Directors, like all other code, should handle NULL BACKEND arguments

sanely, in this case by failing VCL initialization.

Fixes #2036

bin/varnishtest/tests/r02036.vtc

+varnishtest "NULL directors in vcl_init"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {} -start
+
+varnish v1 -errvcl "NULL backend cannot be added" {
+	import directors;
+
+	probe cacheprobe {
+		.initial = 0;
+	}
+
+	backend b01 {
+		.host = "127.0.0.2";
+		.probe = cacheprobe;
+	}
+
+	sub vcl_init {
+		new hash = directors.hash();
+		hash.add_backend(b01,1.0);
+
+		new rr = directors.round_robin();
+		rr.add_backend(hash.backend("text"));
+	}
+}

lib/libvmod_directors/fall_back.c

@@ -122,7 +122,7 @@ vmod_fallback_add_backend(VRT_CTX,
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(fb, VMOD_DIRECTORS_FALLBACK_MAGIC);
-	(void)vdir_add_backend(fb->vd, be, 0.0);
+	vdir_add_backend(ctx, fb->vd, be, 0.0);
 }
 
 VCL_VOID __match_proto__()
@@ -131,7 +131,7 @@ vmod_fallback_remove_backend(VRT_CTX,
 {
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(fb, VMOD_DIRECTORS_FALLBACK_MAGIC);
-	vdir_remove_backend(fb->vd, be, &fb->cur);
+	vdir_remove_backend(ctx, fb->vd, be, &fb->cur);
 }
 
 VCL_BACKEND __match_proto__()

lib/libvmod_directors/hash.c

@@ -81,7 +81,7 @@ vmod_hash_add_backend(VRT_CTX,
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(rr, VMOD_DIRECTORS_HASH_MAGIC);
-	(void)vdir_add_backend(rr->vd, be, w);
+	vdir_add_backend(ctx, rr->vd, be, w);
 }
 
 VCL_VOID __match_proto__()
@@ -91,7 +91,7 @@ vmod_hash_remove_backend(VRT_CTX,
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(rr, VMOD_DIRECTORS_HASH_MAGIC);
-	vdir_remove_backend(rr->vd, be, NULL);
+	vdir_remove_backend(ctx, rr->vd, be, NULL);
 }
 
 VCL_BACKEND __match_proto__()

lib/libvmod_directors/random.c

@@ -109,7 +109,7 @@ vmod_random_add_backend(VRT_CTX,
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(rr, VMOD_DIRECTORS_RANDOM_MAGIC);
-	(void)vdir_add_backend(rr->vd, be, w);
+	vdir_add_backend(ctx, rr->vd, be, w);
 }
 
 VCL_VOID vmod_random_remove_backend(VRT_CTX,
@@ -117,7 +117,7 @@ VCL_VOID vmod_random_remove_backend(VRT_CTX,
 {
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(rr, VMOD_DIRECTORS_RANDOM_MAGIC);
-	vdir_remove_backend(rr->vd, be, NULL);
+	vdir_remove_backend(ctx, rr->vd, be, NULL);
 }
 
 VCL_BACKEND __match_proto__()

lib/libvmod_directors/round_robin.c

@@ -118,7 +118,7 @@ vmod_round_robin_add_backend(VRT_CTX,
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(rr, VMOD_DIRECTORS_ROUND_ROBIN_MAGIC);
-	(void)vdir_add_backend(rr->vd, be, 0.0);
+	vdir_add_backend(ctx, rr->vd, be, 0.0);
 }
 
 VCL_VOID __match_proto__()
@@ -127,7 +127,7 @@ vmod_round_robin_remove_backend(VRT_CTX,
 {
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(rr, VMOD_DIRECTORS_ROUND_ROBIN_MAGIC);
-	vdir_remove_backend(rr->vd, be, NULL);
+	vdir_remove_backend(ctx, rr->vd, be, NULL);
 }
 
 VCL_BACKEND __match_proto__()

lib/libvmod_directors/vdir.c

@@ -113,12 +113,16 @@ vdir_unlock(struct vdir *vd)
 }
 
 
-unsigned
-vdir_add_backend(struct vdir *vd, VCL_BACKEND be, double weight)
+void
+vdir_add_backend(VRT_CTX, struct vdir *vd, VCL_BACKEND be, double weight)
 {
 	unsigned u;
 
 	CHECK_OBJ_NOTNULL(vd, VDIR_MAGIC);
+	if (be == NULL) {
+		VRT_fail(ctx, "NULL backend cannot be added");
+		return;
+	}
 	AN(be);
 	vdir_wrlock(vd);
 	if (vd->n_backend >= vd->l_backend)
@@ -129,17 +133,18 @@ vdir_add_backend(struct vdir *vd, VCL_BACKEND be, double weight)
 	vd->weight[u] = weight;
 	vd->total_weight += weight;
 	vdir_unlock(vd);
-	return (u);
 }
 
 void
-vdir_remove_backend(struct vdir *vd, VCL_BACKEND be, unsigned *cur)
+vdir_remove_backend(VRT_CTX, struct vdir *vd, VCL_BACKEND be, unsigned *cur)
 {
 	unsigned u, n;
 
 	CHECK_OBJ_NOTNULL(vd, VDIR_MAGIC);
-	if (be == NULL)
+	if (be == NULL) {
+		VRT_fail(ctx, "NULL backend cannot be removed");
 		return;
+	}
 	CHECK_OBJ(be, DIRECTOR_MAGIC);
 	vdir_wrlock(vd);
 	for (u = 0; u < vd->n_backend; u++)

lib/libvmod_directors/vdir.h

@@ -47,8 +47,8 @@ void vdir_delete(struct vdir **vdp);
 void vdir_rdlock(struct vdir *vd);
 void vdir_wrlock(struct vdir *vd);
 void vdir_unlock(struct vdir *vd);
-unsigned vdir_add_backend(struct vdir *, VCL_BACKEND be, double weight);
-void vdir_remove_backend(struct vdir *, VCL_BACKEND be, unsigned *cur);
+void vdir_add_backend(VRT_CTX, struct vdir *, VCL_BACKEND, double weight);
+void vdir_remove_backend(VRT_CTX, struct vdir *, VCL_BACKEND, unsigned *cur);
 unsigned vdir_any_healthy(struct vdir *, const struct busyobj *,
     double *changed);
 VCL_BACKEND vdir_pick_be(struct vdir *, double w, const struct busyobj *);