Add template processing for notify channels

README.md

@@ -87,6 +87,8 @@ However, multiple signals can map to the same channel.
 The channel type `notify` is for sending messages to popular messaging
 platforms.
 
+You must specify a URL and, optionally, a message template.
+
 `orb-ag` uses `shoutrrr` for sending notifications.  Use the following
 URL formats for these different services.  Additional details are
 available from the [`shoutrrr`
@@ -113,7 +115,29 @@ documentation](https://containrrr.dev/shoutrrr/v0.8/services/overview/).
 | Telegram    | `telegram://token@telegram?chats=@channel-1[,chat-id-1,...]`                               |
 | Zulip Chat  | `zulip://bot-mail:bot-key@zulip-domain/?stream=name-or-id&topic=name`                     |
 
-The URL format for generic webhooks is described at
+URLs are actually go templates that are processed before use.  Data
+that you can provide to the template engine includes:
+- `.Timestamp` : the RFC3339 formatted timestamp for the matching log entry
+- `.PID`: the process ID for the observed process
+- `.Logline`: the matching log line
+
+Similarly, the message sent may also be a template.  If no `template`
+is specified in the channel definition, then the logline is used as
+the message.  If a `template` is specified, then the template is
+processed with the same data as above before sending.
+
+As an example, here's a channel that sends an email containing json
+data, and the observed PID is in the email subject line:
+
+```
+  - name: "email-on-startup"
+    type: "notify"
+    url:  "smtp://EMAIL@gmail.com:PASSWORD@smtp.gmail.com:587/?from=EMAIL@gmail.com&to=EMAIL@gmail.com&subject=Starting%20process%20{{.PID}}!"
+    template: "{ \"timestamp\": \"{{.Timestamp}}\", \"message\": \"{{.Logline}}\" }"
+```
+
+Generic webhooks and handled specially by `shoutrrr`.  Their URL
+format is described at
 [https://containrrr.dev/shoutrrr/v0.8/services/generic/](https://containrrr.dev/shoutrrr/v0.8/services/generic/).
 
 ### Sending Kafka messages

orb-ag.go

@@ -41,6 +41,7 @@ import (
 	"regexp"
 	"sync"
 	"syscall"
+	"text/template"
 	"time"
 )
 
@@ -49,12 +50,13 @@ var restart bool = true
 var observed_cmd *exec.Cmd
 
 type Channel struct {
-	Name   string `yaml:"name"`
-	Type   string `yaml:"type"`
-	URL    string `yaml:"url"`
-	Topic  string `yaml:"topic"`
-	Broker string `yaml:"broker"`
-	Shell  string `yaml:"shell"`
+	Name     string `yaml:"name"`
+	Type     string `yaml:"type"`
+	URL      string `yaml:"url"`
+	Template string `yaml:"template"`
+	Topic    string `yaml:"topic"`
+	Broker   string `yaml:"broker"`
+	Shell    string `yaml:"shell"`
 }
 
 type Signal struct {
@@ -120,6 +122,12 @@ type Notification struct {
 	Message string
 }
 
+type TemplateData struct {
+	PID       int
+	Logline   string
+	Timestamp string
+}
+
 func loadYAMLConfig(filename string, config *Config) error {
 	bytes, err := ioutil.ReadFile(filename)
 	if err != nil {
@@ -135,14 +143,44 @@ func loadYAMLConfig(filename string, config *Config) error {
 }
 
 func startWorkers(notificationQueue <-chan Notification, numWorkers int, wg *sync.WaitGroup) {
+	var messageString string
+
 	for i := 0; i < numWorkers; i++ {
 		wg.Add(1)
 		go func(workerID int) {
 			defer wg.Done()
 			for notification := range notificationQueue {
+				td := TemplateData{PID: notification.PID,
+					Logline:   notification.Message,
+					Timestamp: time.Now().Format(time.RFC3339)}
 				switch notification.Channel.Type {
 				case "notify":
-					err := shoutrrr.Send(notification.Channel.URL, notification.Message)
+					tmpl, err := template.New("url").Parse(notification.Channel.URL)
+					if err != nil {
+						log.Fatal("org-ab error: can't parse URL template: ", err)
+					}
+					var buffer bytes.Buffer
+					err = tmpl.Execute(&buffer, td)
+					if err != nil {
+						log.Fatal("org-ab error: can't execute URL template: ", err)
+					}
+					urlString := buffer.String()
+					if notification.Channel.Template != "" {
+						tmpl, err := template.New("msg").Parse(notification.Channel.Template)
+						if err != nil {
+							log.Fatal("org-ab error: can't parse template: ", err)
+						}
+						var buffer bytes.Buffer
+						err = tmpl.Execute(&buffer, td)
+						if err != nil {
+							log.Fatal("org-ab error: can't execute URL template: ", err)
+						}
+						messageString = buffer.String()
+					} else {
+						messageString = notification.Message
+					}
+
+					err = shoutrrr.Send(urlString, messageString)
 					if err != nil {
 						log.Println("org-ab warning: failed sending notification: ", err)
 					}