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

This is a back port of d7f8b531.

Conflicts:
	bin/varnishd/cache/cache_fetch.c
	doc/changes.rst
	include/tbl/bo_flags.h

bin/varnishd/cache/cache_fetch.c

@@ -998,16 +998,25 @@ VBF_Fetch(struct worker *wrk, struct req *req, struct objcore *oc,
 	AN(oc->flags & OC_F_BUSY);
 	CHECK_OBJ_ORNULL(oldoc, OBJCORE_MAGIC);
 
+	bo = VBO_GetBusyObj(wrk, req);
+	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_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");
+	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");
 	}
 
-	bo = VBO_GetBusyObj(wrk, req);
-	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_MAGIC);
 	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);
 
@@ -1020,9 +1029,6 @@ VBF_Fetch(struct worker *wrk, struct req *req, struct objcore *oc,
 
 	AN(bo->vcl);
 
-	if (mode == VBF_PASS)
-		bo->do_pass = 1;
-
 	bo->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

@@ -2,6 +2,7 @@ varnishtest "1399 race issue with bg-fetches"
 
 server s1 {
 	rxreq
+	expect req.http.Is-bg == "false"
 	txresp -bodylen 1
 
 	sema r1 sync 2
@@ -17,11 +18,15 @@ 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

+======================================
+Varnish Cache 4.1.9-beta1 (unreleased)
+======================================
+
+* Added ``bereq.is_bgfetch`` which is true for background fetches.
+
 ================================
 Varnish Cache 4.1.8 (2017-08-02)
 ================================

include/tbl/bo_flags.h

@@ -40,5 +40,6 @@ BO_FLAG(abandon,	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, "")
 
 /*lint -restore */

lib/libvcc/generate.py

@@ -440,6 +440,13 @@ sp_variables = [
 		backend.  Not available in pipe mode.
 		"""
 	),
+	('bereq.is_bgfetch',
+		'BOOL',
+		('backend', ),
+		(), """
+		True for background fetches.
+		"""
+	),
 	('beresp',
 		'HTTP',
 		( 'backend_response', 'backend_error'),