impl/memory: Implement Queries

impl/memory/taskqueue_data.go

Index: impl/memory/taskqueue_data.go
diff --git a/impl/memory/taskqueue_data.go b/impl/memory/taskqueue_data.go
index 4bc24a1a7436a074e6b3071393818ef83146b426..25d1db682b45d579dd282d3fdbe3d88fd676bdc0 100644
--- a/impl/memory/taskqueue_data.go
+++ b/impl/memory/taskqueue_data.go
@@ -196,9 +196,14 @@ var _ interface {
 	tq.Testable
 } = (*txnTaskQueueData)(nil)
 
-func (t *txnTaskQueueData) canApplyTxn(obj memContextObj) bool         { return false }
-func (t *txnTaskQueueData) applyTxn(context.Context, memContextObj)    { panic("impossible") }
-func (t *txnTaskQueueData) mkTxn(*ds.TransactionOptions) memContextObj { panic("impossible") }
+func (t *txnTaskQueueData) canApplyTxn(obj memContextObj) bool { return false }
+func (t *txnTaskQueueData) applyTxn(context.Context, memContextObj) {
+	impossible(fmt.Errorf("cannot apply nested transaction"))
+}
+func (t *txnTaskQueueData) mkTxn(*ds.TransactionOptions) memContextObj {
+	impossible(fmt.Errorf("cannot start nested transaction"))
+	return nil
+}
 
 func (t *txnTaskQueueData) endTxn() {
 	if atomic.LoadInt32(&t.closed) == 1 {