vcl: Introduce new obj_stale variable

obj_stale variable gives access to the stale object we had in cache
when doing a 304 revalidation. It is only readable from vcl_backend_refresh
subroutine.

bin/varnishd/cache/cache_vrt.c

@@ -257,6 +257,8 @@ VRT_selecthttp(VRT_CTX, enum gethdr_e where)
 	case HDR_RESP:
 		hp = ctx->http_resp;
 		break;
+	case HDR_OBJ_STALE:
+		/* FALLTHROUGH */
 	case HDR_OBJ:
 		hp = NULL;
 		break;
@@ -281,6 +283,13 @@ VRT_GetHdr(VRT_CTX, VCL_HEADER hs)
 		return (HTTP_GetHdrPack(ctx->req->wrk, ctx->req->objcore,
 		    hs->what));
 	}
+
+	if (hs->where == HDR_OBJ_STALE) {
+		CHECK_OBJ_NOTNULL(ctx->bo, BUSYOBJ_MAGIC);
+		CHECK_OBJ_NOTNULL(ctx->bo->stale_oc, OBJCORE_MAGIC);
+		return (HTTP_GetHdrPack(ctx->bo->wrk, ctx->bo->stale_oc,
+		    hs->what));
+	}
 	hp = VRT_selecthttp(ctx, hs->where);
 	CHECK_OBJ_NOTNULL(hp, HTTP_MAGIC);
 	if (!http_GetHdr(hp, hs->what, &p))

bin/varnishd/cache/cache_vrt_var.c

@@ -546,6 +546,7 @@ VRT_r_##which##_reason(VRT_CTX)				\
 }
 
 VRT_OC_VAR_R(obj, req, REQ_MAGIC, objcore);
+VRT_OC_VAR_R(obj_stale, bo, BUSYOBJ_MAGIC, stale_oc);
 
 /*--------------------------------------------------------------------*/
 
@@ -774,9 +775,13 @@ VRT_r_##which##_##fld(VRT_CTX)					\
 
 /*lint -save -e835 */	// Zero right hand arg to '-'
 
+VRT_DO_EXP_R(obj_stale, ctx->bo->stale_oc, ttl,
+    ttl_now(ctx) - ctx->bo->stale_oc->t_origin)
 VRT_DO_EXP_R(obj, ctx->req->objcore, ttl,
     ttl_now(ctx) - ctx->req->objcore->t_origin)
+VRT_DO_EXP_R(obj_stale, ctx->bo->stale_oc, grace, 0)
 VRT_DO_EXP_R(obj, ctx->req->objcore, grace, 0)
+VRT_DO_EXP_R(obj_stale, ctx->bo->stale_oc, keep, 0)
 VRT_DO_EXP_R(obj, ctx->req->objcore, keep, 0)
 VRT_DO_EXP_L(beresp, ctx->bo->fetch_objcore, ttl,
     ttl_now(ctx) - ctx->bo->fetch_objcore->t_origin)
@@ -808,6 +813,7 @@ VRT_DO_TIME_R(resp, req, t_resp)
 VRT_DO_TIME_R(bereq, bo, t_first)
 VRT_DO_TIME_R(beresp, bo, t_resp)
 VRT_DO_TIME_R(obj, req->objcore, t_origin)
+VRT_DO_TIME_R(obj_stale, bo->stale_oc, t_origin)
 
 /*--------------------------------------------------------------------
  */
@@ -822,6 +828,7 @@ VRT_r_##which##_##age(VRT_CTX)					\
 	return (ttl_now(ctx) - oc->t_origin);			\
 }
 
+VRT_DO_AGE_R(obj_stale, ctx->bo->stale_oc)
 VRT_DO_AGE_R(obj, ctx->req->objcore)
 VRT_DO_AGE_R(beresp, ctx->bo->fetch_objcore)
 
@@ -985,6 +992,18 @@ VRT_r_obj_hits(VRT_CTX)
 
