LogDog: Add Butler stream server package.

client/logdog/butlerlib/streamclient/client.go

Index: client/logdog/butlerlib/streamclient/client.go
diff --git a/client/logdog/butlerlib/streamclient/client.go b/client/logdog/butlerlib/streamclient/client.go
new file mode 100644
index 0000000000000000000000000000000000000000..65d1f55179141756e7758832edbe7b08cf2eb1f1
--- /dev/null
+++ b/client/logdog/butlerlib/streamclient/client.go
@@ -0,0 +1,133 @@
+// 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 (
+	"encoding/json"
+	"fmt"
+	"io"
+	"strings"
+	"sync"
+
+	"github.com/luci/luci-go/client/logdog/butlerlib/streamproto"
+)
+
+type clientFactory func(string) (Client, error)
+
+var (
+	// ProtocolFrameHeaderMagic is the number at the beginning of streams that
+	// identifies the stream handshake version.
+	ProtocolFrameHeaderMagic = []byte("BTLR1\x1E")

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

If and when will this change, and what would it change to?

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

No idea - some other byte string that doesn't begin with these characters :)

This serves two purposes:
- to disambiguate a Butler stream from some happenstance string of bytes (they
probably won't start with these characters), and
- to allow an upgrade to the wire format, if one is ever needed. e.g., a switch
to something other than recordio/JSON.

So TBH probably never, but it's not a bad idea to build this in from day one.

+
+	// defaultRegistry is the default protocol registry.
+	defaultRegistry = &protocolRegistry{}
+)
+
+type protocolRegistry struct {
+	sync.Mutex
+
+	// protocols is the set of registered protocols. Each client should register
+	// via registerProtocol in its init() method.
+	protocols map[string]clientFactory
+}
+
+func (r *protocolRegistry) register(name string, f clientFactory) {
+	r.Lock()
+	defer r.Unlock()
+
+	if _, ok := r.protocols[name]; ok {
+		panic(fmt.Errorf("streamclient: protocol already registered for [%s]", name))
+	}
+	if r.protocols == nil {
+		r.protocols = make(map[string]clientFactory)
+	}
+	r.protocols[name] = f
+}
+
+func (r *protocolRegistry) newClient(path string) (Client, error) {

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

It's not clear from just the name and type what the format of path should be so
it probably deserves a comment. (after reading the public interface this makes
more sense, so perhaps changing ordering would help also)

+	parts := strings.SplitN(path, ":", 2)
+	params := ""
+	if len(parts) == 2 {
+		params = parts[1]
+	}
+
+	if f, ok := r.protocols[parts[0]]; ok {

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

Do we not need to hold a reader lock when reading from a shared map? Do we only
lock in register() because we can instantiate a new map?

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

As long as nothing's writing, we don't need to hold the lock to read. The design
has the services be registered on "init()", so this is safe.

However, the lock is cheap, and I don't enforce anything against runtime
protocol registration, so locking here makes sense. TBH I added the lock
last-minute once I split out the registry so I could register test protocols,
and I forgot to add it here.

+		return f(params)
+	}
+	return nil, fmt.Errorf("streamclient: no protocol registered for [%s]", parts[0])
+}
+
+func registerProtocol(name string, f clientFactory) {
+	defaultRegistry.register(name, f)
+}
+
+// Client is a client to a LogDog Butler StreamServer. A Client will connect
+// to a StreamServer, negotiate a stream configuration, and return an active
+// stream object that can be written to.
+type Client interface {

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

Are there any style guidelines around ordering of public and private code in a
file? As a user I would want to see this at the top (along with New() and
NewStream()).

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

Guidelines are for readability. I split the registry into a different file.

+	// NewStream creates a new stream with the supplied stream properties.
+	NewStream(f streamproto.Flags) (Stream, error)
+}
+
+// streamFactory is a factory method to generate an io.WriteCloser stream for
+// the current clientImpl.
+type streamFactory func() (io.WriteCloser, error)
+
+// clientImpl is an implementation of the Client interface using a net.Conn
+// factory to generate an individual stream.
+type clientImpl struct {
+	// network is the connection path to the stream server.
+	factory streamFactory
+}
+
+// New instantiates a new Client instance. This type of instance will be parsed
+// from the supplied path string, which takes the form:
+//   <protocol>:<protocol-specific-spec>
+//
+// Supported protocols and their respective specs are:
+//   - `tcp4:[port]` (The stream server is listening on localhost at the specified
+//     port.)
+func New(path string) (Client, error) {
+	return defaultRegistry.newClient(path)
+}
+
+func (c *clientImpl) NewStream(f streamproto.Flags) (Stream, error) {
+	p := f.Properties()
+	if err := p.Validate(); err != nil {
+		return nil, fmt.Errorf("streamclient: invalid stream properties: %s", err)
+	}
+
+	client, err := c.factory()
+	if err != nil {
+		return nil, err
+	}
+	passing := false
+	defer func() {
+		// If we haven't written out the connection, close.
+		if !passing {
+			client.Close()
+		}
+	}()
+
+	data, err := json.Marshal(f)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal properties JSON: %s", err)
+	}
+
+	// Perform the handshake: magic + size(data) + data.
+	s := &streamImpl{
+		Properties:  p,
+		WriteCloser: client,
+	}
+	if _, err := s.writeRaw(ProtocolFrameHeaderMagic); err != nil {
+		return nil, fmt.Errorf("failed to write magic number: %s", err)
+	}
+	if err := s.writeRecord(data); err != nil {
+		return nil, fmt.Errorf("failed to write properties: %s", err)
+	}
+
+	passing = true
+	return s, nil
+}