maint: update-guix-package: Prevent accidentally breaking guix pull.

Fixes <https://issues.guix.gnu.org/43893>.

This changes the 'update-guix-package' tool so that it:

1. Always uses a clean checkout to compute the hash of the updated 'guix'
package.
2. Ensures the commit used in the updated 'guix' package definition has already
been pushed upstream.

* build-aux/update-guix-package.scm (%savannah-guix-git-repo-push-url): New
variable.
(with-input-pipe-to-string, with-temporary-git-worktree): New syntaxes.
(find-origin-remote, git-add-worktree): New procedures.
(commit-already-pushed?): New predicate.
(main): Check the commit used has already been pushed upstream and compute the
hash from a clean checkout.
* doc/contributing.texi (Updating the Guix Package): Document it.
* .dir-locals.el (scheme-mode): Fix indentation of with-temporary-git-worktree.

1 files changed, 43 insertions, 0 deletions

diff --git a/doc/contributing.texi b/doc/contributing.texi
index af3601442e..11a932a9bf 100644
--- a/doc/contributing.texi
+++ b/doc/contributing.texi
@@ -28,6 +28,7 @@ choice.
 * Submitting Patches::          Share your work.
 * Tracking Bugs and Patches::   Using Debbugs.
 * Commit Access::               Pushing to the official repository.
+* Updating the Guix Package::   Updating the Guix package definition.
 @end menu
 
 @node Building from Git
@@ -1323,3 +1324,45 @@ only push their own awesome changes, but also offer some of their time
 @emph{reviewing} and pushing other people's changes.  As a committer,
 you're welcome to use your expertise and commit rights to help other
 contributors, too!
+
+@node Updating the Guix Package
+@section Updating the Guix Package
+
+@cindex update-guix-package, updating the guix package
+It is sometimes desirable to update the @code{guix} package itself (the
+package defined in @code{(gnu packages package-management)}), for
+example to make new daemon features available for use by the
+@code{guix-service-type} service type.  In order to simplify this task,
+the following command can be used:
+
+@example
+make update-guix-package
+@end example
+
+The @code{update-guix-package} make target will use the last known
+@emph{commit} corresponding to @code{HEAD} in your Guix checkout,
+compute the hash of the Guix sources corresponding to that commit and
+update the @code{commit}, @code{revision} and hash of the @code{guix}
+package definition.
+
+To validate that the updated @code{guix} package hashes are correct and
+that it can be built successfully, the following command can be run from
+the directory of your Guix checkout:
+
+@example
+./pre-inst-env guix build guix
+@end example
+
+To guard against accidentally updating the @code{guix} package to a
+commit that others can't refer to, a check is made that the commit used
+has already been pushed to the Savannah-hosted Guix git repository.
+
+This check can be disabled, @emph{at your own peril}, by setting the
+@code{GUIX_ALLOW_ME_TO_USE_PRIVATE_COMMIT} environment variable.
+
+To build the resulting 'guix' package when using a private commit, the
+following command can be used:
+
+@example
+./pre-inst-env guix build guix --with-git-url=guix=$PWD
+@end example