Fix streaming Reader race condition (#55)

* Fix streaming Reader race condition

- There was a race condition between creating a `streaming.Reader` with
  `NewReader` and calling its `Subscribe` where events could be consumed
  before there is any channel to send them to.
- To fix this, the default behavior is now to only start the reader's
  `read` goroutine when `Subscribe` is called after it has inserted a
  channel that will receive events.
- Alternatively, there is now an option `WithReaderStartExplicitly`
  which will only start the `read` goroutine when the new `Start` is
  called allowing multiple calls to `Subscribe` before receiving events.

* Remove option to start explicitly

Author: douglaswth

streaming/reader.go

@@ -42,6 +42,8 @@ type (
 		bufferSize int
 		// channels to send notifications
 		chans []chan *Event
+		// startOnce is used to ensure the reader is started only once.
+		startOnce sync.Once
 		// donechan is the reader donechan channel.
 		donechan chan struct{}
 		// streamschan notifies the reader when streams are added or
@@ -114,9 +116,6 @@ func newReader(stream *Stream, opts ...options.Reader) (*Reader, error) {
 		rdb:           stream.rdb,
 	}
 
-	reader.wait.Add(1)
-	pulse.Go(reader.logger, reader.read)
-
 	return reader, nil
 }
 
@@ -127,6 +126,7 @@ func (r *Reader) Subscribe() <-chan *Event {
 	r.lock.Lock()
 	defer r.lock.Unlock()
 	r.chans = append(r.chans, c)
+	r.start()
 	return c
 }
 
@@ -209,6 +209,14 @@ func (r *Reader) IsClosed() bool {
 	return r.closed
 }
 
+// start starts the reader's read goroutine if it is not already running.
+func (r *Reader) start() {
+	r.startOnce.Do(func() {
+		r.wait.Add(1)
+		pulse.Go(r.logger, r.read)
+	})
+}
+
 // read reads events from the streams and sends them to the reader channel.
 func (r *Reader) read() {
 	ctx := context.Background()