Log workspace overflow in activate().

src/tests/workspace.vtc

+varnishtest "log workspace overflow in pesi.activate()"
+
+server s1 {
+	rxreq
+	txresp -body {<esi:include src="/include"/>}
+	# Include bereq never sent due to ws overflow.
+} -start
+
+varnish v1 -vcl+backend {
+	import ${vmod_pesi};
+	import ${vmod_pesi_debug};
+	import vtc;
+	include "debug.inc.vcl";
+
+	sub vcl_backend_response {
+		set beresp.do_esi = true;
+	}
+
+	sub vcl_deliver {
+		# Not enough left over for "pesi"
+		vtc.workspace_alloc(client, -3);
+		pesi.activate();
+	}
+} -start
+
+logexpect l1 -v v1 -g vxid -d 0 -q {Error ~ "^vdp pesi"} {
+        expect 0 * Begin {^req \d+ rxreq$}
+        expect * = Error {^vdp pesi: workspace overflow in activate}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+} -start
+
+logexpect l1 -wait

src/vmod_pesi.c

@@ -180,14 +180,18 @@ pesi_filter_on_ws(struct ws *ws, pesi_filter_list_t *func, void *arg)
 {
 	struct vsb vsb[1];
 	const char *p;
+	struct req *req;
 
 	AN(func);
-	AN(arg);
+	CAST_OBJ_NOTNULL(req, arg, REQ_MAGIC);
 	WS_VSB_new(vsb, ws);
 	func(arg, vsb);
 	p = WS_VSB_finish(vsb, ws, NULL);
-	if (p == NULL)
+	if (p == NULL) {
 		p = "";
+		VSLb(req->vsl, SLT_Error, "vdp pesi: workspace overflow "
+		     "in activate()");
+	}
 	return (p);
 }