Add a signal_close callback method to stevedores, which can be used to

signal background threads to stop in preparation for the coming close
callback.

bin/varnishd/storage/stevedore.c

@@ -436,6 +436,15 @@ STV_close(void)
 {
 	struct stevedore *stv;
 
+	/* Signal intent to close */
+	VTAILQ_FOREACH(stv, &stv_stevedores, list)
+		if (stv->signal_close != NULL)
+			stv->signal_close(stv);
+	stv = stv_transient;
+	if (stv->signal_close != NULL)
+		stv->signal_close(stv);
+
+	/* Close each in turn */
 	VTAILQ_FOREACH(stv, &stv_stevedores, list)
 		if (stv->close != NULL)
 			stv->close(stv);

bin/varnishd/storage/storage.h

@@ -47,6 +47,7 @@ typedef void storage_free_f(struct storage *);
 typedef struct object *storage_allocobj_f(struct stevedore *, struct busyobj *,
     struct objcore **, unsigned ltot, const struct stv_objsecrets *);
 typedef void storage_close_f(const struct stevedore *);
+typedef void storage_signal_close_f(const struct stevedore *);
 
 /* Prototypes for VCL variable responders */
 #define VRTSTVTYPE(ct) typedef ct storage_var_##ct(const struct stevedore *);
@@ -69,6 +70,7 @@ struct stevedore {
 	storage_free_f		*free;		/* --//-- */
 	storage_close_f		*close;		/* --//-- */
 	storage_allocobj_f	*allocobj;	/* --//-- */
+	storage_signal_close_f	*signal_close;	/* --//-- */
 
 	struct lru		*lru;