[BUG] using has-root-span with root.duration_ms in a rule causing rules to be skipped

[howardyoo]

Versions

• Go: N/A
• Refinery: 3.0

Steps to reproduce

1. Define a rule

rules:
  RulesVersion: 2

  Samplers:
    __default__:
      RulesBasedSampler:
        Rules:
          ...
          - Name: Small Fast Traces
            Scope: span
            Conditions:
              - Operator: has-root-span
                Value: true
              - Field: root.duration_ms
                Operator: '<'
                Value: 1000
                Datatype: int
              - Field: "?.NUM_DESCENDANTS"
                Operator: '<='
                Value: 2
                Datatype: int
            Sampler:
              EMADynamicSampler:
                GoalSampleRate: 1000
                FieldList:
                  - service.name
                  - name
           ...

Notice that the rule has has-root-span: true
and also root.duration_ms on certain condition. When testing the refinery, even though a trace would have the matching condition, the rule would not be triggered, and the trace would go into the default sampler.

However, when I removed the has-root-span condition, the sampling would work fine, getting triggered with trace having shorter duration than 1000 and also num of descendants less than 2.

Additional context
Logically, the above condition should work, as the subject trace has a root span, as well as descendants of less than 2, and the duration_ms of the root span is less than 1000. Therefore, it feels likely to be a bug.

[VinozzZ]

Thanks for raising this — I agree the current semantics are a bit confusing.

has-root-span is a trace-level check, so pairing it with scope: span (which evaluates per span) isn’t a meaningful combination.

Here’s the intended model:

• If you want to match or make assertions about the root span, write a trace-scoped rule and reference root fields explicitly with root.. Example: conditions like root.http.status_code, root.name, etc.
• If any condition is a not-exists check on a root. field, also include has-root-span: true. That ensures a root span is present before we assert that a field is missing from it.

So:

• “Match only on root span data” → scope: trace + root. in all conditions
• “Assert a root field is missing” → same, plus has-root-span: true

We should update the docs to reflect this more explicitly, since the current wording can imply that scope: span + has-root-span is valid. I’ll follow up with a doc fix.