LogDog: Add Butler stream server package.

client/logdog/butlerlib/streamclient/stream.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 streamclient
+
+import (
+        "errors"
+        "io"
+
+        "github.com/luci/luci-go/client/logdog/butlerlib/streamproto"
+        "github.com/luci/luci-go/common/logdog/protocol"
+        "github.com/luci/luci-go/common/recordio"
+)
+
+// Stream is an individual LogDog Butler stream.
+type Stream interface {

[estaab, 2015/11/06 22:09:44]

So the caller doesn't control when opening of the stream happens?

I imagine error handling would be a little harder with all classes of errors
happening at the first call to WriteDatagram(). With an Open() all
initialization errors can be dealt with once. wdyt?

[dnj (Google), 2015/11/07 16:47:42]

Hmm?

The caller creates a new stream via "Client.New", which creates the stream and
performs the handshake.

+        io.WriteCloser
+
+        // WriteDatagram writes a LogDog Butler streaming datagram to the underlying
+        // Writer.
+        WriteDatagram([]byte) error
+}
+
+// streamImpl is the standard implementation of the Stream interface.
+type streamImpl struct {
+        *streamproto.Properties
+        io.WriteCloser
+
+        // rioW is a recordio.Writer bound to the WriteCloser. This will be
+        // initialized on the first writeRecord invocation.
+        rioW recordio.Writer
+}
+
+var _ Stream = (*streamImpl)(nil)

[estaab, 2015/11/06 22:09:44]

What does this do?

[dnj (Google), 2015/11/07 16:47:42]

This is a compile-time assert that the "streamImpl" class implements the Stream
interface.

+
+func (s *streamImpl) WriteDatagram(dg []byte) error {
+        if !s.isDatagramStream() {
+                return errors.New("not a datagram stream")
+        }
+
+        return s.writeRecord(dg)
+}
+
+func (s *streamImpl) Write(data []byte) (int, error) {
+        if s.isDatagramStream() {
+                return 0, errors.New("cannot use Write with datagram stream")
+        }
+
+        return s.writeRaw(data)
+}
+
+func (s *streamImpl) writeRaw(data []byte) (int, error) {
+        return s.WriteCloser.Write(data)
+}
+
+func (s *streamImpl) writeRecord(r []byte) error {
+        if s.rioW == nil {
+                s.rioW = recordio.NewWriter(s.WriteCloser)

[estaab, 2015/11/06 22:09:44]

Can this fail?

[dnj (Google), 2015/11/07 16:47:41]

Nope, this is a data structure initialization, so just fields populated. Usage
of the writer can fail (see below), but creation cannot*.

* Well, short of OOM.

+        }
+        if _, err := s.rioW.Write(r); err != nil {
+                return err
+        }
+        return s.rioW.Flush()
+}
+
+func (s *streamImpl) isDatagramStream() bool {
+        return s.StreamType == protocol.LogStreamDescriptor_DATAGRAM
+}