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fix(core): Defer TwP sampling by reading trace state from the scope#21549
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andreiborza wants to merge 5 commits into
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fix(core): Defer TwP sampling by reading trace state from the scope#21549andreiborza wants to merge 5 commits into
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In Tracing-without-Performance (spans disabled), a root placeholder previously froze a negative sampling decision in the DSC, which suppressed downstream sampling instead of leaving the decision to a performance-enabled service further along the trace. The scope is the source of truth for a TwP placeholder's trace state: - `getTraceData` reads the sampling decision from the scope (deferred for a new trace, the upstream decision for a continued trace) while keeping the placeholder's stable span id, so the outgoing `sentry-trace` header omits the flag instead of asserting `-0`. - `getDynamicSamplingContextFromSpan` resolves a placeholder's DSC from its captured scope (continued traces keep the incoming DSC; new traces derive it from the client). A new (head-of-trace) TwP trace does not stamp a local `transaction` in its DSC; continued traces still propagate the upstream decision and DSC. No DSC is written to the scope at span start, preserving the browser's "scope stays DSC-free between navigations" behavior.
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mydea
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Actually pretty smart to put that into the DSC into the scope for TwP.
Not approving just yet because of my comment and the Symbol comment of Francesco.
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In Tracing-without-Performance (spans disabled), a root placeholder previously froze a negative sampling decision in the DSC, which suppressed downstream sampling instead of leaving the decision to a performance-enabled service further along the trace.
The scope is the source of truth for a TwP placeholder's trace state:
getTraceDatareads the sampling decision from the scope (deferred for a new trace, the upstream decision for acontinued trace), so the outgoing
sentry-traceheader omits the flag instead of asserting-0. The span id comes from the scope'spropagationSpanId(a fresh id is generated when the scope has none).getDynamicSamplingContextFromSpanresolves a placeholder's DSC from its captured scope (continued traces keep the incoming DSC; new traces derive it from the client).The scope is only consulted for genuine TwP placeholders. A non-recording span in tracing mode, the child of an unsampled span, or an ignored span carries an explicit negative decision and keeps propagating
-0viaspanToTraceHeader.A new (head-of-trace) TwP trace does not stamp a local
transactionin its DSC; continued traces still propagate the upstream decision and DSC.No DSC is written to the scope at span start, preserving the browser's "scope stays DSC-free between navigations" behavior.
This is an alternative to #21406