Samplers never receive the parent's tracestate, and the composite sampler erases it

Describe your environment

Labels: bug, sdk, trace, sampling, spec-compliance
Affected packages: opentelemetry-sdk
Found on: main @ 0a5d76b6
Environment: CPython 3.12

What happened?

Two independent omissions on the same path.

Tracer.start_span calls should_sample(context, trace_id, name, kind, attributes, links) and never passes the seventh parameter, trace_state. Separately, ParentBased.should_sample accepts trace_state but does not forward it to its delegate. Between them, no sampler ever receives a tracestate, even though the Sampler ABC declares the parameter and the comment at the call site says the sampler may modify it.

A third, related bug: _update_trace_state returns early when the incoming tracestate is empty, so SamplingIntent.update_trace_state is silently skipped for every root span.

Steps to Reproduce

from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace._sampling_experimental import (
    composable_always_on, composable_parent_threshold, composite_sampler,
)
from opentelemetry.trace import set_span_in_context
from opentelemetry.trace.propagation.tracecontext import TraceContextTextMapPropagator

carrier = {
    "traceparent": "00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01",
    "tracestate": "vendora=alpha,ot=th:8",
}
remote = TraceContextTextMapPropagator().extract(carrier)

provider = TracerProvider(
    sampler=composite_sampler(composable_parent_threshold(composable_always_on()))
)
span = provider.get_tracer("t").start_span("child", context=remote)

out = {}
TraceContextTextMapPropagator().inject(out, context=set_span_in_context(span))
print("incoming:", carrier["tracestate"])
print("outgoing:", out.get("tracestate"))

Expected Result

The sampler receives the parent's tracestate, and vendor entries survive into the child span's tracestate.

Actual Result

# what the sampler actually receives
bare sampler               trace_state = None
ParentBased(sampler)       trace_state = None

# end-to-end propagation through a composite sampler
incoming tracestate : vendora=alpha,ot=th:8
outgoing tracestate : None
vendora preserved   : False

# control - the default sampler
outgoing tracestate : vendora=alpha,ot=th:8
vendora preserved   : True

Additional context

Two distinct consequences.

First, the W3C consistent probability sampling implementation in _sampling_experimental is inert. _ComposableParentThreshold reads the parent threshold from tracestate and always sees None, so it falls back to the sampled flag with threshold_reliable=False. The rv random value is never honoured either, so consistent sampling across a trace cannot work and span-to-metrics estimation loses its adjusted counts.

Second, and worse for anyone not using that module: _CompositeSampler rebuilds the outgoing tracestate from the parameter it was handed. Given None, it emits a fresh tracestate containing only ot - silently discarding every vendor entry from the incoming request. Tracestate is how other tracing systems carry their own context through a trace, so this breaks interoperability for third parties that have nothing to do with sampling.

Any custom sampler that relies on the documented trace_state parameter is also affected, including one wrapped in ParentBased - which is the default composition.

Would you like to implement a fix?

Yes

Tip

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Inspect Tracer.start_span, ParentBased.should_sample, _update_trace_state, and _CompositeSampler, including the related logic in _sampling_experimental. Run the provided reproduction first. Done means samplers receive the parent tracestate, root-span updates are applied, and composite sampling preserves vendor entries in outgoing tracestate.