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/event.go

@@ -0,0 +1,119 @@
+// 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 (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"strings"
+	"time"
+)
+
+func wrapNewlines(w io.Writer, prefix []byte, value []byte) (sum int64, err error) {
+	if len(value) == 0 {
+		return
+	}
+	n := 0
+	last := 0
+	for j := bytes.IndexByte(value, '\n'); j > -1; j = bytes.IndexByte(value[last:], '\n') {
+		n, err = w.Write(prefix)
+		sum += int64(n)
+		if err != nil {
+			return
+		}
+		n, err = w.Write(value[last : last+j+1])
+		sum += int64(n)
+		if err != nil {
+			return
+		}
+		last += j + 1
+	}
+	n, err = w.Write(prefix)
+	sum += int64(n)
+	if err != nil {
+		return
+	}
+	n, err = w.Write(value[last:])
+	sum += int64(n)
+	if err != nil {
+		return
+	}
+	n, err = w.Write([]byte("\n"))
+	sum += int64(n)
+	return
+}
+
+// Event is an eventsource event, not all fields need to be set
+type Event struct {
+	// Name represents the value of the event: tag in the stream
+	Name string
+	// Data is either JSONified []byte or interface{} that can be JSONd
+	Data interface{}
+	// ID represents the ID of an event
+	ID string
+	// Retry tells the receiver only to attempt to reconnect to the source after this time
+	Retry time.Duration
+}
+
+// WriteTo writes data to w until there's no more data to write or when an error occurs.
+// The return value n is the number of bytes written. Any error encountered during the write is also returned.
+func (e *Event) WriteTo(w io.Writer) (int64, error) {
+	sum := int64(0)
+	nint := 0
+	n, err := wrapNewlines(w, []byte("event: "), []byte(e.Name))
+	sum += n
+	if err != nil {
+		return sum, err
+	}
+
+	if e.Data != nil {
+		var data []byte
+		switch v := e.Data.(type) {
+		case []byte:
+			data = v
+		case string:
+			data = []byte(v)
+		default:
+			var err error
+			data, err = json.Marshal(e.Data)
+			if err != nil {
+				return sum, err
+			}
+		}
+		n, err := wrapNewlines(w, []byte("data: "), data)
+		sum += n
+		if err != nil {
+			return sum, err
+		}
+
+	}
+
+	n, err = wrapNewlines(w, []byte("id: "), []byte(e.ID))
+	sum += n
+	if err != nil {
+		return sum, err
+	}
+
+	if e.Retry != 0 {
+		nint, err = fmt.Fprintf(w, "retry: %d\n", int64(e.Retry/time.Millisecond))
+		sum += int64(nint)
+		if err != nil {
+			return sum, err
+		}
+	}
+
+	nint, err = w.Write([]byte("\n"))
+	sum += int64(nint)
+
+	return sum, err
+}
+
+func (e *Event) String() string {
+	buf := new(strings.Builder)
+	_, _ = e.WriteTo(buf)
+	return buf.String()
+}