Wait for `run` goroutine to exit before `Close`

The underlying Loggers Close() function can be called with the the
run() goroutine still writing to the driver. This is causing the
fluentd-golang-logger to panic cause it doesn't defensively check
for the closing of the channel before writing to it.
It relies on the docker daemon to keep the contract of not calling Log()
if Close() has already been called.

Contributions by: James Johnston <james.johnston@thumbtack.com>
                  Nathan Wong <nathanw@thumbtack.com>

Signed-off-by: Anuj Varma <anujvarma@thumbtack.com>

daemon/logger/ring.go

@@ -19,6 +19,7 @@ type RingLogger struct {
 	l         Logger
 	logInfo   Info
 	closeFlag int32
+	wg        sync.WaitGroup
 }
 
 var _ SizedLogger = &RingLogger{}
@@ -42,6 +43,7 @@ func newRingLogger(driver Logger, logInfo Info, maxSize int64) *RingLogger {
 		l:       driver,
 		logInfo: logInfo,
 	}
+	l.wg.Add(1)
 	go l.run()
 	return l
 }
@@ -93,6 +95,7 @@ func (r *RingLogger) setClosed() {
 func (r *RingLogger) Close() error {
 	r.setClosed()
 	r.buffer.Close()
+	r.wg.Wait()
 	// empty out the queue
 	var logErr bool
 	for _, msg := range r.buffer.Drain() {
@@ -118,6 +121,7 @@ func (r *RingLogger) Close() error {
 // logger.
 // This is run in a goroutine when the RingLogger is created
 func (r *RingLogger) run() {
+	defer r.wg.Done()
 	for {
 		if r.closed() {
 			return