condfetch: Handle failed stale_oc without a boc

The assertion that the stale objcore of a conditional fetch cannot be
failed unless it was streaming is incorrect. Between the moment when
we grab the stale objcore in HSH_Lookup and the moment we try to use
it after vcl_backend_response, the backend fetch may have completed or
failed.

Instead, we need to treat an ongoing fetch and a failed fetch as
separate checks since the latter may happen with or without a boc.

bin/varnishd/cache/cache_fetch.c

@@ -800,14 +800,18 @@ vbf_stp_condfetch(struct worker *wrk, struct busyobj *bo)
 		HSH_DerefBoc(bo->wrk, stale_oc);
 		stale_boc = NULL;
 		if (stale_state != BOS_FINISHED) {
-			(void)VFP_Error(bo->vfc, "Template object failed");
-			vbf_cleanup(bo);
-			wrk->stats->fetch_failed++;
-			return (F_STP_FAIL);
+			assert(stale_state == BOS_FAILED);
+			AN(stale_oc->flags & OC_F_FAILED);
 		}
 	}
+
 	AZ(stale_boc);
-	AZ(stale_oc->flags & OC_F_FAILED);
+	if (stale_oc->flags & OC_F_FAILED) {
+		(void)VFP_Error(bo->vfc, "Template object failed");
+		vbf_cleanup(bo);
+		wrk->stats->fetch_failed++;
+		return (F_STP_FAIL);
+	}
 
 	if (vbf_beresp2obj(bo)) {
 		vbf_cleanup(bo);

bin/varnishtest/tests/c00105.vtc

+varnishtest "Failed post-streaming revalidation"
+
+barrier b1 cond 2
+barrier b2 sock 2
+barrier b3 sock 2
+
+server s1 {
+	rxreq
+	txresp -nolen -hdr {Etag: "abc"} -hdr "Content-Length: 100"
+	barrier b1 sync
+	barrier b2 sync
+} -start
+
+server s2 {
+	rxreq
+	expect req.http.If-None-Match == {"abc"}
+	txresp -status 304 -nolen -hdr {Etag: "abc"} -hdr "Content-Length: 100"
+} -start
+
+varnish v1 -vcl+backend {
+	import directors;
+	import vtc;
+
+	sub vcl_recv {
+		if (req.http.backend == "s2") {
+			set req.backend_hint = s2;
+		}
+	}
+
+	sub vcl_backend_response {
+		if (beresp.was_304) {
+			vtc.barrier_sync("${b2_sock}");
+			vtc.barrier_sync("${b3_sock}");
+		}
+		set beresp.ttl = 1ms;
+	}
+} -start
+
+client c1 {
+	txreq -hdr "backend: s1"
+	rxresphdrs
+	expect resp.status == 200
+	expect_close
+} -start
+
+barrier b1 sync
+
+# ensure stale_oc
+delay 0.01
+
+client c2 {
+	txreq -hdr "backend: s2"
+	rxresphdrs
+	expect resp.status == 200
+	expect_close
+} -start
+
+client c1 -wait
+barrier b3 sync
+client c2 -wait