datastorecache: Add Locker field, public settings.

appengine/datastorecache/lock.go

Index: appengine/datastorecache/lock.go
diff --git a/appengine/datastorecache/lock.go b/appengine/datastorecache/lock.go
new file mode 100644
index 0000000000000000000000000000000000000000..c816e47c8c79045f8455bdb82774c980b2b2782e
--- /dev/null
+++ b/appengine/datastorecache/lock.go
@@ -0,0 +1,62 @@
+// Copyright 2016 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 datastorecache
+
+import (
+	"strings"
+
+	"github.com/luci/luci-go/appengine/memlock"
+
+	"github.com/luci/gae/service/info"
+
+	"golang.org/x/net/context"
+)
+
+// ErrFailedToLock is a sentinel error returned by Locker.TryWithLock if the
+// lock is already held by another entity.
+var ErrFailedToLock = memlock.ErrFailedToLock
+
+// Locker is an interface to a generic locking function.
+type Locker interface {
+	// TryWithLock blocks on acquiring a lock for the specified key, invokes the
+	// supplied function while holding the lock, and releases the lock before
+	// returning.
+	//
+	// If the lock is already held, TryWithLock should return ErrFailedToLock.
+	// Otherwise, TryWithLock will forward the return value of fn.
+	TryWithLock(c context.Context, key string, fn func(context.Context) error) error
+}
+
+// memLocker is a Locker implementation that uses the memlock library.
+type memLocker struct {
+	clientID string
+}
+
+// MemLocker returns a Locker instance that uses a memcache lock bound to the
+// current request ID.
+func MemLocker(c context.Context) Locker {
+	return &memLocker{
+		clientID: strings.Join([]string{
+			"datastore_cache",
+			info.RequestID(c),
+		}, "\x00"),
+	}
+}
+
+func (m *memLocker) TryWithLock(c context.Context, key string, fn func(context.Context) error) error {
+	switch err := memlock.TryWithLock(c, key, m.clientID, fn); err {
+	case memlock.ErrFailedToLock:
+		return ErrFailedToLock
+	default:
+		return err
+	}
+}
+
+// nopLocker is a Locker instance that performs no actual locking.
+type nopLocker struct{}
+
+func (nopLocker) TryWithLock(c context.Context, key string, fn func(context.Context) error) error {
+	return fn(c)
+}