LogDog: Add butler protocol reader/write library.

client/logdog/butlerproto/doc.go

Index: client/logdog/butlerproto/doc.go
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+// 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 butlerproto implements the LogDog Butler wire protocol. This protocol
+// wraps Butler messages that are published to Cloud Pub/Sub for LogDog
+// consumption.
+//
+// The protocol begins with a set of header bytes to identify and parameterize
+// the remaining data, followed by the data itself.
+//
+// Note that the Pub/Sub layer is assumed to provide both a total length (so no
+// need to length-prefix) and integrity (so no need to checksum).
+//
+// Header
+//
+// The header is a fixed four bytes which positively identify the message as a
+// Butler Pub/Sub message and describe the remaining data. Variant parameters
+// can use different magic numbers to identify different parameters.
+//
+// Two magic numbers are currently defined:
+//   - 0x10 0x6d 0x06 0x00 (LogDog protocol ButlerLogBundle Raw)
+//   - 0x10 0x6d 0x06 0x62 (LogDog protocol ButlerLogBundle GZip compressed)
+//
+// Data
+//
+// The data component is described by the header, and consists of all data in
+// the Pub/Sub message past the last header byte.
+package butlerproto