Interface update, per-method Contexts.

service/datastore/transaction.go

Index: service/datastore/transaction.go
diff --git a/service/datastore/transaction.go b/service/datastore/transaction.go
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+++ b/service/datastore/transaction.go
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+// 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 datastore
+
+import (
+	"golang.org/x/net/context"
+)
+
+// Transaction is a generic interface used to describe a Datastore transaction.
+//
+// The nil Transaction represents no transaction context.
+//
+// TODO: Add some functionality here. Ideas include:
+//	- Active() bool: is the transaction currently active?
+//	- AffectedGroups() []*ds.Key: list the groups that have been referenced in
+//	  this Transaction so far.
+type Transaction interface{}
+
+// WithoutTransaction returns a Context that isn't bound to a transaction.
+//
+// This can be useful to perform non-transactional tasks given only a Context
+// that is bound to a transaction.
+func WithoutTransaction(c context.Context) context.Context {
+	return Raw(c).WithoutTransaction()
+}
+
+// CurrentTransaction returns a reference to the current Transaction, or nil
+// if the Context does not have a current Transaction.
+func CurrentTransaction(c context.Context) Transaction {
+	return Raw(c).CurrentTransaction()
+}