maint: Generate doc/version[-LANG].texi using `mdate-from-git.scm'.

This replaces Automake's `build-aux/mdate-sh' with our own
`build-aux/mdate-from-git.scm' to use reproducible timestamps from Git
instead.

* build-aux/mdate-from-git.scm: New script.
* bootstrap: Use it to replace build-aux/mdate-sh.
* Makefile.am (EXTRA_DIST): Add it.

Change-Id: I17d0a7de9ffea397129c0db1728f86e28a4e245f

Makefile.am

@@ -725,6 +725,7 @@ EXTRA_DIST +=						\
   build-aux/convert-xref.scm				\
   build-aux/generate-authors.scm			\
   build-aux/git-version-gen				\
+  build-aux/mdate-from-git.scm				\
   build-aux/test-driver.scm				\
   build-aux/update-NEWS.scm				\
   build-aux/update-guix-package.scm			\

bootstrap

@@ -24,4 +24,10 @@ for lang in ${langs}; do
     fi
 done
 
-exec autoreconf -vfi
+autoreconf -vfi
+
+# Replace Automake's build-aux/mdate-sh with build-aux/mdate-from-git, our
+# own, reproducible version.
+chmod +w build-aux/mdate-sh
+rm -f build-aux/mdate-sh
+ln -s mdate-from-git.scm build-aux/mdate-sh

build-aux/mdate-from-git.scm

@@ -0,0 +1,89 @@
+#! /bin/sh
+# -*-scheme-*-
+export LANG=C LANGUAGE=C LC_TIME=C
+export TZ=UTC0
+exec guile --no-auto-compile -L $srcdir -C $srcdir -e '(mdate-from-git)' -s "$0" "$@"
+!#
+
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2024 Janneke Nieuwenhuizen <janneke@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; This program 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.
+;;;
+;;; This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;;;; Commentary:
+;;;
+;;; Usage: mdate-from-git.scm FILE
+;;;
+;;; This script is compatible with Automake's `mdate-sh' but uses the timestamp
+;;; from Git instead of from the file system.  Also, it can be appended to
+;;; mdate-sh.
+
+;;; As a special exception for Guix, it caters for doc/guix.LANG.texi files that
+;;; are not stored in Git, by using po/doc/guix-manual.LANG.po for the Git
+;;; timestamp.  Test doing something like:
+;;;
+;;; build-aux/mdate-from-git.scm doc/guix.de.texi
+;;;
+;;;; Code:
+
+(define-module (mdate-from-git)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 popen)
+  #:use-module (ice-9 rdelim)
+  #:use-module (ice-9 regex)
+  #:export (main))
+
+(define (pipe-command command)
+  (let* ((port (apply open-pipe* OPEN_READ command))
+         (output (read-string port)))
+    (close-port port)
+    output))
+
+(define (guix.LANG.texi->guix-manual.LANG.po file-name)
+  "Translated manuals doc/guix.LANG.texi are not tracked in Git and are
+generated from po/doc/guix-manual.LANG.po.  For such an untraced .TEXI file,
+return its .PO counterpart."
+  (let ((m (string-match "doc/guix.([^.]+).texi" file-name)))
+    (if (not m) file-name
+        (let ((lang (match:substring m 1)))
+          (format #f "po/doc/guix-manual.~a.po" lang)))))
+
+
+;;;
+;;; Entry point.
+;;;
+(define (main args)
+  (match args
+    ((script file-name)
+     (let* ((command `("git" "ls-files" "--error-unmatch" "--" ,file-name))
+            (tracked? (zero? (with-error-to-port (%make-void-port "w")
+                               (lambda _
+                                 (with-output-to-port (%make-void-port "w")
+                                   (lambda _ (apply system* command)))))))
+            (file-name (if tracked? file-name
+                           (guix.LANG.texi->guix-manual.LANG.po file-name)))
+            (command `("git" "log" "--pretty=format:%ct" "-n1" "--" ,file-name))
+            (timestamp (with-error-to-port  (%make-void-port "w")
+                         (lambda _ (pipe-command command))))
+            (source-date-epoch (or (getenv "SOURCE_DATE_EPOCH") "1"))
+            (timestamp (if (string-null? timestamp) source-date-epoch
+                           timestamp))
+            (time (gmtime (string->number timestamp)))
+            (d-m-y (strftime "%-d %B %Y" time)))
+       (display d-m-y)))
+    (_
+     (format (current-error-port) "Usage: mdate-from-git.scm FILE\n")
+     (exit 2))))