fix bereq rollback with retry

When retrying after a rollback, we might have unread bytes of the
backend response body. As we need to finalize the director for rollback,
all we can do in this case is to close the connection.

Spotted by reza, who also provided the test case, thank you.

Closes #3353

bin/varnishd/cache/cache_fetch.c

@@ -106,6 +106,11 @@ void Bereq_Rollback(struct busyobj *bo)
 {
 	CHECK_OBJ_NOTNULL(bo, BUSYOBJ_MAGIC);
 
+	if (bo->htc != NULL &&
+	    bo->htc->body_status != BS_NONE &&
+	    bo->htc->body_status != BS_TAKEN)
+		bo->htc->doclose = SC_RESP_CLOSE;
+
 	vbf_cleanup(bo);
 	VCL_TaskLeave(bo->privs);
 	VCL_TaskEnter(bo->privs);

bin/varnishtest/tests/r03353.vtc

+varnishtest "Test rollback and retry"
+
+server s1 {
+	rxreq
+	txresp -nolen -hdr "Content-Length: 3"
+	expect_close
+	accept
+
+	rxreq
+	txresp -body xxx
+} -start
+
+varnish v1 -vcl+backend {
+	import std;
+
+	sub vcl_backend_response {
+		if (bereq.retries == 0) {
+			std.rollback(bereq);
+		}
+	}
+
+	sub vcl_backend_error {
+		if (bereq.retries == 0) {
+			return (retry);
+		}
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+} -run