Don't change the C-L if we detect a failed stream.

Previously we would read the response Content-Length from a failed oc,
which would make the error response valid. Now, if this is detected,
we don't touch the Content-Length.

bin/varnishd/cache/cache_req_fsm.c

@@ -425,7 +425,7 @@ cnt_transmit(struct worker *wrk, struct req *req)
 	status = http_GetStatus(req->resp);
 	head = !strcmp(req->http0->hd[HTTP_HDR_METHOD].b, "HEAD");
 
-	if (boc != NULL)
+	if (boc != NULL || (req->objcore->flags & (OC_F_FAILED)))
 		req->resp_len = clval;
 	else
 		req->resp_len = ObjGetLen(req->wrk, req->objcore);
@@ -477,6 +477,11 @@ cnt_transmit(struct worker *wrk, struct req *req)
 
 	HSH_Cancel(wrk, req->objcore, boc);
 
+	if (!req->doclose && (req->objcore->flags & OC_F_FAILED))
+		/* The object we delivered failed due to a streaming error.
+		 * Fail the request. */
+		req->doclose = SC_TX_ERROR;
+
 	if (boc != NULL)
 		HSH_DerefBoc(wrk, req->objcore);
 

bin/varnishtest/tests/r03560.vtc

+varnishtest "A backend connection error gets returned as a valid short response"
+
+barrier b1 sock 2
+
+server s1 {
+	rxreq
+	txresp -nolen -hdr "Content-Length: 10"
+	barrier b1 sync
+	send "12345"
+	# Early connection close error
+} -start
+
+varnish v1 -vcl+backend {
+	import vtc;
+
+	sub vcl_deliver {
+		# Make sure we are streaming and give the backend time to error out
+		vtc.barrier_sync("${b1_sock}");
+		vtc.sleep(1s);
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresphdrs
+	expect resp.http.Content-Length == "10"
+	recv 5
+	expect_close
+} -run