Add text/template support to core/v2.Handler "commands"

[calebhailey]

Feature Suggestion

Parse text/template templates for the core/v2.Handler command field. I haven't considered whether it might also be useful to support templates in other Handler fields.

Possible Implementation

Just do it ™

In practice, Handler templates would be evaluated just prior to command execution using the incoming Sensu event as the source data.

Context

The Sensu Plugin SDK added support for handler templating in April 2019. This functionality has been fully documented since September 2020[^1].

With a plugin that supports templating (e.g. the Sensu Pagerduty Handler), it is possible to do the following:

spec:
  type: pipe
  command: >-
    sensu-pagerduty-handler
    --dedup-key-template "{{.Entity.Namespace}}-{{.Entity.Name}}-{{.Check.Name}}"
    --status-map "{\"info\":[0],\"warning\": [1],\"critical\": [2],\"error\": [3,127]}"
    --summary-template "[{{.Entity.Namespace}}] {{.Entity.Name}}/{{.Check.Name}}: {{.Check.State}}"
    --details-template "{{.Check.Output}}\n\n{{.Check}}"

However, the templates are limited to command flags which have enabled support for templating (via the sensu-plugin-sdk).

So in the following two examples using a text/template conditional statement, only the first example works:

spec:
  type: pipe
  command: >-
    sensu-slack-handler
    --channel ${MATTERMOST_CHANNEL}
    --username SensuGo
    --webhook-url ${MATTERMOST_WEBHOOK_URL}   
    --description-template '{{ if (index .Check.Annotations "sensu.io/notifications/warning") }}{{ index .Check.Annotations "sensu.io/notifications/warning" }}\n\n[namespace: {{ .Entity.Namespace }}]{{ else }}{{ .Check.Output }}\n\n[namespace: {{ .Entity.Namespace }}]{{ end }}'

spec:
  type: pipe
  command: >-
    sensu-slack-handler
    --channel ${MATTERMOST_CHANNEL}
    --username SensuGo
    --webhook-url ${MATTERMOST_WEBHOOK_URL}   
    {{ if (index .Check.Annotations "sensu.io/notifications/warning") }}
      --description-template {{ index .Check.Annotations "sensu.io/notifications/warning" }}\n\n[namespace: {{ .Entity.Namespace }}]
    {{ else }}
      --description-template {{ .Check.Output }}\n\n[namespace: {{ .Entity.Namespace }}]
    {{ end }}

What's interesting is that, if we were to natively support text/template for core/v2.Handler commands, we could effectively drop support for handler templates from the plugin SDK and both of these examples would "just work" ™.

[^1]: this documentation also helped clarify that Handler Templates are distinct from Sensu Tokens.