Add pkg/gate, to limit concurrency.

Change-Id: I0a75e0caa7bcf16f7b7b93ca7366cfe0c9a4cbd2

pkg/gate/gate.go

@@ -0,0 +1,43 @@
+/*
+Copyright 2013 Google 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 gate provides a mechanism to control a maximum number of
+// operations happening at once.
+package gate
+
+// A Gate limits concurrency.
+type Gate struct {
+	c chan struct{}
+}
+
+// New returns a new gate that will only permit max operations at once.
+func New(max int) *Gate {
+	return &Gate{make(chan struct{}, max)}
+}
+
+// Start starts an operation, blocking until the gate has room.
+func (g *Gate) Start() {
+	g.c <- struct{}{}
+}
+
+// Done finishes an operation.
+func (g *Gate) Done() {
+	select {
+	case <-g.c:
+	default:
+		panic("Done called more than Start")
+	}
+}