counter: Add a helper for counting streamed readers

This makes it easy to get the size and digest from a single streamed
Put, since you need both to construct a valid image-spec Descriptor.

Signed-off-by: W. Trevor King <wking@tremily.us>

counter/counter.go

@@ -0,0 +1,32 @@
+// Copyright 2017 casengine contributors
+//
+// 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 counter defines a byte-counting writer.  One use case is measuring the size of content being streamed into CAS.
+package counter
+
+type Counter struct {
+	count uint64
+}
+
+// Write implements io.Writer for Counter.
+func (c *Counter) Write(p []byte) (n int, err error) {
+	c.count += uint64(len(p))
+	return len(p), nil
+}
+
+// Count returns the number of bytes which have been written to this
+// Counter.
+func (c *Counter) Count() (n uint64) {
+	return c.count
+}

counter/counter_test.go

@@ -0,0 +1,38 @@
+// Copyright 2017 casengine contributors
+//
+// 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 counter defines a byte-counting writer.  One use case is measuring the size of content being streamed into CAS.
+package counter
+
+import (
+	"io"
+	"io/ioutil"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test(t *testing.T) {
+	bodyIn := "Hello, World!"
+	counter := &Counter{}
+	reader := strings.NewReader(bodyIn)
+	countedReader := io.TeeReader(reader, counter)
+	bodyOut, err := ioutil.ReadAll(countedReader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, bodyIn, string(bodyOut))
+	assert.Equal(t, uint64(len(bodyIn)), counter.Count())
+}