 /*--------------------------------------------------------------------*/
 
+VCL_INT
+VRT_r_obj_stale_hits(VRT_CTX)
+{
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	CHECK_OBJ_NOTNULL(ctx->bo, BUSYOBJ_MAGIC);
+	CHECK_OBJ_NOTNULL(ctx->bo->stale_oc, OBJCORE_MAGIC);
+
+	return (ctx->bo->stale_oc->hits);
+}
+
+/*--------------------------------------------------------------------*/
+
 VCL_BOOL
 VRT_r_resp_is_streaming(VRT_CTX)
 {

bin/varnishtest/tests/b00083.vtc

@@ -0,0 +1,77 @@
+varnishtest "Test obj_stale vcl variables"
+
+server s1 {
+	rxreq
+	txresp -hdr "Etag: abcd" -hdr "from-bo: true" -bodylen 10
+	rxreq
+	expect req.http.if-none-match == "abcd"
+	txresp -status 304
+} -start
+
+varnish v1 -vcl+backend {
+
+	sub vcl_backend_response {
+		set beresp.http.vbresp = "true";
+		set beresp.ttl = 0.01s;
+		set beresp.grace = 0s;
+		set beresp.keep = 10m;
+		set beresp.http.was-304 = beresp.was_304;
+	}
+
+	sub vcl_backend_refresh {
+		set beresp.http.vbref = "true";
+
+		set beresp.http.http = obj_stale.http.from-bo;
+		set beresp.http.age = obj_stale.age;
+		set beresp.http.can_esi = obj_stale.can_esi;
+		set beresp.http.grace = obj_stale.grace;
+		set beresp.http.hits = obj_stale.hits;
+		set beresp.http.keep = obj_stale.keep;
+		set beresp.http.proto = obj_stale.proto;
+		set beresp.http.reason = obj_stale.reason;
+		set beresp.http.status = obj_stale.status;
+		set beresp.http.storage = obj_stale.storage;
+		set beresp.http.time = obj_stale.time;
+		set beresp.http.ttl = obj_stale.ttl;
+		set beresp.http.uncacheable = obj_stale.uncacheable;
+
+		return (merge);
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+
+	expect resp.http.was-304 == false
+	expect resp.http.vbref == <undef>
+	expect resp.http.vbresp == true
+	expect resp.http.from-bo == true
+} -run
+
+delay 0.01
+
+client c2 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.http.was-304 == true
+	expect resp.http.vbresp == true
+	expect resp.http.vbref == true
+	expect resp.http.from-bo == true
+
+	expect resp.http.http == true
+	expect resp.http.age == 0
+	expect resp.http.can_esi == false
+	expect resp.http.grace == 0.000
+	expect resp.http.hits == 0
+	expect resp.http.keep == 600.000
+	expect resp.http.proto == HTTP/1.1
+	expect resp.http.reason == OK
+	expect resp.http.status == 200
+	expect resp.http.storage == storage.s0
+	expect resp.http.time != <undef>
+	expect resp.http.ttl != <undef>
+	expect resp.http.uncacheable == false
+} -run

doc/sphinx/reference/vcl_var.rst

@@ -1358,6 +1358,167 @@ beresp.was_304
 	to our conditional fetch from the backend and turned
 	that into ``beresp.status = 200``
 
