Add basic logging to file support

This currently logs to a json file with the stream type.  This is slow
and hard on the cpu and memory so we need to swich this over to
something like protobufs for the binary logs but this is just a start.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

log.go

@@ -0,0 +1,113 @@
+package containerd
+
+import (
+	"encoding/json"
+	"io"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+
+	"github.com/Sirupsen/logrus"
+)
+
+type logConfig struct {
+	BundlePath string
+	LogSize    int64 // in bytes
+	Stdin      io.WriteCloser
+	Stdout     io.ReadCloser
+	Stderr     io.ReadCloser
+}
+
+func newLogger(i *logConfig) (*logger, error) {
+	l := &logger{
+		config:   i,
+		messages: make(chan *Message, DefaultBufferSize),
+	}
+	hout := &logHandler{
+		stream:   "stdout",
+		messages: l.messages,
+	}
+	herr := &logHandler{
+		stream:   "stderr",
+		messages: l.messages,
+	}
+	l.wg.Add(2)
+	go func() {
+		defer l.wg.Done()
+		io.Copy(hout, i.Stdout)
+	}()
+	go func() {
+		defer l.wg.Done()
+		io.Copy(herr, i.Stderr)
+	}()
+	return l, l.start()
+}
+
+type Message struct {
+	Stream    string    `json:"stream"`
+	Timestamp time.Time `json:"timestamp"`
+	Data      []byte    `json:"data"`
+}
+
+type logger struct {
+	config   *logConfig
+	f        *os.File
+	wg       sync.WaitGroup
+	messages chan *Message
+}
+
+type logHandler struct {
+	stream   string
+	messages chan *Message
+}
+
+func (h *logHandler) Write(b []byte) (int, error) {
+	h.messages <- &Message{
+		Stream:    h.stream,
+		Timestamp: time.Now(),
+		Data:      b,
+	}
+	return len(b), nil
+}
+
+func (l *logger) start() error {
+	f, err := os.OpenFile(
+		filepath.Join(l.config.BundlePath, "logs.json"),
+		os.O_CREATE|os.O_WRONLY|os.O_APPEND,
+		0655,
+	)
+	if err != nil {
+		return err
+	}
+	l.f = f
+	l.wg.Add(1)
+	go func() {
+		l.wg.Done()
+		enc := json.NewEncoder(f)
+		for m := range l.messages {
+			if err := enc.Encode(m); err != nil {
+				logrus.WithField("error", err).Error("write log message")
+			}
+		}
+	}()
+	return nil
+}
+
+func (l *logger) Close() (err error) {
+	for _, c := range []io.Closer{
+		l.config.Stdin,
+		l.config.Stdout,
+		l.config.Stderr,
+	} {
+		if cerr := c.Close(); err == nil {
+			err = cerr
+		}
+	}
+	close(l.messages)
+	l.wg.Wait()
+	if ferr := l.f.Close(); err == nil {
+		err = ferr
+	}
+	return err
+}