Fail if there is no body

src/Makefile.am

@@ -41,6 +41,7 @@ AM_VTC_LOG_FLAGS = \
 TESTS = \
 	zfr_iter_test \
 	vtc/vmod_zipflow.vtc \
+	vtc/zero.vtc \
 	vtc/empty.vtc \
 	vtc/coverage.vtc \
 	vtc/sub.vtc \

src/vmod_zipflow.c

@@ -616,6 +616,11 @@ vdp_zipflow_init(VRT_CTX, struct vdp_ctx *vdc, void **priv, struct objcore *oc)
 		return (1);
 	}
 
+	if (vdc->req->resp_len == 0) {
+		VSLb(vdc->vsl, SLT_Error, "zipflow: need body bytes");
+		return (-1);
+	}
+
 	if (zft->body_resp) {
 		zfr->bundle = 0;
 		WS_TOP_ALLOC_OBJ(ctx, zft->zis, ZFR_ITER_MAGIC);

src/vmod_zipflow.vcc

@@ -112,6 +112,11 @@ Example: Read URLs to bundle into the ZIP from the request body
 VCL INTERFACE REFERENCE
 =======================
 
+**NOTE** Varnish-Cache does not support content generated by Varnish
+Delivery Processors if there is not response body to be sent. This
+means that, on the top level, a response body must always be present,
+as illustrated by ``set resp.body = " ";`` in the examples above.
+
 $Function VOID subreq(STRING url, STRING host=0)
 
 $Restrict client

src/vtc/zero.vtc

+varnishtest "test vmod-zipflow: zero response cant work"
+
+varnish v1 -vcl {
+	import zipflow;
+
+	backend proforma none;
+
+	sub vcl_recv {
+		return (synth(200));
+	}
+
+	sub vcl_synth {
+		if (zipflow.is_subreq()) {
+			synthetic("file content");
+		} else {
+			set resp.filters += " zipflow";
+			zipflow.subreq("/file");
+			synthetic("");
+		}
+		return (deliver);
+	}
+} -start
+
+client c1 {
+	txreq
+	expect_close
+} -run