oss_17_jaeger_debug_and_parentbased_trace_state

[ocelotl]

Jaeger propagator must set only the SAMPLED bit (not force DEBUG) on inject; ParentBased sampler must forward incoming trace_state (Linear OSS-17, findings P4/T4).

Branch / worktree: oss_17_jaeger_debug_and_parentbased_trace_state

Linear issue: https://linear.app/dash0/issue/OSS-17/p4-t4-jaeger-forces-debug-bit-on-all-sampled-spans-parentbased-drops

[ocelotl]

🔒 Internal (dash0) — not for upstream.

• Linear: https://linear.app/dash0/issue/OSS-17/p4-t4-jaeger-forces-debug-bit-on-all-sampled-spans-parentbased-drops (findings P4/T4)
• Slug/branch/worktree: oss_17_jaeger_debug_and_parentbased_trace_state
• Commit: 16648be08 · PR: oss_17_jaeger_debug_and_parentbased_trace_state #33
• Validation: jaeger 18 passed, sdk sampling+trace 124 passed; scope jaeger + sdk + changelog.
• Reviewer note: one pre-existing test (test_debug_flag_set) asserted the OLD buggy forced-DEBUG behavior and was replaced with test_debug_flag_not_set_for_sampled_span.

[ocelotl]

📣 Public-facing draft — to be opened upstream in open-telemetry/opentelemetry-python. No internal references; copy verbatim.

Title: Jaeger propagator forces DEBUG flag on inject, and ParentBased sampler drops incoming trace_state

Two related trace-propagation/sampling correctness bugs.

1. Jaeger propagator forces the DEBUG flag on inject

propagator/opentelemetry-propagator-jaeger/src/opentelemetry/propagators/jaeger/__init__.py (inject):

trace_flags = span_context.trace_flags
if trace_flags.sampled:
    trace_flags |= self.DEBUG_FLAG   # DEBUG_FLAG = 0x02

Every normally-sampled span injected with the Jaeger propagator has the Jaeger DEBUG flag (0x02) set in the uber-trace-id header. In Jaeger the debug flag is a forced-keep signal, not a normal sampling flag, so routine sampled traffic instructs downstream Jaeger components to force-keep the trace, defeating downstream sampling. Extract already masks with flags & TraceFlags.SAMPLED, so inject/extract are asymmetric. Injecting a sampled span should set only the SAMPLED bit (0x01).

2. ParentBased sampler drops the incoming trace_state

opentelemetry-sdk/src/opentelemetry/sdk/trace/sampling.py (ParentBased.should_sample) selects a delegate and calls it without trace_state; additionally the SDK call site in opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py (Tracer.start_span) never passes trace_state. So the trace_state parameter documented on Sampler.should_sample is dead for any custom delegate under ParentBased (it always receives None).

Spec specification/trace/sdk.md lists trace_state among ShouldSample inputs; a composite sampler must forward its inputs to the delegate.

Proposed fix

• Jaeger inject: trace_flags = TraceFlags(span_context.trace_flags & TraceFlags.SAMPLED); drop the DEBUG OR.
• ParentBased: forward trace_state=trace_state to the delegate.
• SDK start_span: derive trace_state from the parent span context and pass it into should_sample.