fix inconsistency with .backend() arguments affecting state

src/vmod_cluster.c

@@ -482,8 +482,8 @@ vmod_cluster_backend(VRT_CTX,
 
 	if (resolve == LAZY &&
 	    (ctx->method & cluster_methods) == 0) {
-		VRT_fail(ctx, "cluster.backend(resolve=LAZY)"
-		    " can not be called here");
+		VRT_fail(ctx, "cluster.backend(resolve=%s)"
+		    " can not be called here", arg->resolve);
 		return NULL;
 	}
 
@@ -493,7 +493,7 @@ vmod_cluster_backend(VRT_CTX,
 	char pstk[param_sz(pr, pr->nblack + 1)];
 	nblack = pr->nblack;
 
-	if (resolve == SHALLOW || resolve == DEEP)
+	if ((ctx->method & cluster_methods) == 0)
 		spc = pstk;
 
 	if (arg->valid_deny && arg->deny != NULL &&

src/vmod_cluster.vcc

@@ -183,22 +183,27 @@ documentation:
   backend
 
 * for ``resolve=DEEP`` the actual backend which the `cluster` or
-  `real`backend resolve to. Only differs from ``resolve=SHALLOW`` for
+  `real` backend resolve to. Only differs from ``resolve=SHALLOW`` for
   director backends.
 
 ``resolve=NOW`` is identical to ``resolve=DEEP`` as a transitional
 interface and will be removed in a future version.
 
-The optional `deny`, `real` and `uncacheable_direct` arguments have
-the same effect as calling the methods :ref:`func_cluster.deny`,
-:ref:`func_cluster.set_real` and
-:ref:`func_cluster.set_uncacheable_direct` before the `.backend()`
-method with ``resolve=LAZY``.
+The optional `deny`, `real` and `uncacheable_direct` arguments behave
+differently depending on context:
 
-Use of these arguments with ``resolve=LAZY`` is only allowed in
-``vcl_backend_fetch {}`` and ``vcl_init {}``.
+* in ``vcl_backend_fetch {}`` and ``vcl_init {}``, they have the same
+  effect as calling the methods :ref:`func_cluster.deny`,
+  :ref:`func_cluster.set_real` and
+  :ref:`func_cluster.set_uncacheable_direct` before the `.backend()`
+  method - in other words, they affect future method calls on the same
+  cluster object also.
 
-Calling with ``resolve=DEEP`` and ``resolve=SHALLOW`` is valid anywhere.
+* Outside ``vcl_backend_fetch {}`` and ``vcl_init {}``, the `deny`,
+  `real` and `uncacheable_direct` arguments only affect the current
+  return value.
+
+  Also, they cannot be used together with ``resolve=LAZY``.
 
 SEE ALSO
 ========