fix(core): App start data lost when first transaction is unsampled (#5756)

* fix(core): Skip unsampled spans when recording first root span ID for 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 <[email protected]>

* fix(core): Update CHANGELOG entry wording

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

Author: antonis

CHANGELOG.md

@@ -45,6 +45,7 @@
 
 ### Fixes
 
+- App start data not attached to sampled transactions when preceded by unsampled transactions ([#5756](https://github.com/getsentry/sentry-react-native/pull/5756))
 - Resolve relative `SOURCEMAP_FILE` paths against the project root in the Xcode build script ([#5730](https://github.com/getsentry/sentry-react-native/pull/5730))
 - Fixes the issue with unit mismatch in `adjustTransactionDuration` ([#5740](https://github.com/getsentry/sentry-react-native/pull/5740))
 - Handle `inactive` state for spans ([#5742](https://github.com/getsentry/sentry-react-native/pull/5742))

packages/core/src/js/tracing/integrations/appStart.ts

@@ -7,6 +7,7 @@ import {
   getCurrentScope,
   SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN,
   SentryNonRecordingSpan,
+  spanIsSampled,
   startInactiveSpan,
   timestampInSeconds,
 } from '@sentry/core';
@@ -241,6 +242,10 @@ export const appStartIntegration = ({
       return;
     }
 
+    if (!spanIsSampled(rootSpan)) {
+      return;
+    }
+
     setFirstStartedActiveRootSpanId(rootSpan.spanContext().spanId);
   };
 

packages/core/test/tracing/integrations/appStart.test.ts

@@ -5,6 +5,7 @@ import {
   getIsolationScope,
   SEMANTIC_ATTRIBUTE_SENTRY_OP,
   SEMANTIC_ATTRIBUTE_SENTRY_ORIGIN,
+  SentryNonRecordingSpan,
   setCurrentClient,
   startInactiveSpan,
   timestampInSeconds,
@@ -463,6 +464,50 @@ describe('App Start Integration', () => {
       });
     });
 
+    it('Does not lock firstStartedActiveRootSpanId to unsampled root span', async () => {
+      mockAppStart({ cold: true });
+
+      const integration = appStartIntegration();
+      const client = new TestClient({
+        ...getDefaultTestClientOptions(),
+        enableAppStartTracking: true,
+        tracesSampleRate: 1.0,
+      });
+      setCurrentClient(client);
+      integration.setup(client);
+      integration.afterAllSetup(client);
+
+      // Simulate an unsampled root span starting first
+      const unsampledSpan = new SentryNonRecordingSpan();
+      client.emit('spanStart', unsampledSpan);
+
+      // Then a sampled root span starts
+      const sampledSpan = startInactiveSpan({
+        name: 'Sampled Root Span',
+        forceTransaction: true,
+      });
+      const sampledSpanId = sampledSpan.spanContext().spanId;
+
+      // Process a transaction event matching the sampled span
+      const event = getMinimalTransactionEvent();
+      event.contexts!.trace!.span_id = sampledSpanId;
+
+      const actualEvent = await processEventWithIntegration(integration, event);
+
+      // App start should be attached to the sampled transaction
+      const appStartSpan = (actualEvent as TransactionEvent)?.spans?.find(
+        ({ description }) => description === 'Cold Start',
+      );
+      expect(appStartSpan).toBeDefined();
+      expect(appStartSpan).toEqual(
+        expect.objectContaining({
+          description: 'Cold Start',
+          op: APP_START_COLD_OP,
+        }),
+      );
+      expect((actualEvent as TransactionEvent)?.measurements?.[APP_START_COLD_MEASUREMENT]).toBeDefined();
+    });
+
     it('Adds Cold App Start Span to Active Span', async () => {
       const [timeOriginMilliseconds, appStartTimeMilliseconds] = mockAppStart({ cold: true });