[type/__debconf_set_selections] Synchronise objects

Works around locking error:

	debconf: DbDriver "config": /var/cache/debconf/config.dat is locked by another process: Resource temporarily unavailable

cdist/conf/type/__debconf_set_selections/explorer/state

@@ -41,10 +41,15 @@ test -s "${linesfile}" || {
 	exit 0
 }
 
+# assert __type_explorer is set (because it is used by the Perl script)
+: "${__type_explorer:?}"
+
 /usr/bin/perl -- - "${linesfile}" <<'EOF'
 use strict;
 use warnings "all";
 
+use Fcntl qw(:DEFAULT :flock);
+
 use Debconf::Db;
 use Debconf::Question;
 
@@ -78,7 +83,20 @@ sub state {
 	}
 }
 
-Debconf::Db->load(readonly => 'true');
+
+# Load Debconf DB but manually lock on the state explorer script,
+# because Debconf aborts immediately if executed concurrently.
+# This is not really an ideal solution because the Debconf DB could be locked by
+# another process (e.g. apt-get), but no way to achieve this could be found.
+# If you know how to, please provide a patch.
+my $lockfile = "%ENV{'__type_explorer'}/state";
+if (open my $lock_fh, '+<', $lockfile) {
+   flock $lock_fh, LOCK_EX or die "Cannot lock $lockfile";
+}
+{
+	Debconf::Db->load(readonly => 'true');
+}
+
 
 while (<>) {
 	# Read and process lines (taken from debconf-set-selections)