Interface update, per-method Contexts.

impl/memory/transaction.go

Index: impl/memory/transaction.go
diff --git a/impl/memory/transaction.go b/impl/memory/transaction.go
new file mode 100644
index 0000000000000000000000000000000000000000..5b0f44c24d8e7ad65c682ce706ea85bc266a88b0
--- /dev/null
+++ b/impl/memory/transaction.go
@@ -0,0 +1,44 @@
+// Copyright 2015 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package memory
+
+import (
+	"sync/atomic"
+
+	ds "github.com/luci/gae/service/datastore"
+
+	"github.com/luci/luci-go/common/errors"
+
+	"golang.org/x/net/context"
+)
+
+type transactionImpl struct {
+	// boolean 0 or 1, use atomic.*Int32 to access.
+	closed int32
+	isXG   bool
+}
+
+func (ti *transactionImpl) close() error {
+	if !atomic.CompareAndSwapInt32(&ti.closed, 0, 1) {
+		return errors.New("transaction is already closed")
+	}
+	return nil
+}
+
+func (ti *transactionImpl) valid() error {
+	if atomic.LoadInt32(&ti.closed) == 1 {
+		return errors.New("transaction has expired")
+	}
+	return nil
+}
+
+func assertTxnValid(c context.Context) error {
+	t := ds.CurrentTransaction(c)
+	if t == nil {
+		return nil
+	}
+
+	return t.(*transactionImpl).valid()
+}