Don't add another Connection: header if we already have one,

but do insist on it being "close" if we need that.

Test case by daghf

Fixes #1613

bin/varnishd/http1/cache_http1_deliver.c

@@ -142,8 +142,13 @@ V1D_Deliver(struct req *req, struct busyobj *bo)
 
 	VSLb(req->vsl, SLT_Debug, "RES_MODE %x", req->res_mode);
 
-	http_SetHeader(req->resp,
-	    req->doclose ? "Connection: close" : "Connection: keep-alive");
+	if (req->doclose) {
+		if (!http_HdrIs(req->resp, H_Connection, "close")) {
+			http_Unset(req->resp, H_Connection);
+			http_SetHeader(req->resp, "Connection: close");
+		}
+	} else if (!http_GetHdr(req->resp, H_Connection, NULL))
+		http_SetHeader(req->resp, "Connection: keep-alive");
 
 	VDP_push(req, v1d_bytes, NULL, 1);
 

bin/varnishtest/tests/r01613.vtc

+varnishtest "Extra Connection header erroneously inserted."
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v2 -vcl+backend {
+	sub vcl_deliver {
+		set resp.http.Connection = "close";
+	}
+} -start
+
+varnish v1 -vcl {
+	import ${vmod_std};
+
+	backend b {
+		.host = "${v2_addr}";
+		.port = "${v2_port}";
+	}
+
+	sub vcl_backend_response {
+		std.collect(beresp.http.Connection);
+		set beresp.http.foo = beresp.http.Connection;
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.http.foo == "close"
+} -run