Work around arguably wrong event order in Varnish-Cache

Ref varnishcache/varnish-cache#4037

Closes #107

src/tests/r00107.vtc

@@ -0,0 +1,50 @@
+varnishtest "regression test issue #107 dom_release() from unref_domains"
+
+feature cmd "getent hosts localhost www.localhost img.localhost"
+
+server s1 {
+       rxreq
+       expect req.http.Host == localhost
+       txresp
+
+       rxreq
+       expect req.http.Host == www.localhost
+       txresp
+
+       rxreq
+       expect req.http.Host == www.localhost
+       txresp
+} -start
+
+varnish v1 -vcl+backend {
+	import ${vmod_dynamic};
+
+	sub vcl_init {
+		new d1 = dynamic.director(
+			port = "${s1_port}",
+			ttl = 1m,
+			domain_usage_timeout = 900ms);
+	}
+
+	sub vcl_recv {
+		set req.backend_hint = d1.backend(req.http.Host);
+		return (pass);
+	}
+
+	sub vcl_backend_error {
+		# the director may resolve ::1 first
+		return (retry);
+	}
+} -start
+
+client c1 {
+	txreq -hdr "Host: localhost"
+	rxresp
+	expect resp.status == 200
+} -run
+
+delay 1
+
+varnish v1 -vcl {backend proforma none;}
+
+varnish v1 -cliok "vcl.state vcl1 cold"

src/vmod_dynamic.c

@@ -942,6 +942,11 @@ dynamic_search(struct vmod_dynamic_director *obj, const char *addr,
  * We need to keep our backends around until the last reference to a domain is
  * lost, otherwise it would stop working
  */
+
+// XXX workaround #107 / VC#4037
+static void v_matchproto_(vdi_event_f)
+dom_event(VCL_BACKEND dir, enum vcl_event_e ev);
+
 static void v_matchproto_(vdi_release_f)
 dom_release(VCL_BACKEND dir)
 {
@@ -951,6 +956,10 @@ dom_release(VCL_BACKEND dir)
 	CHECK_OBJ_NOTNULL(dir, DIRECTOR_MAGIC);
 	CAST_OBJ_NOTNULL(dom, dir->priv, DYNAMIC_DOMAIN_MAGIC);
 
+	// XXX workaround #107 / VC#4037
+	if (dom->thread)
+		dom_event(dir, VCL_EVENT_COLD);
+
 	AZ(dom->thread);
 	assert(dom->status == DYNAMIC_ST_READY);