fix(core): App start data lost when first transaction is unsampled#5756
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fix(core): App start data lost when first transaction is unsampled#5756
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… app start When `tracesSampleRate < 1.0`, the first root span could be unsampled, permanently locking `firstStartedActiveRootSpanId` to a span that would never reach `processEvent`. This caused app start data to be lost for the entire session. Now we check `spanIsSampled()` before locking the ID, so app start attaches to the first sampled root span instead. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Semver Impact of This PR⚪ None (no version bump detected) 📋 Changelog PreviewThis is how your changes will appear in the changelog.
Plus 11 more 🤖 This preview updates automatically when you update the PR. |
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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q: what about users with sample rate near 0, aren't they going to lose the app start data anyway? |
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Author
Good point @lucas-zimerman 👍 |
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iOS (legacy) Performance metrics 🚀
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| Revision | Plain | With Sentry | Diff |
|---|---|---|---|
| ea3e26e+dirty | 1229.13 ms | 1228.46 ms | -0.67 ms |
| 80e4616+dirty | 1221.32 ms | 1225.64 ms | 4.32 ms |
| 818a608+dirty | 1205.76 ms | 1208.00 ms | 2.24 ms |
| 77061ed+dirty | 1233.16 ms | 1234.88 ms | 1.71 ms |
| bef3709+dirty | 1222.07 ms | 1220.24 ms | -1.83 ms |
| a206511+dirty | 1185.00 ms | 1186.35 ms | 1.35 ms |
| 74979ac+dirty | 1210.49 ms | 1213.31 ms | 2.82 ms |
| a2bb688+dirty | 1223.53 ms | 1232.90 ms | 9.37 ms |
| 8a868fe+dirty | 1221.50 ms | 1230.78 ms | 9.28 ms |
| d590428+dirty | 1211.77 ms | 1220.51 ms | 8.75 ms |
App size
| Revision | Plain | With Sentry | Diff |
|---|---|---|---|
| ea3e26e+dirty | 3.41 MiB | 4.58 MiB | 1.17 MiB |
| 80e4616+dirty | 3.38 MiB | 4.60 MiB | 1.22 MiB |
| 818a608+dirty | 2.63 MiB | 3.91 MiB | 1.28 MiB |
| 77061ed+dirty | 2.63 MiB | 3.98 MiB | 1.34 MiB |
| bef3709+dirty | 3.38 MiB | 4.78 MiB | 1.40 MiB |
| a206511+dirty | 3.41 MiB | 4.67 MiB | 1.25 MiB |
| 74979ac+dirty | 3.38 MiB | 4.60 MiB | 1.22 MiB |
| a2bb688+dirty | 2.63 MiB | 3.99 MiB | 1.36 MiB |
| 8a868fe+dirty | 3.38 MiB | 4.60 MiB | 1.22 MiB |
| d590428+dirty | 3.38 MiB | 4.78 MiB | 1.39 MiB |
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📢 Type of change
📜 Description
When
tracesSampleRate < 1.0, theappStartIntegrationlocksfirstStartedActiveRootSpanIdto the first root span regardless of whether it is sampled. SinceprocessEventonly runs for sampled events, if the first root span is unsampled, app start data is permanently lost for that session — subsequent sampled transactions fail the span ID match check.This PR adds a
spanIsSampled()guard inrecordFirstStartedActiveRootSpanIdso the ID only locks to the first sampled root span. The existing age/duration guards still protect against stale app starts.💡 Motivation and Context
Users with
tracesSampleRate < 1.0were losing Cold/Warm Start data whenever the first transaction happened to be unsampled (which is statistically likely at low sample rates). This is a data loss bug — the app start is measured by the native layer but never attached to any event.Fixes #5757
💚 How did you test it?
SentryNonRecordingSpan(unsampled) followed by a real sampled span, verifies app start attaches to the sampled transactionyarn test,yarn build,yarn lint)📝 Checklist
sendDefaultPIIis enabled🔮 Next steps
🤖 Generated with Claude Code