Add EventSource support (#11235)

If the browser supports EventSource switch to use this instead of
polling notifications.

Signed-off-by: Andrew Thornton art27@cantab.net

routers/events/events.go

@@ -0,0 +1,112 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package events
+
+import (
+	"net/http"
+	"time"
+
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/eventsource"
+	"code.gitea.io/gitea/modules/graceful"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/routers/user"
+)
+
+// Events listens for events
+func Events(ctx *context.Context) {
+	// FIXME: Need to check if resp is actually a http.Flusher! - how though?
+
+	// Set the headers related to event streaming.
+	ctx.Resp.Header().Set("Content-Type", "text/event-stream")
+	ctx.Resp.Header().Set("Cache-Control", "no-cache")
+	ctx.Resp.Header().Set("Connection", "keep-alive")
+	ctx.Resp.Header().Set("X-Accel-Buffering", "no")
+	ctx.Resp.WriteHeader(http.StatusOK)
+
+	// Listen to connection close and un-register messageChan
+	notify := ctx.Req.Context().Done()
+	ctx.Resp.Flush()
+
+	shutdownCtx := graceful.GetManager().ShutdownContext()
+
+	uid := ctx.User.ID
+
+	messageChan := eventsource.GetManager().Register(uid)
+
+	unregister := func() {
+		eventsource.GetManager().Unregister(uid, messageChan)
+		// ensure the messageChan is closed
+		for {
+			_, ok := <-messageChan
+			if !ok {
+				break
+			}
+		}
+	}
+
+	if _, err := ctx.Resp.Write([]byte("\n")); err != nil {
+		log.Error("Unable to write to EventStream: %v", err)
+		unregister()
+		return
+	}
+
+	timer := time.NewTicker(30 * time.Second)
+
+loop:
+	for {
+		select {
+		case <-timer.C:
+			event := &eventsource.Event{
+				Name: "ping",
+			}
+			_, err := event.WriteTo(ctx.Resp)
+			if err != nil {
+				log.Error("Unable to write to EventStream for user %s: %v", ctx.User.Name, err)
+				go unregister()
+				break loop
+			}
+			ctx.Resp.Flush()
+		case <-notify:
+			go unregister()
+			break loop
+		case <-shutdownCtx.Done():
+			go unregister()
+			break loop
+		case event, ok := <-messageChan:
+			if !ok {
+				break loop
+			}
+
+			// Handle logout
+			if event.Name == "logout" {
+				if ctx.Session.ID() == event.Data {
+					_, _ = (&eventsource.Event{
+						Name: "logout",
+						Data: "here",
+					}).WriteTo(ctx.Resp)
+					ctx.Resp.Flush()
+					go unregister()
+					user.HandleSignOut(ctx)
+					break loop
+				}
+				// Replace the event - we don't want to expose the session ID to the user
+				event = (&eventsource.Event{
+					Name: "logout",
+					Data: "elsewhere",
+				})
+			}
+
+			_, err := event.WriteTo(ctx.Resp)
+			if err != nil {
+				log.Error("Unable to write to EventStream for user %s: %v", ctx.User.Name, err)
+				go unregister()
+				break loop
+			}
+			ctx.Resp.Flush()
+		}
+	}
+	timer.Stop()
+}