Expose to VCL whether or not a fetch is a background fetch (bgfetch)

The use case are cluster requests: Intra-cluster bgfetches should
trigger a synchronous fetch on the peer-varnish in order to avoid
additional short-lived / expired objects being created. Or, in
other words, a bgfetch should actually get a fresh object and not
one in grace from another cache.

Merges #2376

bin/varnishd/cache/cache_fetch.c

@@ -1020,20 +1020,28 @@ VBF_Fetch(struct worker *wrk, struct req *req, struct objcore *oc,
 	AN(oc->flags & OC_F_BUSY);
 	CHECK_OBJ_ORNULL(oldoc, OBJCORE_MAGIC);
 
-
-	switch (mode) {
-	case VBF_PASS:		how = "pass"; break;
-	case VBF_NORMAL:	how = "fetch"; break;
-	case VBF_BACKGROUND:	how = "bgfetch"; break;
-	default:		WRONG("Wrong fetch mode");
-	}
-
 	bo = VBO_GetBusyObj(wrk, req);
 	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_MAGIC);
 
 	boc = HSH_RefBoc(oc);
 	CHECK_OBJ_NOTNULL(boc, BOC_MAGIC);
 
+	switch (mode) {
+	case VBF_PASS:
+		how = "pass";
+		bo->do_pass = 1;
+		break;
+	case VBF_NORMAL:
+		how = "fetch";
+		break;
+	case VBF_BACKGROUND:
+		how = "bgfetch";
+		bo->is_bgfetch = 1;
+		break;
+	default:
+		WRONG("Wrong fetch mode");
+	}
+
 	VSLb(bo->vsl, SLT_Begin, "bereq %u %s", VXID(req->vsl->wid), how);
 	VSLb(req->vsl, SLT_Link, "bereq %u %s", VXID(bo->vsl->wid), how);
 
@@ -1044,9 +1052,6 @@ VBF_Fetch(struct worker *wrk, struct req *req, struct objcore *oc,
 
 	AN(bo->vcl);
 
-	if (mode == VBF_PASS)
-		bo->do_pass = 1;
-
 	oc->boc->vary = req->vary_b;
 	req->vary_b = NULL;
 

bin/varnishd/cache/cache_vrt_var.c

@@ -202,6 +202,14 @@ VRT_r_beresp_##field(VRT_CTX)						\
 
 /*--------------------------------------------------------------------*/
 
+unsigned
+VRT_r_bereq_is_bgfetch(VRT_CTX)
+{
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	CHECK_OBJ_NOTNULL(ctx->bo, BUSYOBJ_MAGIC);
+	return (ctx->bo->is_bgfetch);
+}
+
 unsigned
 VRT_r_bereq_uncacheable(VRT_CTX)
 {

bin/varnishtest/tests/r01399.vtc

@@ -4,6 +4,7 @@ barrier b1 cond 2
 
 server s1 {
 	rxreq
+	expect req.http.Is-bg == "false"
 	txresp -bodylen 1
 
 	barrier b1 sync
@@ -19,11 +20,15 @@ server s1 {
 	# And see if it has all its marbles still...
 	rxreq
 	expect req.url == "/"
+	expect req.http.Is-bg == "true"
 	txresp -bodylen 2
 
 } -start
 
 varnish v1 -vcl+backend {
+	sub vcl_backend_fetch {
+		set bereq.http.Is-bg = bereq.is_bgfetch;
+	}
 	sub vcl_backend_response {
 		set beresp.do_stream = false;
 		set beresp.ttl = 2s;

doc/changes.rst

@@ -19,6 +19,8 @@ Varnish Cache Trunk (ongoing)
   ``MAIN.client_req``. VSM consumers should be changed to use the
   latter if necessary.
 
+* Added ``bereq.is_bgfetch`` which is true for background fetches.
+
 ================================
 Varnish Cache 5.1.2 (2017-04-07)
 ================================

include/tbl/bo_flags.h

@@ -39,6 +39,7 @@ BO_FLAG(uncacheable,	0, 0, "")
 BO_FLAG(is_gzip,	0, 0, "")
 BO_FLAG(is_gunzip,	0, 0, "")
 BO_FLAG(was_304,	1, 0, "")
+BO_FLAG(is_bgfetch,	0, 0, "")
 #undef BO_FLAG
 
 /*lint -restore */

lib/libvcc/generate.py

@@ -478,6 +478,13 @@ sp_variables = [
 		backend.  Not available in pipe mode.
 		"""
 	),
+	('bereq.is_bgfetch',
+		'BOOL',
+		('backend', ),
+		(), """
+		True for background fetches.
+		"""
+	),
 	('beresp',
 		'HTTP',
 		('backend_response', 'backend_error'),