add .cluster_selected method

src/Makefile.am

@@ -27,13 +27,14 @@ AM_VTC_LOG_FLAGS = \
 	-p vmod_path="$(abs_builddir)/.libs:$(vmoddir)"
 
 TESTS = \
-	vtc/cfg.vtc		\
-	vtc/cld.vtc		\
-	vtc/deep.vtc		\
-	vtc/deep_stk.vtc	\
-	vtc/direct.vtc	\
-	vtc/shallow.vtc		\
-	vtc/lazy.vtc		\
+	vtc/cfg.vtc			\
+	vtc/cld.vtc			\
+	vtc/cluster_selected.vtc	\
+	vtc/deep.vtc			\
+	vtc/deep_stk.vtc		\
+	vtc/direct.vtc			\
+	vtc/shallow.vtc			\
+	vtc/lazy.vtc			\
 	vtc/lazy_shard.vtc
 
 # Documentation

src/vmod_cluster.c

@@ -507,8 +507,8 @@ vmod_cluster_resolve(VRT_CTX, VCL_BACKEND dir)
 static VCL_BACKEND
 cluster_choose(VRT_CTX,
     struct vmod_cluster_cluster *vc,
-    enum resolve_e resolve,
-    struct VARGS(cluster_backend) *arg)
+    enum resolve_e resolve, enum decision_e *decision,
+    struct VARGS(cluster_cluster_selected) *arg)
 {
 	int modify = arg->valid_deny || arg->valid_real ||
 	    arg->valid_uncacheable_direct;
@@ -517,11 +517,14 @@ cluster_choose(VRT_CTX,
 	void *spc = NULL;
 	int nblack;
 
+	if (decision != NULL)
+		*decision = D_NULL;
+
 	if (! modify) {
 		if (resolve == LAZY)
 			return (vc->dir);
 		pr = cluster_task_param_r(ctx, vc);
-		return (decide(ctx, pr, resolve, NULL));
+		return (decide(ctx, pr, resolve, decision));
 	}
 
 	AN(modify);
@@ -565,7 +568,7 @@ cluster_choose(VRT_CTX,
 	if (resolve == LAZY)
 		return (vc->dir);
 
-	return (decide(ctx, pr, resolve, NULL));
+	return (decide(ctx, pr, resolve, decision));
 }
 
 VCL_BACKEND
@@ -573,9 +576,42 @@ vmod_cluster_backend(VRT_CTX,
     struct vmod_cluster_cluster *vc,
     struct VARGS(cluster_backend) *arg)
 {
-	return (cluster_choose(ctx, vc, parse_resolve_e(arg->resolve), arg));
+	struct VARGS(cluster_cluster_selected)
+	    carg[1] = {{
+			.valid_deny = arg->valid_deny,
+			.valid_real = arg->valid_real,
+			.valid_uncacheable_direct = arg->valid_uncacheable_direct,
+			.deny = arg->deny,
+			.real = arg->real,
+			.uncacheable_direct = arg->uncacheable_direct
+		}};
+	return (cluster_choose(ctx, vc, parse_resolve_e(arg->resolve), NULL, carg));
 }
 
+VCL_BOOL
+vmod_cluster_cluster_selected(VRT_CTX,
+    struct VPFX(cluster_cluster) *vc,
+    struct VARGS(cluster_cluster_selected) *arg)
+{
+	enum decision_e decision;
+	VCL_BACKEND b;
+
+	if (ctx->method != VCL_MET_BACKEND_FETCH) {
+		VRT_fail(ctx,
+		    "cluster.cluster_selected can not be called here");
+		return (0);
+	}
+
+	b = cluster_choose(ctx, vc, CLD, &decision, arg);
+
+	if (decision == D_NULL || b == NULL)
+		return (0);
+
+	assert(b != vc->dir);
+	VRT_l_bereq_backend(ctx, b);
+
+	return (decision == D_CLUSTER);
+}
 /*
  * layered directors may not be prepared to resolve outside a VCL task, so when
  * called from the cli (no method, no vcl), just return healthy if either the

src/vmod_cluster.vcc

@@ -225,6 +225,31 @@ differently depending on context:
 
   Also, they cannot be used together with ``resolve=LAZY``.
 
+$Method BOOL .cluster_selected(
+	[ BACKEND deny ], [ BACKEND real ],
+	[ BOOL uncacheable_direct ])
+
+The indended use case is::
+
+	if (xcluster.cluster_selected(...) {
+		# prep the cluster request
+		return (fetch);
+	}
+
+which is almost identical to::
+
+	set bereq.backend = xcluster.backend(resolve=CLD, ...);
+	if (bereq.backend != xcluster.get_real()) {
+		# prep the cluster request
+		return (fetch);
+	}
+
+Behaviour differs for the case that the ``NULL`` backend would be set,
+in which case ``bereq.backend`` is not modified;
+
+This method may only be called from ``vcl_backend_fetch {}`` and fail
+the vcl otherwise.
+
 SEE ALSO
 ========
 vcl\(7),varnishd\(1)

src/vtc/cluster_selected.vtc

+varnishtest "test .cluster_selected"
+
+varnish v1 -vcl {
+    import cluster;
+    import directors;
+
+    backend s1 { .host = "${bad_backend}";}
+    backend s2 { .host = "${bad_backend}";}
+    backend s3 { .host = "${bad_backend}";}
+
+    sub vcl_init {
+	new rr = directors.round_robin();
+	rr.add_backend(s1);
+	rr.add_backend(s2);
+
+	new real = directors.round_robin();
+	real.add_backend(s3);
+
+	new cl = cluster.cluster(rr.backend(), deny=s2, real=real.backend());
+    }
+    sub vcl_backend_fetch {
+	set bereq.http.c1 = cl.cluster_selected();
+	set bereq.http.b1 = bereq.backend;
+	set bereq.http.c2 = cl.cluster_selected();
+	set bereq.http.b2 = bereq.backend;
+    }
+    sub vcl_backend_error {
+	set beresp.status = 200;
+	set beresp.http.c1 = bereq.http.c1;
+	set beresp.http.b1 = bereq.http.b1;
+	set beresp.http.c2 = bereq.http.c2;
+	set beresp.http.b2 = bereq.http.b2;
+    }
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.http.c1 == true
+	expect resp.http.b1 == s1
+	expect resp.http.c2 == false
+	expect resp.http.b2 == real
+} -run