add NilJournal; fix call sites to ChainStore constructor.

journal/nil.go

@@ -0,0 +1,16 @@
+package journal
+
+type nilJournal struct{}
+
+// nilj is a singleton nil journal.
+var nilj Journal = &nilJournal{}
+
+func NilJournal() Journal {
+	return nilj
+}
+
+func (n *nilJournal) RegisterEventType(_, _ string) EventType { return EventType{} }
+
+func (n *nilJournal) AddEntry(_ EventType, _ interface{}) {}
+
+func (n *nilJournal) Close() error { return nil }

journal/sugar.go

@@ -3,7 +3,13 @@ package journal
 // MaybeAddEntry is a convenience function that evaluates if the EventType is
 // enabled, and if so, it calls the supplier to create the entry and
 // subsequently journal.AddEntry on the provided journal to record it.
+//
+// This is safe to call with a nil Journal, either because the value is nil,
+// or because a journal obtained through NilJournal() is in use.
 func MaybeAddEntry(journal Journal, evtType EventType, supplier func() interface{}) {
+	if journal == nil || journal == nilj {
+		return
+	}
 	if !evtType.Enabled() {
 		return
 	}