lambda (commit _)
+                    (and (string-contains (commit-message commit)
+                                          message)
+                         (return commit)))
+                  #f
+                  repository)
+    (error "commit not found" message)))