amtool check-config validate templates

[mac2000]

will be so nice if amtool will not only check the correctness of the configuration file but also given templates

repro:

alertmanager.yml

global:
  slack_api_url: https://hooks.slack.com/services/xxxx
route:
  receiver: demo

receivers:
  - name: demo
    slack_configs:
    - title: '{{ if .CommonLabels.alertname }}{{ .CommonLabels.alertname }}{{ if .GroupLabels.alertname }}{{ .GroupLabels.alertname }}{{ else }}alertmanager{{ end }}'

docker run --rm -v $PWD:/code -w /code --entrypoint=amtool prom/alertmanager check-config alertmanager.yml

will say that everything fine

but sooner or later you will realize that you are not receiving alerts not because everything is fine but because a silly typo in the template 🤷‍♂️

from golang perspective such template will raise an error template: :1: unexpected EOF:

package main

import (
	"html/template"
)

func main() {
	_, err := template.New("").Parse(`{{ if .CommonLabels.alertname }}{{ .CommonLabels.alertname }}{{ if .GroupLabels.alertname }}{{ .GroupLabels.alertname }}{{ else }}alertmanager{{ end }}`)
	if err != nil {
		panic(err)
	}

}

so theoretically amtool can perform such check on each field that can use templates and warn us, so we know about the problem before applying changes to alertmanager

[mac2000]

I'm not a go lang developer but here is what I'm trying to achieve:

package main

import (
	"html/template"
	"log"

	"github.com/prometheus/alertmanager/config"
)

func main() {
	cfg, err := config.LoadFile("alertmanager.yml")
	if err != nil {
		log.Fatalln(err.Error())
	}

	tpl := template.New("test")
	if len(cfg.Templates) > 0 {
		tpl, err = tpl.ParseFiles(cfg.Templates...)
		if err != nil {
			log.Fatalln(err.Error())
		}
	}

	for _, receiver := range cfg.Receivers {
		for _, slack := range receiver.SlackConfigs {
			_, err = tpl.Parse(slack.Title)
			if err != nil {
				log.Fatalf("%s.slack.title: %s", receiver.Name, err.Error())
			}
			_, err = tpl.Parse(slack.Text)
			if err != nil {
				log.Fatalf("%s.slack.text: %s", receiver.Name, err.Error())
			}
		}
	}
}

with mistake from above we will get (can not say that error is meaningful but still it is better than nothing):

2022/05/05 08:48:17 demo.slack.title: template: test:1: unexpected EOF

wondering if there is a way to somehow just render everything instead of checking each and every templatable fields

[r3code]

This might help

alertmanager/README.md

Line 303 in 9f683fc

Then you can test out how a template would look like with example by using this command:

it allows rendering a template