Add leases manager interface

Add leases manager to the leases package and use the
interface on the client and service.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

leases/id.go

@@ -0,0 +1,43 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package leases
+
+import (
+	"encoding/base64"
+	"fmt"
+	"math/rand"
+	"time"
+)
+
+// WithRandomID sets the lease ID to a random unique value
+func WithRandomID() Opt {
+	return func(l *Lease) error {
+		t := time.Now()
+		var b [3]byte
+		rand.Read(b[:])
+		l.ID = fmt.Sprintf("%d-%s", t.Nanosecond(), base64.URLEncoding.EncodeToString(b[:]))
+		return nil
+	}
+}
+
+// WithID sets the ID for the lease
+func WithID(id string) Opt {
+	return func(l *Lease) error {
+		l.ID = id
+		return nil
+	}
+}