[master] 1df203a75 When streaming, send headers if no body data available yet

[Nils Goroll]

commit 1df203a7574b4c6f6853f94c1e80da70a7af7849
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Mon Mar 1 19:09:29 2021 +0100

    When streaming, send headers if no body data available yet
    
    This is a TTFB optimization: I noticed that, when streaming, we would
    only sometimes send headers while waiting for more body data from the
    backend, depending on the amount of body data read ahead when reading
    the backend headers.
    
    The flush_head debug bit would avoid the delay, but it leads to an extra
    flush in all cases, even when we could send some body data down the line
    with the headers.
    
    This trivial patch peeks into the busy object to determine if the
    first ObjWaitExtend() is likely to block and, if so, issues a flush,
    which will cause the headers to be sent in addition to any initial data
    VDPs might have to send.

diff --git a/bin/varnishd/storage/storage_simple.c b/bin/varnishd/storage/storage_simple.c
index 314da53fe..62d03f638 100644
--- a/bin/varnishd/storage/storage_simple.c
+++ b/bin/varnishd/storage/storage_simple.c
@@ -280,6 +280,8 @@ sml_iterator(struct worker *wrk, struct objcore *oc,
 	l = 0;
 
 	u = 0;
+	if (boc->len_so_far == 0)
+		func(priv, OBJ_ITER_FLUSH, NULL, 0);
 	while (1) {
 		ol = len;
 		nl = ObjWaitExtend(wrk, oc, ol);