Hit-for-pass objects are not IMS candidates

Original test case by fgsch

Fixes: #1858

bin/varnishd/cache/cache_fetch.c

@@ -644,9 +644,10 @@ vbf_stp_fetch(struct worker *wrk, struct busyobj *bo)
 	if (bo->do_gzip || bo->do_gunzip)
 		ObjSetFlag(bo->wrk, bo->fetch_objcore, OF_CHGGZIP, 1);
 
-	if (http_IsStatus(bo->beresp, 200) && (
-	    http_GetHdr(bo->beresp, H_Last_Modified, &p) ||
-	    http_GetHdr(bo->beresp, H_ETag, &p)))
+	if (!(bo->fetch_objcore->flags & OC_F_PASS) &&
+	    http_IsStatus(bo->beresp, 200) && (
+	      http_GetHdr(bo->beresp, H_Last_Modified, &p) ||
+	      http_GetHdr(bo->beresp, H_ETag, &p)))
 		ObjSetFlag(bo->wrk, bo->fetch_objcore, OF_IMSCAND, 1);
 
 	if (bo->htc->body_status != BS_NONE)

bin/varnishtest/tests/r01858.vtc

+varnishtest "Test a hit-for-pass does not issue an IMS request"
+
+server s1 {
+	rxreq
+	txresp \
+	    -hdr {Etag: "foo"} \
+	    -body "foo"
+	rxreq
+	expect req.http.if-none-match == <undef>
+	txresp \
+	    -hdr {Etag: "bar"} \
+	    -body "bar"
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_backend_response {
+		if (beresp.status == 200) {
+			set beresp.ttl = 1s;
+			set beresp.uncacheable = true;
+			return (deliver);
+		}
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.body == "foo"
+
+	delay 1.5
+
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.body == "bar"
+} -run
+