.

go/zodb/storage/demo/demo.go

@@ -55,15 +55,25 @@ type Storage struct {
 // baseMutatedError is reported when Storage.base is detected to change.
 type baseMutatedError struct {
 	baseAt0  zodb.Tid
-//	baseHead zodb.Tid
-	baseEvent zodb.Event
+	baseHead zodb.Tid
 }
 
 func (e *baseMutatedError) Error() string {
-//	return fmt.Sprintf("base.head mutated from @%s to @%s", e.baseAt0, e.baseHead)
-	return fmt.Sprintf("base mutated from @%s: %s", e.baseAt0, e.baseEvent)
+	return fmt.Sprintf("base.head mutated from @%s to @%s", e.baseAt0, e.baseHead)
 }
 
+// baseError is reported on error detected on Storage.base .
+type baseError struct {
+	err error
+}
+
+func (e *baseError) Error() string {
+	return fmt.Sprintf("base: error: %s", e.err)
+}
+
+func (e *baseError) Cause() error  { return e.err }
+func (e *baseError) Unwrap() error { return e.err }
+
 // watcher detects base mutation and proxies δ events to user.
 // it runs as separate goroutine.
 func (d *Storage) watcher(ctx context.Context) error {
@@ -85,7 +95,18 @@ func (d *Storage) watcher(ctx context.Context) error {
 				continue
 			}
 
-			edown := &baseMutatedError{d.baseAt0, event}
+			var edown error
+			switch event := event.(type) {
+			case *zodb.EventCommit:
+				edown = &baseMutatedError{d.baseAt0, event.Tid}
+
+			case *zodb.EventError:
+				edown = &baseError{event.Err}
+
+			default:
+				edown = fmt.Errorf("base: unexpected event %T", event)
+			}
+
 			ev := &zodb.EventError{edown} // XXX + context
 			if d.watchq != nil {
 				d.watchq <- ev
@@ -101,6 +122,7 @@ func (d *Storage) watcher(ctx context.Context) error {
 				continue
 			}
 
+			// XXX +select on close
 			d.watchq <- event // !nil becase d.δWatchq != nil
 		}
 	}
@@ -143,7 +165,7 @@ func (d *Storage) Sync(ctx context.Context) (_ zodb.Tid, err error) {
 			return err
 		}
 		if baseHead != d.baseAt0 {
-			return &baseMutatedError{d.baseAt0, fmt.Errorf("to @%s", baseHead)}
+			return &baseMutatedError{d.baseAt0, baseHead}
 		}
 		return nil
 	})