debian: Backup all databases with user percolated data

We want to backup all databases that have possible user data, in case
something went wrong, or one of the databases got lost or similar.

debian/changelog

@@ -90,6 +90,7 @@ dpkg (1.17.7) UNRELEASED; urgency=low
     kept being supported to handle existing bzip2 compressed .deb files.
   * Use GNU tar's --no-unquote when using -T in dpkg-deb to avoid mangling
     filenames. Reported by Niels Thykier <niels@thykier.net>. Closes: #743687
+  * Backup all databases with user data, not just the status database.
 
   [ Updated dpkg translations ]
   * German (Sven Joachim).

debian/dpkg.cron.daily

@@ -1,10 +1,30 @@
 #!/bin/sh
 
-# Backup the 7 last versions of dpkg's status file
+dbdir=/var/lib/dpkg
+
+# Backup the 7 last versions of dpkg databases containing user data.
 if cd /var/backups ; then
-    if ! cmp -s dpkg.status.0 /var/lib/dpkg/status ; then
-            cp -p /var/lib/dpkg/status dpkg.status
-            savelog -c 7 dpkg.status >/dev/null
+    # We backup all relevant database files if any has changed, so that
+    # the rotation number always contains an internally consistent set.
+    dbchanged=no
+    dbfiles="arch status diversions statoverride"
+    for db in $dbfiles ; do
+        if ! cmp -s dpkg.${db}.0 $dbdir/$db ; then
+            dbchanged=yes
+            break;
+        fi
+    done
+    if [ "$dbchanged" = "yes" ] ; then
+        for db in $dbfiles ; do
+            cp -p $dbdir/$db dpkg.$db
+            savelog -c 7 dpkg.$db >/dev/null
+        done
     fi
-fi
 
+    # The alternatives database is independent from the dpkg database.
+    if ! test -e alternatives.tar.0 ||
+       ! tar -df alternatives.tar.0 -C $dbdir alternatives >/dev/null ; then
+        tar -cf alternatives.tar -C $dbdir alternatives
+        savelog -c 7 -J alternatives.tar >/dev/null
+    fi
+fi