Feature: Web Panic Reporting via hooks

This PR is for issue of "email after registry webapp panic" #41, improving my
previous design (closed).
It use self setting up hooks, to catch panic in web application.
And, send email in hooks handle directly, to no use new http server and
handler.

Signed-off-by: xiekeyang <keyangxie@126.com>

registry/handlers/hooks.go

@@ -0,0 +1,53 @@
+package handlers
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"strings"
+	"text/template"
+
+	"github.com/Sirupsen/logrus"
+)
+
+// logHook is for hooking Panic in web application
+type logHook struct {
+	LevelsParam []string
+	Mail        *mailer
+}
+
+// Fire forwards an error to LogHook
+func (hook *logHook) Fire(entry *logrus.Entry) error {
+	addr := strings.Split(hook.Mail.Addr, ":")
+	if len(addr) != 2 {
+		return errors.New("Invalid Mail Address")
+	}
+	host := addr[0]
+	subject := fmt.Sprintf("[%s] %s: %s", entry.Level, host, entry.Message)
+
+	html := `
+	{{.Message}}
+
+	{{range $key, $value := .Data}}
+	{{$key}}: {{$value}}
+	{{end}}
+	`
+	b := bytes.NewBuffer(make([]byte, 0))
+	t := template.Must(template.New("mail body").Parse(html))
+	if err := t.Execute(b, entry); err != nil {
+		return err
+	}
+	body := fmt.Sprintf("%s", b)
+
+	return hook.Mail.sendMail(subject, body)
+}
+
+// Levels contains hook levels to be catched
+func (hook *logHook) Levels() []logrus.Level {
+	levels := []logrus.Level{}
+	for _, v := range hook.LevelsParam {
+		lv, _ := logrus.ParseLevel(v)
+		levels = append(levels, lv)
+	}
+	return levels
+}