By using a fresh copy of the last commit, we ensure the computed hash is
stable in the face of local edits.  This change also computes the hash
externally from the store, which allows to verify that the hashes are valid
using, e.g.:

 #FIXME: This doesn't work (recursion?)
./pre-inst-env guix build guix --with-git-url=guix=file://$PWD

* build-aux/update-guix-package.scm (git-add-worktree): New procedure.
(main): Use it to checkout a clean copy of the used commit, and compute the
hash from it.  Print a user warning after completion.
---
 build-aux/update-guix-package.scm | 59 +++++++++++++------------------
 1 file changed, 25 insertions(+), 34 deletions(-)

diff --git a/build-aux/update-guix-package.scm b/build-aux/update-guix-package.scm
index f695e91cfd..b609e57b8f 100644
--- a/build-aux/update-guix-package.scm
+++ b/build-aux/update-guix-package.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017, 2018 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -29,6 +30,7 @@
              (guix utils)
              (guix base32)
              (guix build utils)
+             (guix scripts hash)
              (gnu packages package-management)
              (ice-9 match))
 
@@ -101,44 +103,33 @@ COMMIT."
       (exp
        (error "'guix' package definition is not as expected" exp)))))
 
+(define (git-add-worktree directory commit-ish)
+  "Create a new git worktree at DIRECTORY, detached on commit COMMIT-ISH."
+  (invoke "git" "worktree" "add" "--detach" directory commit-ish))
+
 
 (define (main . args)
   (match args
     ((commit version)
-     (with-store store
-       (let* ((source   (add-to-store store
-                                      "guix-checkout" ;dummy name
-                                      #t "sha256" %top-srcdir
-                                      #:select? version-controlled?))
-              (hash     (query-path-hash store source))
-              (location (package-definition-location))
-              (old-hash (content-hash-value
-                          (origin-hash (package-source guix)))))
-         (edit-expression location
-                          (update-definition commit hash
-                                             #:old-hash old-hash
-                                             #:version version))
-
-         ;; Re-add SOURCE to the store, but this time under the real name used
-         ;; in the 'origin'.  This allows us to build the package without
-         ;; having to make a real checkout; thus, it also works when working
-         ;; on a private branch.
-         (reload-module
-          (resolve-module '(gnu packages package-management)))
-
-         (let* ((source (add-to-store store
-                                      (origin-file-name (package-source guix))
-                                      #t "sha256" source))
-                (root   (store-path-package-name source)))
-
-           ;; Add an indirect GC root for SOURCE in the current directory.
-           (false-if-exception (delete-file root))
-           (symlink source root)
-           (add-indirect-root store
-                              (string-append (getcwd) "/" root))
-
-           (format #t "source code for commit ~a: ~a (GC root: ~a)~%"
-                   commit source root)))))
+     (with-directory-excursion %top-srcdir
+       (call-with-temporary-directory
+        (lambda (tmp-directory)
+          (let* ((dummy (git-add-worktree tmp-directory commit))
+                 (hash (nix-base32-string->bytevector
+                        (string-trim-both
+                         (with-output-to-string
+		           (lambda ()
+		             (guix-hash "-rx" tmp-directory))))))
+                 (location (package-definition-location))
+                 (old-hash (content-hash-value
+                            (origin-hash (package-source guix)))))
+            (edit-expression location
+                             (update-definition commit hash
+                                                #:old-hash old-hash
+                                                #:version version))
+            (format #t "Updated Guix to commit ~s.  You must ensure this
+commit hash exists in the public repository, else 'guix pull' will break.
+Beware of 'git rebase'~%" commit))))))
     ((commit)
      ;; Automatically deduce the version and revision numbers.
      (main commit #f))))
-- 
2.28.0