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

modules/eventsource/messenger.go

@@ -0,0 +1,78 @@
+// 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 eventsource
+
+import "sync"
+
+// Messenger is a per uid message store
+type Messenger struct {
+	mutex    sync.Mutex
+	uid      int64
+	channels []chan *Event
+}
+
+// NewMessenger creates a messenger for a particular uid
+func NewMessenger(uid int64) *Messenger {
+	return &Messenger{
+		uid:      uid,
+		channels: [](chan *Event){},
+	}
+}
+
+// Register returns a new chan []byte
+func (m *Messenger) Register() <-chan *Event {
+	m.mutex.Lock()
+	// TODO: Limit the number of messengers per uid
+	channel := make(chan *Event, 1)
+	m.channels = append(m.channels, channel)
+	m.mutex.Unlock()
+	return channel
+}
+
+// Unregister removes the provider chan []byte
+func (m *Messenger) Unregister(channel <-chan *Event) bool {
+	m.mutex.Lock()
+	defer m.mutex.Unlock()
+	for i, toRemove := range m.channels {
+		if channel == toRemove {
+			m.channels = append(m.channels[:i], m.channels[i+1:]...)
+			close(toRemove)
+			break
+		}
+	}
+	return len(m.channels) == 0
+}
+
+// UnregisterAll removes all chan []byte
+func (m *Messenger) UnregisterAll() {
+	m.mutex.Lock()
+	defer m.mutex.Unlock()
+	for _, channel := range m.channels {
+		close(channel)
+	}
+	m.channels = nil
+}
+
+// SendMessage sends the message to all registered channels
+func (m *Messenger) SendMessage(message *Event) {
+	m.mutex.Lock()
+	defer m.mutex.Unlock()
+	for i := range m.channels {
+		channel := m.channels[i]
+		select {
+		case channel <- message:
+		default:
+		}
+	}
+}
+
+// SendMessageBlocking sends the message to all registered channels and ensures it gets sent
+func (m *Messenger) SendMessageBlocking(message *Event) {
+	m.mutex.Lock()
+	defer m.mutex.Unlock()
+	for i := range m.channels {
+		m.channels[i] <- message
+	}
+}