+obj_stale
+---------
+
+This is the stale object we had in cache. It cannot be modified.
+
+.. _obj_stale.age:
+
+obj_stale.age
+
+	Type: DURATION
+
+	Readable from: vcl_backend_refresh
+
+	The age of the stale object.
+
+
+.. _obj_stale.can_esi:
+
+obj_stale.can_esi
+
+	Type: BOOL
+
+	Readable from: vcl_backend_refresh
+
+	If the stale object can be ESI processed, that is if setting
+	``resp.do_esi`` or adding ``esi`` to ``resp.filters`` in
+	``vcl_deliver {}`` would cause the response body to be ESI
+	processed.
+
+
+.. _obj_stale.grace:
+
+obj_stale.grace
+
+	Type: DURATION
+
+	Readable from: vcl_backend_refresh
+
+	The stale object's grace period in seconds.
+
+
+.. _obj_stale.hits:
+
+obj_stale.hits
+
+	Type: INT
+
+	Readable from: vcl_backend_refresh
+
+	The count of cache-hits on this stale object.
+
+	In `vcl_deliver` a value of 0 indicates a cache miss.
+
+
+.. _obj_stale.http:
+
+obj_stale.http.*
+
+	Type: HEADER
+
+	Readable from: vcl_backend_refresh
+
+	The HTTP headers stored in the stale object.
+
+	See req.http_ for general notes.
+
+
+.. _obj_stale.keep:
+
+obj_stale.keep
+
+	Type: DURATION
+
+	Readable from: vcl_backend_refresh
+
+	The stale object's keep period in seconds.
+
+
+.. _obj_stale.proto:
+
+obj_stale.proto
+
+	Type: STRING
+
+	Readable from: vcl_backend_refresh
+
+	The HTTP protocol version stored in the stale object.
+
+
+.. _obj_stale.reason:
+
+obj_stale.reason
+
+	Type: STRING
+
+	Readable from: vcl_backend_refresh
+
+
+	The HTTP reason phrase stored in the stale object.
+
+
+.. _obj_stale.status:
+
+obj_stale.status
+
+	Type: INT
+
+	Readable from: vcl_backend_refresh
+
+
+	The HTTP status code stored in the stale object.
+
+	More information in the `HTTP response status`_ section.
+
+
+.. _obj_stale.storage:
+
+obj_stale.storage
+
+	Type: STEVEDORE
+
+	Readable from: vcl_backend_refresh
+
+	The storage backend where this stale object is stored.
+
+
+.. _obj_stale.time:
+
+obj_stale.time
+
+	Type: TIME
+
+	Readable from: vcl_backend_refresh
+
+	The time the stale object was created from the perspective of the
+	server which generated it. This will roughly be equivalent to
+	``now`` - ``obj.age``.
+
+
+.. _obj_stale.ttl:
+
+obj_stale.ttl
+
+	Type: DURATION
+
+	Readable from: vcl_backend_refresh
+
+	The stale object's time to live, in seconds.
+
+
+.. _obj_stale.uncacheable:
+
+obj_stale.uncacheable
+
+	Type: BOOL
+
+	Readable from: vcl_backend_refresh
+
+	Whether the stale object is uncacheable (pass, hit-for-pass or
+	hit-for-miss).
+
 
 obj
 ---

include/vrt.h

@@ -58,6 +58,19 @@
  * binary/load-time compatible, increment MAJOR version
  *
  * NEXT (2024-03-15)
+ * 	VRT_r_obj_stale_age() added
+ *	VRT_r_obj_stale_can_esi() added
+ *	VRT_r_obj_stale_grace() added
+ *	VRT_r_obj_stale_hits() added
+ *	VRT_r_obj_stale_keep() added
+ *	VRT_r_obj_stale_proto() added
+ *	VRT_r_obj_stale_reason() added
+ *	VRT_r_obj_stale_status() added
+ *	VRT_r_obj_stale_storage() added
+ *	VRT_r_obj_stale_time() added
+ *	VRT_r_obj_stale_ttl() added
+ *	VRT_r_obj_stale_uncacheable() added
+ *	enum gethdr_e has new value HDR_OBJ_STALE
  * 18.0 (2023-09-15)
  *	[cache_filter.h] struct vdp gained priv1 member
  *	VRT_trace() added
@@ -641,7 +654,8 @@ enum gethdr_e {
 	HDR_RESP,
 	HDR_OBJ,
 	HDR_BEREQ,
-	HDR_BERESP
+	HDR_BERESP,
+	HDR_OBJ_STALE
 };
 
 struct gethdr_s {