fix(core): Avoid prolonging idle span when starting standalone span (#16928)

When we we start and send a web vital
standalone span while an idlespan (e.g. pageload) was still running,
we'd restart the idle span's child span timeout (i.e. prolong its
duration potentially). Is is unintended because by definition the
standalone span should not be associated with a potentially ongoing idle
span and its tree.

This can happen, for example when users hide the page while the pageload
span is still active, causing web vitals to be reported.

Author: Lms24

packages/core/src/tracing/idleSpan.ts

@@ -17,6 +17,7 @@ import {
 import { timestampInSeconds } from '../utils/time';
 import { freezeDscOnSpan, getDynamicSamplingContextFromSpan } from './dynamicSamplingContext';
 import { SentryNonRecordingSpan } from './sentryNonRecordingSpan';
+import { SentrySpan } from './sentrySpan';
 import { SPAN_STATUS_ERROR } from './spanstatus';
 import { startInactiveSpan } from './trace';
 
@@ -326,7 +327,12 @@ export function startIdleSpan(startSpanOptions: StartSpanOptions, options: Parti
       // or if this is the idle span itself being started,
       // or if the started span has already been closed,
       // we don't care about it for activity
-      if (_finished || startedSpan === span || !!spanToJSON(startedSpan).timestamp) {
+      if (
+        _finished ||
+        startedSpan === span ||
+        !!spanToJSON(startedSpan).timestamp ||
+        (startedSpan instanceof SentrySpan && startedSpan.isStandaloneSpan())
+      ) {
         return;
       }
 

packages/core/test/lib/tracing/idleSpan.test.ts

@@ -586,6 +586,28 @@ describe('startIdleSpan', () => {
       expect(spanToJSON(idleSpan).status).not.toEqual('deadline_exceeded');
       expect(spanToJSON(idleSpan).timestamp).toBeDefined();
     });
+
+    it("doesn't reset the timeout for standalone spans", () => {
+      const idleSpan = startIdleSpan({ name: 'idle span' }, { finalTimeout: 99_999 });
+      expect(idleSpan).toBeDefined();
+
+      // Start any span to cancel idle timeout
+      startInactiveSpan({ name: 'span' });
+
+      // Wait some time
+      vi.advanceTimersByTime(TRACING_DEFAULTS.childSpanTimeout - 1000);
+      expect(spanToJSON(idleSpan).status).not.toEqual('deadline_exceeded');
+      expect(spanToJSON(idleSpan).timestamp).toBeUndefined();
+
+      // new standalone span should not reset the timeout
+      const standaloneSpan = startInactiveSpan({ name: 'standalone span', experimental: { standalone: true } });
+      expect(standaloneSpan).toBeDefined();
+
+      // Wait for timeout to exceed
+      vi.advanceTimersByTime(1001);
+      expect(spanToJSON(idleSpan).status).not.toEqual('deadline_exceeded');
+      expect(spanToJSON(idleSpan).timestamp).toBeDefined();
+    });
   });
 
   describe('disableAutoFinish', () => {