Allow shard.reconfigure() to work on empty directors

Also move the lock up to cover more operations.

bin/varnishtest/tests/d06001.vtc

+varnishtest "Empty shard reconfig"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+	import directors;
+
+	sub vcl_init {
+		new shard1 = directors.shard();
+		new shard2 = directors.shard();
+
+		shard1.reconfigure();
+
+		shard2.add_backend(s1);
+		shard2.reconfigure();
+	}
+
+	sub vcl_recv {
+		set req.backend_hint = shard2.backend();
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+} -run

lib/libvmod_directors/shard_cfg.c

@@ -616,14 +616,19 @@ shardcfg_reconfigure(VRT_CTX, struct vmod_priv *priv,
 		return (0);
 	}
 
+	sharddir_wrlock(shardd);
+
 	change = shard_change_get(ctx, priv, shardd);
-	if (change == NULL)
+	if (change == NULL) {
+		sharddir_unlock(shardd);
 		return (0);
+	}
 
-	if (VSTAILQ_FIRST(&change->tasks) == NULL)
+	if (VSTAILQ_FIRST(&change->tasks) == NULL) {
+		shard_change_finish(change);
+		sharddir_unlock(shardd);
 		return (1);
-
-	sharddir_wrlock(shardd);
+	}
 
 	shardcfg_apply_change(ctx, shardd, change, replicas);
 	shard_change_finish(change);