Collect PerformanceCollectionData only in sampled transactions

[stefanosiano]

📜 Description

performance data collection is now started only when the transaction is sampled

💡 Motivation and Context

We were running the performance collection even when the transaction was not sampled, so the data was lost.
Avoiding it will lower the overhead of performance data collection

💚 How did you test it?

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[seer-by-sentry]

Bug: compositePerformanceCollector fails to start for transactions with deferred sampling decisions that are later marked as sampled.
_{Severity: CRITICAL | Confidence: 0.98}

🔍 Detailed Analysis

When a transaction is created with a deferred sampling decision (samplingDecision = null), the SentryTracer constructor evaluates Boolean.TRUE.equals(isSampled()). Since isSampled() returns null, this condition is false, preventing compositePerformanceCollector.start() from being called. If the transaction is later sampled via setSamplingDecision(new TracesSamplingDecision(true)), the performance collector remains unstarted, resulting in a loss of performance data for that transaction. Additionally, compositePerformanceCollector.stop() is unconditionally called during finish(), potentially on an uninitialized collector.

💡 Suggested Fix

Modify the SentryTracer constructor to start compositePerformanceCollector if isSampled() is null, or add logic within setSamplingDecision() to start the collector if it wasn't previously started and the transaction becomes sampled.

🤖 Prompt for AI Agent

Fix this bug. In sentry/src/main/java/io/sentry/SentryTracer.java at lines 94-96: When a
transaction is created with a deferred sampling decision (`samplingDecision = null`),
the `SentryTracer` constructor evaluates `Boolean.TRUE.equals(isSampled())`. Since
`isSampled()` returns `null`, this condition is `false`, preventing
`compositePerformanceCollector.start()` from being called. If the transaction is later
sampled via `setSamplingDecision(new TracesSamplingDecision(true))`, the performance
collector remains unstarted, resulting in a loss of performance data for that
transaction. Additionally, `compositePerformanceCollector.stop()` is unconditionally
called during `finish()`, potentially on an uninitialized collector.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[stefanosiano]

this is the only case why we didn't do it before
should we ignore it?

[romtsn]

hmm, yeah I think this is valid actually. Probably we can't simply just do that without a breaking change - but even then I think we'd have to discuss this. I'd imagine if we ever do tail-based sampling (and we probably will), setSamplingDecision will be useful for that. So maybe this PR has to be backlogged for now

[stefanosiano]

got it
on the other side, if the sample rate is like 0.01, we'd run the performance collection 10k times more than needed 😅
Let's discuss it in the next sync

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	300.92 ms	342.73 ms	41.81 ms
Size	1.58 MiB	2.12 MiB	551.52 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
ee747ae	382.73 ms	435.41 ms	52.68 ms
96449e8	361.30 ms	423.39 ms	62.09 ms
ee747ae	374.71 ms	455.18 ms	80.47 ms
3699cd5	423.60 ms	495.52 ms	71.92 ms
b3d8889	371.84 ms	447.49 ms	75.65 ms
27d7cf8	369.82 ms	422.62 ms	52.80 ms
d364ace	411.72 ms	430.81 ms	19.10 ms
7314dbe	437.83 ms	505.64 ms	67.81 ms
ce0a49e	532.00 ms	609.96 ms	77.96 ms
3998a95	415.94 ms	478.54 ms	62.60 ms

App size

Revision	Plain	With Sentry	Diff
ee747ae	1.58 MiB	2.10 MiB	530.95 KiB
96449e8	1.58 MiB	2.11 MiB	539.35 KiB
ee747ae	1.58 MiB	2.10 MiB	530.95 KiB
3699cd5	1.58 MiB	2.10 MiB	533.45 KiB
b3d8889	1.58 MiB	2.10 MiB	535.07 KiB
27d7cf8	1.58 MiB	2.12 MiB	549.42 KiB
d364ace	1.58 MiB	2.11 MiB	539.75 KiB
7314dbe	1.58 MiB	2.10 MiB	533.45 KiB
ce0a49e	1.58 MiB	2.10 MiB	532.94 KiB
3998a95	1.58 MiB	2.10 MiB	532.96 KiB

Previous results on branch: stefanosiano/enh/collect-performance-sampled-profile

Startup times

Revision	Plain	With Sentry	Diff
a5d8753	343.00 ms	383.61 ms	40.61 ms
c4bc643	310.58 ms	365.24 ms	54.66 ms

App size

Revision	Plain	With Sentry	Diff
a5d8753	1.58 MiB	2.12 MiB	549.44 KiB
c4bc643	1.58 MiB	2.12 MiB	549.44 KiB

[markushi]

LGTM!