FFM-2096 Add Interface for hooking into SSE Events

[jcox250]

What

• Creates an Interface that clients can implement in order or receive the SSE Events that the SDK gets from Feature Flags
• Adds an option func so clients can pass their own EventStreamListener in, default behaviour is to not have one.

Why

In order to implement streaming in the Proxy we need to be able to get the SSE events after the SDK has updated the cache. This change only forwards the Event to the EventListener after the cache has been updated.

Testing

Did the following locally

• Created a basic EventListener in the Proxy and passed it in to the embedded SDK using WithEventStreamListener.
• Changed the Proxy stream handler to just return the Grip Control headers for pushpin
• Ran the Proxy and pointed and SDK at it that was running in streaming mode
• Toggled flags on/off and the SDK got the event and then fetched the new value from the Proxy's cache

Basic Event Listener

type EventListener struct {
	gpc *gripcontrol.GripPubControl
}

func NewEventListener() EventListener {
	gpc := gripcontrol.NewGripPubControl([]map[string]interface{}{
		{
			"control_uri": "http://localhost:5561",
		},
	})
	return EventListener{gpc}
}

func (e EventListener) Pub(ctx context.Context, event stream.Event) error {
	fmt.Println("Got SSE Event From SDK")

	h := hash.NewSha256()
	channel := h.Hash(event.APIKey)

	content := fmt.Sprintf("event: *\ndata: %s\n\n", event.Event.Data)
	if err := e.gpc.PublishHttpStream(channel, content, "", ""); err != nil {
		fmt.Printf("Error sending data: %s, to pushpin with err %s", event.Event.Data, err)
	}

	fmt.Printf("Pushing event to pushpin %+v", event)
	return nil
}

Basic Stream Handler

h.router.GET("/stream", func(c echo.Context) error {
	w := c.Response().Writer
	apiKey := c.Request().Header.Get("API-Key")
	h := hash.NewSha256()

	w.Header().Add("Content-Type", "text/event-stream")
	w.Header().Add("Grip-Hold", "stream")
	w.Header().Add("Grip-Channel", h.Hash(apiKey))
	w.Header().Add("Grip-Keep-Alive", "format=cstring,timeout=30")

	return nil
})

Update
After the refactor I got my Proxy to use the latest commit on this branch, reran the same manual tests and streaming still works as expected

[davejohnston]

LGTM