storage: add watch ID to identify watchings

One watcher includes multiple watchings, and their events are
sent out through one channel. For the received event, user would like to
know which watching it belongs to.

Introduce a watch ID. When watching on some key, user will get a watch
ID. The watch ID is attached to all events that is observed by this
watch.

storage/watcher_test.go

@@ -0,0 +1,64 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// 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 storage
+
+import "testing"
+
+// TestWatcherWatchID tests that each watcher provides unique watch ID,
+// and the watched event attaches the correct watch ID.
+func TestWatcherWatchID(t *testing.T) {
+	s := WatchableKV(newWatchableStore(tmpPath))
+	defer cleanup(s, tmpPath)
+
+	w := s.NewWatcher()
+	defer w.Close()
+
+	idm := make(map[int64]struct{})
+
+	// synced watchings
+	for i := 0; i < 10; i++ {
+		id, cancel := w.Watch([]byte("foo"), false, 0)
+		if _, ok := idm[id]; ok {
+			t.Errorf("#%d: id %d exists", i, id)
+		}
+		idm[id] = struct{}{}
+
+		s.Put([]byte("foo"), []byte("bar"))
+
+		ev := <-w.Chan()
+		if ev.WatchID != id {
+			t.Errorf("#%d: watch id in event = %d, want %d", i, ev.WatchID, id)
+		}
+
+		cancel()
+	}
+
+	s.Put([]byte("foo2"), []byte("bar"))
+	// unsynced watchings
+	for i := 10; i < 20; i++ {
+		id, cancel := w.Watch([]byte("foo2"), false, 1)
+		if _, ok := idm[id]; ok {
+			t.Errorf("#%d: id %d exists", i, id)
+		}
+		idm[id] = struct{}{}
+
+		ev := <-w.Chan()
+		if ev.WatchID != id {
+			t.Errorf("#%d: watch id in event = %d, want %d", i, ev.WatchID, id)
+		}
+
+		cancel()
+	}
+}