added email notification text content support (prometheus#934)

* added email notification text content support (configuration text)
added default email notification text content default value empty
converted email notification from html to multipart/alternatives with support to both html (first) and text (last)
ignore intellij IDE .idea working folder

* removed specific editor .gitignore entries

* renamed TEXT to Text as it's not an acronym
added TODO note to refactor multipart code to use standard go library

* refactored to use standard go mime/multipart library for text and html parts and bounderies

* use multipart createPart returned writer
added error handling while creating parts
removed unnecessary quotes from boundry string

* removed unnecessary comments

config/notifiers.go

@@ -33,6 +33,7 @@ var (
 			VSendResolved: false,
 		},
 		HTML: `{{ template "email.default.html" . }}`,
+		Text: ``,
 	}
 
 	// DefaultEmailSubject defines the default Subject header of an Email.
@@ -142,6 +143,7 @@ type EmailConfig struct {
 	AuthIdentity string            `yaml:"auth_identity,omitempty" json:"auth_identity,omitempty"`
 	Headers      map[string]string `yaml:"headers,omitempty" json:"headers,omitempty"`
 	HTML         string            `yaml:"html,omitempty" json:"html,omitempty"`
+	Text         string            `yaml:"text,omitempty" json:"text,omitempty"`
 	RequireTLS   *bool             `yaml:"require_tls,omitempty" json:"require_tls,omitempty"`
 
 	// Catches all undefined fields and must be empty after parsing.

notify/impl.go

@@ -20,9 +20,9 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"io"
 	"io/ioutil"
 	"mime"
+	"mime/multipart"
 	"net"
 	"net/http"
 	"net/mail"
@@ -40,6 +40,7 @@ import (
 	"github.com/prometheus/alertmanager/config"
 	"github.com/prometheus/alertmanager/template"
 	"github.com/prometheus/alertmanager/types"
+	"net/textproto"
 )
 
 type notifierConfig interface {
@@ -365,23 +366,51 @@ func (n *Email) Notify(ctx context.Context, as ...*types.Alert) (bool, error) {
 		fmt.Fprintf(wc, "%s: %s\r\n", header, mime.QEncoding.Encode("utf-8", value))
 	}
 
-	fmt.Fprintf(wc, "Content-Type: text/html; charset=UTF-8\r\n")
+	buffer := &bytes.Buffer{}
+	multipartWriter := multipart.NewWriter(buffer)
+
 	fmt.Fprintf(wc, "Date: %s\r\n", time.Now().Format(time.RFC1123Z))
+	fmt.Fprintf(wc, "Content-Type: multipart/alternative;  boundary=%s\r\n", multipartWriter.Boundary())
+
 
 	// TODO: Add some useful headers here, such as URL of the alertmanager
 	// and active/resolved.
 	fmt.Fprintf(wc, "\r\n")
 
-	// TODO(fabxc): do a multipart write that considers the plain template.
+	// Html template
+	w, err := multipartWriter.CreatePart(textproto.MIMEHeader{"Content-Type": {"text/html; charset=UTF-8"}})
+	if err != nil {
+		return false, fmt.Errorf("creating part for html template: %s", err)
+	}
 	body, err := n.tmpl.ExecuteHTMLString(n.conf.HTML, data)
 	if err != nil {
 		return false, fmt.Errorf("executing email html template: %s", err)
 	}
-	_, err = io.WriteString(wc, body)
+	_, err = w.Write([]byte(body))
+	if err != nil {
+		return true, err
+	}
+
+	// Text template
+	// Last alternative based on recommendation in section 7.2.3 of w3 rfc1341 protocol
+	// https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
+	w, err = multipartWriter.CreatePart(textproto.MIMEHeader{"Content-Type": {"text/plain; charset=UTF-8"}})
+	if err != nil {
+		return false, fmt.Errorf("create part for text template: %s", err)
+	}
+	body, err = n.tmpl.ExecuteTextString(n.conf.Text, data)
+	if err != nil {
+		return false, fmt.Errorf("executing email text template: %s", err)
+	}
+	_, err = w.Write([]byte(body))
 	if err != nil {
 		return true, err
 	}
 
+	multipartWriter.Close()
+
+	wc.Write(buffer.Bytes())
+
 	return false, nil
 }