Ring buffer for notifications

Results from BenchmarkListener:
Current code:
1000000	      1540 ns/op	     109 B/op	       1 allocs/op

New:
1000000	      1139 ns/op	      16 B/op	       1 allocs/op

Kubernetes-commit: 1940f5a5d2acce377ab16b5561f3d9002fb2a319

util/buffer/BUILD

@@ -0,0 +1,28 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["ring_growing.go"],
+    visibility = ["//visibility:public"],
+)
+
+go_test(
+    name = "go_default_test",
+    srcs = ["ring_growing_test.go"],
+    library = ":go_default_library",
+    deps = ["//vendor/github.com/stretchr/testify/assert:go_default_library"],
+)
+
+filegroup(
+    name = "package-srcs",
+    srcs = glob(["**"]),
+    tags = ["automanaged"],
+    visibility = ["//visibility:private"],
+)
+
+filegroup(
+    name = "all-srcs",
+    srcs = [":package-srcs"],
+    tags = ["automanaged"],
+    visibility = ["//visibility:public"],
+)

util/buffer/ring_growing.go

@@ -0,0 +1,72 @@
+/*
+Copyright 2017 The Kubernetes 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 buffer
+
+// RingGrowing is a growing ring buffer.
+// Not thread safe.
+type RingGrowing struct {
+	data     []interface{}
+	n        int // Size of Data
+	beg      int // First available element
+	readable int // Number of data items available
+}
+
+// NewRingGrowing constructs a new RingGrowing instance with provided parameters.
+func NewRingGrowing(initialSize int) *RingGrowing {
+	return &RingGrowing{
+		data: make([]interface{}, initialSize),
+		n:    initialSize,
+	}
+}
+
+// ReadOne reads (consumes) first item from the buffer if it is available, otherwise returns false.
+func (r *RingGrowing) ReadOne() (data interface{}, ok bool) {
+	if r.readable == 0 {
+		return nil, false
+	}
+	r.readable--
+	element := r.data[r.beg]
+	r.data[r.beg] = nil // Remove reference to the object to help GC
+	if r.beg == r.n-1 {
+		// Was the last element
+		r.beg = 0
+	} else {
+		r.beg++
+	}
+	return element, true
+}
+
+// WriteOne adds an item to the end of the buffer, growing it if it is full.
+func (r *RingGrowing) WriteOne(data interface{}) {
+	if r.readable == r.n {
+		// Time to grow
+		newN := r.n * 2
+		newData := make([]interface{}, newN)
+		to := r.beg + r.readable
+		if to <= r.n {
+			copy(newData, r.data[r.beg:to])
+		} else {
+			copied := copy(newData, r.data[r.beg:])
+			copy(newData[copied:], r.data[:(to%r.n)])
+		}
+		r.beg = 0
+		r.data = newData
+		r.n = newN
+	}
+	r.data[(r.readable+r.beg)%r.n] = data
+	r.readable++
+}

util/buffer/ring_growing_test.go

@@ -0,0 +1,50 @@
+/*
+Copyright 2017 The Kubernetes 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 buffer
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGrowth(t *testing.T) {
+	t.Parallel()
+	x := 10
+	g := NewRingGrowing(1)
+	for i := 0; i < x; i++ {
+		assert.Equal(t, i, g.readable)
+		g.WriteOne(i)
+	}
+	read := 0
+	for g.readable > 0 {
+		v, ok := g.ReadOne()
+		assert.True(t, ok)
+		assert.Equal(t, read, v)
+		read++
+	}
+	assert.Equalf(t, x, read, "expected to have read %d items: %d", x, read)
+	assert.Zerof(t, g.readable, "expected readable to be zero: %d", g.readable)
+	assert.Equalf(t, g.n, 16, "expected N to be 16: %d", g.n)
+}
+
+func TestEmpty(t *testing.T) {
+	t.Parallel()
+	g := NewRingGrowing(1)
+	_, ok := g.ReadOne()
+	assert.False(t, ok)
+}