Add some minimal debugging for fetch/deliver processors for a start

bin/varnishd/cache/cache_deliver_proc.c

@@ -85,6 +85,9 @@ VDP_push(struct req *req, const struct vdp *vdp, void *priv, int bottom)
 	AN(vdp->name);
 	AN(vdp->func);
 
+	if (DO_DEBUG(DBG_PROCESSORS))
+		VSLb(req->vsl, SLT_Debug, "VDP_push(%s)", vdp->name);
+
 	vdpe = WS_Alloc(req->ws, sizeof *vdpe);
 	if (vdpe == NULL)
 		return;

bin/varnishd/cache/cache_fetch_proc.c

@@ -141,6 +141,9 @@ VFP_Open(struct vfp_ctx *vc)
 	VTAILQ_FOREACH_REVERSE(vfe, &vc->vfp, vfp_entry_s, list) {
 		if (vfe->vfp->init == NULL)
 			continue;
+		if (DO_DEBUG(DBG_PROCESSORS))
+			VSLb(vc->wrk->vsl, SLT_Debug, "VFP_Open(%s)",
+			     vfe->vfp->name);
 		vfe->closed = vfe->vfp->init(vc, vfe);
 		if (vfe->closed != VFP_OK && vfe->closed != VFP_NULL) {
 			(void)VFP_Error(vc, "Fetch filter %s failed to open",

include/tbl/debug_bits.h

@@ -48,6 +48,7 @@ DEBUG_BIT(DROP_POOLS,		drop_pools,	"Drop thread pools (testing)")
 DEBUG_BIT(SLOW_ACCEPTOR,	slow_acceptor,	"Slow down Acceptor")
 DEBUG_BIT(H2_NOCHECK,		h2_nocheck,	"Disable various H2 checks")
 DEBUG_BIT(VMOD_SO_KEEP,		vmod_so_keep,	"Keep copied VMOD libraries")
+DEBUG_BIT(PROCESSORS,		processors,	"Fetch/Deliver processors")
 #undef DEBUG_BIT
 
 /*lint -restore */