vrt_var: Make req.ttl and req.grace unsetable

We continue to use a default value of -1 instead of changing the
default to NAN. These variables can be read from VCL, and VCL does not
want to deal with NAN.

Author: AlveElde

bin/varnishd/cache/cache_vrt_var.c

@@ -561,12 +561,24 @@ VRT_r_req_##nm(VRT_CTX)							\
 	return (ctx->req->elem);					\
 }
 
+#define REQ_VAR_U(nm, elem, val)					\
+									\
+VCL_VOID								\
+VRT_u_req_##nm(VRT_CTX)							\
+{									\
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);				\
+	CHECK_OBJ_NOTNULL(ctx->req, REQ_MAGIC);				\
+	ctx->req->elem = val;						\
+}
+
 REQ_VAR_R(backend_hint, director_hint, VCL_BACKEND)
 
 REQ_VAR_L(ttl, d_ttl, VCL_DURATION, if (!(arg>0.0)) arg = 0;)
 REQ_VAR_R(ttl, d_ttl, VCL_DURATION)
+REQ_VAR_U(ttl, d_ttl, -1)
 REQ_VAR_L(grace, d_grace, VCL_DURATION, if (!(arg>0.0)) arg = 0;)
 REQ_VAR_R(grace, d_grace, VCL_DURATION)
+REQ_VAR_U(grace, d_grace, -1)
 
 VCL_VOID
 VRT_l_req_backend_hint(VRT_CTX, VCL_BACKEND be)

bin/varnishtest/tests/b00084.vtc

@@ -0,0 +1,37 @@
+varnishtest "unset req.ttl and req.grace"
+
+server s1 {
+	rxreq
+	expect req.http.ttl-initial == -1.000
+	expect req.http.grace-initial == -1.000
+	expect req.http.ttl-set == 0.000
+	expect req.http.grace-set == 0.000
+	expect req.http.ttl-unset == -1.000
+	expect req.http.grace-unset == -1.000
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_recv {
+		set req.http.ttl-initial = req.ttl;
+		set req.http.grace-initial = req.grace;
+
+		set req.ttl = 0s;
+		set req.grace = 0s;
+
+		set req.http.ttl-set = req.ttl;
+		set req.http.grace-set = req.grace;
+
+		unset req.ttl;
+		unset req.grace;
+
+		set req.http.ttl-unset = req.ttl;
+		set req.http.grace-unset = req.grace;
+	}
+} -start
+
+client c1 -repeat 2 {
+	txreq
+	rxresp
+	expect resp.status == 200
+} -run

doc/sphinx/reference/vcl_var.rst

@@ -246,11 +246,13 @@ req.grace
 
 	Writable from: client
 
+	Unsetable from: client
 
 	Upper limit on the object grace.
 
 	During lookup the minimum of req.grace and the object's stored
-	grace value will be used as the object's grace.
+	grace value will be used as the object's grace. Setting req.grace
+	to 0s prevents hits on stale objects.
 
 
 .. _req.hash:
@@ -510,8 +512,10 @@ req.ttl
 
 	Writable from: client
 
+	Unsetable from: client
 
 	Upper limit on the object age for cache lookups to return hit.
+	Lookups always miss when req.ttl is set to 0s.
 
 
 .. _req.url: