Child spans inherit the parent’s TraceFlags::kIsSampled bit even when the active sampler returns Decision::DROP

[SurvivorofChaos]

Describe your environment
Ubuntu22.04
OpenTelemetry-cpp v1.22.0

Steps to reproduce

1. Process1: Start a span, and set it to active, as parent, propogate(IsSampled=true)
2. Process2: Start a child span, with custom sampler, which returns Decision::DROP
3. Check the child span trace flag, it would be true

What is the expected behavior?
Per the spec, TraceFlags::kIsSampled must reflect the sampler decision.
Dropped spans should have the sampled bit unset.

What is the actual behavior?
The child span always get the same IsSampled()=true if the parent is sampled, no matter what the custom sampler returns.

Root Cause
The code copies the parent’s flags (see this line) and only sets the sampled bit when ShouldSample returns RECORD_AND_SAMPLE(see this line). It never clears the bit for DROP/RECORD_ONLY. When a drop decision occurs, the SDK still creates a SpanContext with kIsSampled set and wraps it in a NoopSpan, so span->GetContext().IsSampled() remains true.

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[marcalff]

To analyze, confirm the desired behavior with the specs.

[marcalff]

From the sampler spec:

DROP - IsRecording will be false, the Span will not be recorded and all events and attributes will be dropped.

The is_sampled flag is not explicitly mentioned, but here the spec indicates that the combination is_recording = false and is_sampled = true is not allowed, so the sample flag should be unset indeed.

RECORD_ONLY - IsRecording will be true, but the Sampled flag MUST NOT be set.

The sample flag should be cleared then.

RECORD_AND_SAMPLE - IsRecording will be true and the Sampled flag MUST be set.

This part is compliant.

@SurvivorofChaos

Thanks for the report, this looks like a valid bug.