logdog: Add frame read/write library.

common/logdog/frame/writer.go

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package frame
+
+import (
+        "bytes"
+        "encoding/binary"
+        "io"
+)
+
+// WriteFrame writes a single frame to an io.Writer.
+func WriteFrame(w io.Writer, frame []byte) (int, error) {
+        var sizeBuf [binary.MaxVarintLen64]byte
+        sizeBytes := binary.PutUvarint(sizeBuf[:], uint64(len(frame)))
+
+        count, err := w.Write(sizeBuf[:sizeBytes])
+        if err != nil {
+                return count, err
+        }
+
+        amount, err := w.Write(frame)
+        count += amount
+        if err != nil {
+                return count, err
+        }
+
+        return count, nil
+}
+
+// Writer implements the io.Writer interface. Data written to the Writer is
+// translated into a series of frames. Each frame is spearated by a call to
+// Flush.
+//
+// Frame boundaries are created by calling Flush. Flush will always write a
+// frame, even if the frame's data size is zero.
+//
+// Data written over consecutive Write calls belongs to the same frame. It is
+// buffered until a frame boundary is created via Flush().
+type Writer interface {
+        io.Writer
+
+        // Flush writes the buffered frame
+        Flush() error
+}
+
+// writer implements the Writer interface by wrapping an io.Writer.
+type writer struct {
+        inner io.Writer
+        buf   bytes.Buffer
+}
+
+// NewWriter creates a new Writer instance that data as frames to an underlying
+// io.Writer.
+func NewWriter(w io.Writer) Writer {
+        return &writer{
+                inner: w,
+        }
+}
+
+func (w *writer) Write(data []byte) (int, error) {
+        return w.buf.Write(data)
+}
+
+func (w *writer) Flush() error {
+        _, err := WriteFrame(w.inner, w.buf.Bytes())
+        if err != nil {
+                return err
+        }
+
+        w.buf.Reset()
+        return nil
+}