Add a debug facility to force a V1L_Flush after the headers.

bin/varnishd/http1/cache_http1_deliver.c

@@ -150,6 +150,8 @@ V1D_Deliver(struct req *req, struct busyobj *bo)
 	V1L_Reserve(req->wrk, req->ws, &req->sp->fd, req->vsl, req->t_prev);
 
 	req->acct.resp_hdrbytes += HTTP1_Write(req->wrk, req->resp, HTTP1_Resp);
+	if (DO_DEBUG(DBG_FLUSH_HEAD))
+		(void)V1L_Flush(req->wrk);
 
 	ois = OIS_DONE;
 	if (req->wantbody) {

include/tbl/debug_bits.h

@@ -38,3 +38,4 @@ DEBUG_BIT(HASHEDGE,		hashedge,	"Edge cases in Hash")
 DEBUG_BIT(VCLREL,		vclrel,		"Rapid VCL release")
 DEBUG_BIT(LURKER,		lurker,		"VSL Ban lurker")
 DEBUG_BIT(ESI_CHOP,		esi_chop,	"Chop ESI fetch to bits")
+DEBUG_BIT(FLUSH_HEAD,		flush_head,	"Flush after http1 head")