Summary

I believe this PR addresses #13158: although the description is quite vague, it's the closest reference to the issue this PR addresses that I could find.

This PR proposes a refactor of rulefmt to fix a surprising behavior when using YAML anchors/aliases to configure alert rules. Although their use is somewhat discouraged in favour of using proper configuration management tools to generate full YAML, they can still be useful in some situations and the following behavior is unexpected: YAML aliases work everywhere but in the name and expr of a rule (see the Example section).

This is due to the explicit use of yaml.Node for Alert/Record/Expr when decoding a RuleNode.

This PR instead proposes to unmarshal everything into the proper final types, and keep the decoding structs only for validation. As a consequence, Rule becomes the type used in RuleGroup instead of RuleNode, which means its Alert/Record/Expr fields are fully unmarshalled, with aliases resolved if needed. This has the side effect of reducing the creep of YAML types in other modules: for example rules/manager.go used r.Expr.Value, the .Value being YAML-specific.

I'm not sure whether this should considered a breaking change or not: I've chosen not to export RuleNode since it would no longer make sense for other modules to use it now, but that could be changed to make sure nothing breaks.

The first commit adds a set of failing tests. The second commit is the proposed implementation (which fixes the tests).

Example

The following configuration illustrates the issue:

groups:
  - name: my-group-name
    interval: 30s
    rules:
      - alert: &alertname HighAlert
        expr: some expression
        for: 5m
        labels:
          severity: &severity critical
      - alert: *alertname
        expr: some other expression
        for: 5m
        labels:
          severity: *severity

In this example, Prometheus will properly register the first rule, but the second rule will have .Alert == "alertname" even though it will properly have .Labels.Severity == "critical".