Warn the user if /bin/sh is ineffectively diverted (DEP17)

Co-authored-by: Andrej Shadura <andrewsh@debian.org>

Closes: #1078130

debian/clean

@@ -1 +0,0 @@
-debian/dash.templates

debian/control

@@ -17,7 +17,7 @@ Architecture: any
 Multi-Arch: foreign
 Essential: yes
 Priority: required
-Pre-Depends: ${shlibs:Depends}
+Pre-Depends: ${shlibs:Depends}, debconf
 Depends:
  debianutils (>= 5.6-0.1),
  ${misc:Depends},

debian/dash.preinst

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+set -e
+
+if [ "$1" = upgrade ] && dpkg --compare-versions "$2" lt 0.5.12-9~; then
+	# Upgrading from pre-/usr-move DEP17
+	truename=$(dpkg-divert --truename /bin/sh)
+	if [ "$truename" != /bin/sh ] &&
+		[ "$(dpkg-divert --listpackage /bin/sh)_$truename" != "dash_/bin/sh.distrib" ] &&
+		[ "$(dpkg-divert --truename /usr/bin/sh)" = "/usr/bin/sh" ]; then
+		# There is a diversion of /bin/sh and it is not our own
+		# diversion to /bin/sh.distrib that will be removed in
+		# postinst and there is no matching diversion of /usr/bin/sh.
+		. /usr/share/debconf/confmodule
+		db_input critical dash/ineffectivediversion || true
+		db_go || true
+		# We have no other debconf usage in dash, so we can purge rightaway.
+		db_purge || true
+	fi
+fi
+
+#DEBHELPER#
+
+exit 0

debian/dash.templates

@@ -0,0 +1,10 @@
+Template: dash/ineffectivediversion
+Type: error
+_Description: /bin/sh is ineffectively diverted
+ A diversion of /bin/sh was found.
+ .
+ After this update, dash will be recorded in the dpkg database at its new path, /usr/bin/sh. Since there is no diversion set for /usr/bin/sh, the existing diversion for /bin/sh will no longer work.
+ .
+ To keep using your preferred default shell, you need to set up another diversion for /usr/bin/sh. If you don’t do so now, your changes to /bin/sh will be lost after this upgrade.
+ .
+ Once the upgrade is complete, the diversion for /bin/sh will no longer be needed and can be removed.

debian/po/POTFILES.in

@@ -0,0 +1,2 @@
+[encoding: UTF-8]
+[type: gettext/rfc822deb] dash.templates

debian/po/templates.pot

@@ -0,0 +1,55 @@
+# debconf templates for dash
+# Copyright (C) 2024 Debian dash maintainers
+# This file is distributed under the same license as the dash package.
+# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
+#
+msgid ""
+msgstr ""
+"Project-Id-Version: dash\n"
+"Report-Msgid-Bugs-To: dash@packages.debian.org\n"
+"POT-Creation-Date: 2025-01-03 14:21+0100\n"
+"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language-Team: LANGUAGE <LL@li.org>\n"
+"Language: \n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=UTF-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+
+#. Type: error
+#. Description
+#: ../dash.templates:1001
+msgid "/bin/sh is ineffectively diverted"
+msgstr ""
+
+#. Type: error
+#. Description
+#: ../dash.templates:1001
+msgid "A diversion of /bin/sh was found."
+msgstr ""
+
+#. Type: error
+#. Description
+#: ../dash.templates:1001
+msgid ""
+"After this update, dash will be recorded in the dpkg database at its new "
+"path, /usr/bin/sh. Since there is no diversion set for /usr/bin/sh, the "
+"existing diversion for /bin/sh will no longer work."
+msgstr ""
+
+#. Type: error
+#. Description
+#: ../dash.templates:1001
+msgid ""
+"To keep using your preferred default shell, you need to set up another "
+"diversion for /usr/bin/sh. If you don’t do so now, your changes to /bin/sh "
+"will be lost after this upgrade."
+msgstr ""
+
+#. Type: error
+#. Description
+#: ../dash.templates:1001
+msgid ""
+"Once the upgrade is complete, the diversion for /bin/sh will no longer be "
+"needed and can be removed."
+msgstr ""