fetch: A weak LM header does not make an IMS candidate

Avoid marking an object as an IMS candidate when a weak Last-Modified
header is the only validator. In the case where there is an ETag AND a
weak Last-Modified header, add a If-None-Match header, but do not add
an If-Modified-Since header.

bin/varnishd/cache/cache_fetch.c

@@ -274,7 +274,7 @@ vbf_stp_mkbereq(struct worker *wrk, struct busyobj *bo)
 	    ObjCheckFlag(bo->wrk, bo->stale_oc, OF_IMSCAND) &&
 	    (bo->stale_oc->boc != NULL || ObjGetLen(wrk, bo->stale_oc) != 0)) {
 		AZ(bo->stale_oc->flags & (OC_F_HFM|OC_F_PRIVATE));
-		q = HTTP_GetHdrPack(bo->wrk, bo->stale_oc, H_Last_Modified);
+		q = RFC2616_Strong_LM(NULL, wrk, bo->stale_oc);
 		if (q != NULL)
 			http_PrintfHeader(bo->bereq0,
 			    "If-Modified-Since: %s", q);
@@ -700,7 +700,7 @@ vbf_stp_fetch(struct worker *wrk, struct busyobj *bo)
 
 	if (!(oc->flags & OC_F_HFM) &&
 	    http_IsStatus(bo->beresp, 200) && (
-	      http_GetHdr(bo->beresp, H_Last_Modified, NULL) ||
+	      RFC2616_Strong_LM(bo->beresp, NULL, NULL) != NULL ||
 	      http_GetHdr(bo->beresp, H_ETag, NULL)))
 		ObjSetFlag(bo->wrk, oc, OF_IMSCAND, 1);
 

bin/varnishtest/tests/b00079.vtc

+varnishtest "Test backend IMS with weak and strong LM"
+
+server s1 {
+	rxreq
+	txresp -hdr "Last-Modified: Wed, 11 Sep 2013 13:36:55 GMT" -nodate -body "1"
+
+	# When origin does not send a Date, varnish inserts one, prompting IMS
+	rxreq
+	expect req.http.if-modified-since == "Wed, 11 Sep 2013 13:36:55 GMT"
+	txresp -status 304 -hdr "Last-Modified: Wed, 11 Sep 2013 13:36:55 GMT" \
+		-hdr "Date: Wed, 11 Sep 2013 13:36:55 GMT" \
+
+	# LM was the same as Date
+	rxreq
+	expect req.http.if-modified-since == <undef>
+	txresp -hdr "Last-Modified: Wed, 11 Sep 2013 13:36:55 GMT" \
+		-hdr "Date: Wed, 11 Sep 2013 13:36:56 GMT" \
+		-body "2"
+
+	# LM was one second older than Date
+	rxreq
+	expect req.http.if-modified-since == "Wed, 11 Sep 2013 13:36:55 GMT"
+	txresp -status 304 -hdr "Last-Modified: Wed, 11 Sep 2013 13:36:55 GMT" \
+		-hdr "Date: Wed, 11 Sep 2013 13:36:55 GMT" \
+		-hdr {ETag: "foo"}
+
+	# LM was the same as Date, but we had an ETag, prompting INM
+	rxreq
+	expect req.http.if-modified-since == <undef>
+	expect req.http.if-none-match == {"foo"}
+	txresp -status 304 -hdr "Last-Modified: Wed, 11 Sep 2013 13:36:55 GMT" \
+		-hdr "Date: Wed, 11 Sep 2013 13:36:55 GMT" \
+		-hdr {ETag: "foo"}
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_backend_response {
+		set beresp.ttl = 1ms;
+		set beresp.grace = 0s;
+		set beresp.keep = 1m;
+		set beresp.http.was-304 = beresp.was_304;
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.body == "1"
+	expect resp.http.was-304 == "false"
+
+	delay 0.1
+
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.body == "1"
+	expect resp.http.was-304 == "true"
+
+	delay 0.1
+
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.body == "2"
+	expect resp.http.was-304 == "false"
+
+	delay 0.1
+
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.body == "2"
+	expect resp.http.was-304 == "true"
+
+	delay 0.1
+
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.body == "2"
+	expect resp.http.was-304 == "true"
+} -run