Directors are allowed to fail to pick a backend.

Fixes	#1501

bin/varnishd/cache/cache_fetch.c

@@ -181,7 +181,6 @@ vbf_stp_mkbereq(const struct worker *wrk, struct busyobj *bo)
 	CHECK_OBJ_NOTNULL(bo->req, REQ_MAGIC);
 
 	assert(bo->state == BOS_INVALID);
-	AN(bo->director);
 	AZ(bo->vbc);
 	AZ(bo->should_close);
 	AZ(bo->storage_hint);
@@ -255,7 +254,6 @@ vbf_stp_startfetch(struct worker *wrk, struct busyobj *bo)
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
 	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_MAGIC);
 
-	AN(bo->director);
 	AZ(bo->vbc);
 	AZ(bo->should_close);
 	AZ(bo->storage_hint);

bin/varnishd/cache/cache_http1_fetch.c

@@ -300,6 +300,10 @@ V1F_fetch_hdr(struct worker *wrk, struct busyobj *bo, struct req *req)
 	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_MAGIC);
 	htc = &bo->htc;
 
+	if (bo->director == NULL) {
+		VSLb(bo->vsl, SLT_FetchError, "No backend");
+		return (-1);
+	}
 	AN(bo->director);
 
 	hp = bo->bereq;

bin/varnishd/cache/cache_vrt_var.c

@@ -270,9 +270,8 @@ VRT_r_beresp_backend_name(const struct vrt_ctx *ctx)
 		CHECK_OBJ_NOTNULL(ctx->bo->vbc, VBC_MAGIC);
 		return (ctx->bo->vbc->backend->vcl_name);
 	}
-	if (ctx->bo->director != NULL) {
+	if (ctx->bo->director != NULL)
 		return (ctx->bo->director->vcl_name);
-	}
 	return (NULL);
 }
 
@@ -350,7 +349,6 @@ VRT_l_bereq_backend(const struct vrt_ctx *ctx, struct director *be)
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(ctx->bo, BUSYOBJ_MAGIC);
-	AN(ctx->bo->director);
 	ctx->bo->director = be;
 }
 
@@ -360,7 +358,6 @@ VRT_r_bereq_backend(const struct vrt_ctx *ctx)
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(ctx->bo, BUSYOBJ_MAGIC);
-	AN(ctx->bo->director);
 	return (ctx->bo->director);
 }
 

bin/varnishtest/tests/r01501.vtc

+varnishtest "director fails to pick backend"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+
+	import ${vmod_debug};
+
+	sub vcl_recv {
+		set req.backend_hint = debug.no_backend();
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 503
+} -run