Don't create backends on cooling VCLs, return NULL

bin/varnishd/cache/cache_backend.h

@@ -130,5 +130,5 @@ struct vbc *VBT_Get(struct tcp_pool *, double tmo, const struct backend *,
 void VBT_Wait(struct worker *, struct vbc *);
 
 /* cache_vcl.c */
-void VCL_AddBackend(struct vcl *, struct backend *);
+int VCL_AddBackend(struct vcl *, struct backend *);
 void VCL_DelBackend(struct backend *);

bin/varnishd/cache/cache_backend_cfg.c

@@ -65,10 +65,12 @@ struct director *
 VRT_new_backend(VRT_CTX, const struct vrt_backend *vrt)
 {
 	struct backend *b;
+	struct director *d;
 	struct vsb *vsb;
 	struct vcl *vcl;
 	struct tcp_pool *tp = NULL;
 	const struct vrt_backend_probe *vbp;
+	int retval;
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(vrt, VRT_BACKEND_MAGIC);
@@ -123,9 +125,15 @@ VRT_new_backend(VRT_CTX, const struct vrt_backend *vrt)
 	if (vbp != NULL)
 		VBP_Insert(b, vbp, tp);
 
-	VCL_AddBackend(ctx->vcl, b);
+	retval = VCL_AddBackend(ctx->vcl, b);
+
+	if (retval == 0)
+		return (b->director);
 
-	return (b->director);
+	d = b->director;
+	VRT_delete_backend(ctx, &d);
+	AZ(d);
+	return (NULL);
 }
 
 /*--------------------------------------------------------------------

bin/varnishd/cache/cache_vcl.c

@@ -193,12 +193,16 @@ VCL_Rel(struct vcl **vcc)
 
 /*--------------------------------------------------------------------*/
 
-void
+int
 VCL_AddBackend(struct vcl *vcl, struct backend *be)
 {
 
 	CHECK_OBJ_NOTNULL(vcl, VCL_MAGIC);
 	CHECK_OBJ_NOTNULL(be, BACKEND_MAGIC);
+
+	if (vcl->temp == vcl_temp_cooling)
+		return (1);
+
 	Lck_Lock(&vcl_mtx);
 	VTAILQ_INSERT_TAIL(&vcl->backend_list, be, vcl_list);
 	Lck_Unlock(&vcl_mtx);
@@ -208,6 +212,8 @@ VCL_AddBackend(struct vcl *vcl, struct backend *be)
 		VBE_Event(be, VCL_EVENT_WARM);
 	} else if (vcl->temp != vcl_temp_init)
 		WRONG("Dynamic Backends can only be added to warm VCLs");
+
+	return (0);
 }
 
 void

doc/sphinx/reference/directors.rst

@@ -110,7 +110,9 @@ Finally, Varnish will take care of event propagation for *all* native backends,
 but dynamic backends can only be created when the VCL is warm. If your backends
 are created by an independent thread (basically outside of VCL scope) you must
 subscribe to VCL events and watch for VCL state (see
-:ref:`ref-vmod-event-functions`). You are also encouraged to comply with the
+:ref:`ref-vmod-event-functions`). Varnish will panic if you try to create a
+backend on a cold VCL, and ``VRT_new_backend`` will return ``NULL`` if the VCL
+is cooling. You are also encouraged to comply with the
 :ref:`ref_vcl_temperature` in general.