Interface update, per-method Contexts.

service/datastore/transaction.go

+// 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.
+type Transaction interface{}

[dnj, 2016/09/01 15:25:40]

ATM this is just an empty interface. At some point in the future we may want to
give it IDs or actual methods (commit, cancel, etc.). ATM it's 100%
implementation specific.

[iannucci, 2016/09/16 01:01:13]

discussed offline, but I think the only thing we'd want to expose would be query
operations like "Active() bool", "AffectedGroups() []*ds.Key", etc. The explicit
commit/cancel/etc. doesn't seem to be needed for anything that we can think of
right now.

[dnj, 2016/09/16 05:44:43]

Added some comment.

+
+// WithTransaction returns a Context with the supplied datastore Transaction
+// instance installed.
+func WithTransaction(c context.Context, t Transaction) context.Context {
+        return Raw(c).WithTransaction(t)
+}
+
+// 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()
+}