Implement a resp.is_streaming status variable

This variable will be true when the response object's body will be
streamed from the backend.

bin/varnishd/cache/cache_vrt_var.c

@@ -658,6 +658,19 @@ VRT_r_obj_uncacheable(VRT_CTX)
 
 /*--------------------------------------------------------------------*/
 
+unsigned
+VRT_r_resp_is_streaming(VRT_CTX)
+{
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	CHECK_OBJ_NOTNULL(ctx->req, REQ_MAGIC);
+	if (ctx->req->objcore == NULL)
+		return (0);	/* When called from vcl_synth */
+	CHECK_OBJ_NOTNULL(ctx->req->objcore, OBJCORE_MAGIC);
+	return (ctx->req->objcore->busyobj != NULL ? 1 : 0);
+}
+
+/*--------------------------------------------------------------------*/
+
 #define HTTP_VAR(x)						\
 struct http *							\
 VRT_r_##x(VRT_CTX)				\

bin/varnishtest/tests/c00069.vtc

+varnishtest "Test resp.is_streaming"
+
+server s1 {
+	rxreq
+	txresp -nolen -hdr "Content-Length: 10"
+	delay 1
+	send "1234567890"
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_recv {
+		if (req.url == "/synth") {
+			return(synth(200, "OK"));
+		}
+	}
+	sub vcl_synth {
+		set resp.http.streaming = resp.is_streaming;
+	}
+	sub vcl_deliver {
+		set resp.http.streaming = resp.is_streaming;
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.http.streaming == "true"
+
+	delay 0.1
+
+	txreq
+	rxresp
+	expect resp.http.streaming == "false"
+
+	txreq -url /synth
+	rxresp
+	expect resp.http.streaming == "false"
+} -run
+

lib/libvcc/generate.py

@@ -670,6 +670,14 @@ sp_variables = [
 		The corresponding HTTP header.
 		"""
 	),
+        ('resp.is_streaming',
+                'BOOL',
+                ( 'deliver', 'synth', ),
+                ( ), """
+                Returns true when the response will be streamed
+                from the backend.
+                """
+        ),
 	('now',
 		'TIME',
 		( 'all',),