Adding custom template functions

[AurelPaulovic]

Hello, I would like to ask if Alertmanager supports adding custom template functions to alerting templates without having to create a custom build of the Alertmanager codebase. The problem is that Go templates and the functions provided in the alerting templates are very restrictive. Even if I could define some "function" as as a nested template, Go templates don't allow me to assign the output of the nested template into a variable within another template or use in conditionals so there is no way for me to reuse code.

I imagine I could solve my problems if there was a way for me to dynamically define custom template functions or evaluate template within variable assignment or conditionals, i.e. something like non-existing function callNestedTemplate:

{{ define "foo" }}
123
{{ end }}

{{ define "bar" }}
{{ $variable := callNestedTemplate "foo" }}
{{/* .. use $variable */}}
{{ end }}

{{ define "baz" }}
{{ if eq (callNestedTemplate "foo") 123 }}
do something
{{ else }}
do other thing
{{ end }} 
{{ end }}

I found #3174 which looks promising and at least adds support at the level of template.FromGlobs by supplying Option. However, it still seems that there is no way to use this with standard build of Alertmanager using some form of plugin.

[AurelPaulovic]

It looks like I need the functionality of tmpl function from Prometheus' console templates in the Alertmanager templating system. I can't use tmpl in alerting rules because Prometheus does not allow it (and does not allow to use template libraries in alerting rules as well) so I can't just circumvent the lack of support in Alertmanager's templates by computing the required values in annotations in the alerting rules.

tmpl := html_template.New(te.name)
tmpl.Funcs(html_template.FuncMap{
    "tmpl": func(name string, data interface{}) (html_template.HTML, error) {
        var buffer bytes.Buffer
	err := tmpl.ExecuteTemplate(&buffer, name, data)
	return html_template.HTML(buffer.String()), err
    },
})

Having tmpl in Alertmanager would not solve all of my templating issues but would still be tremendously helpful.