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Nov 17, 2025
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ref(browser): Move trace lifecycle listeners to class function #18231

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138 changes: 68 additions & 70 deletions packages/browser/src/profiling/UIProfiler.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -62,76 +62,7 @@ export class UIProfiler {
this._client = client;
this._sessionSampled = sessionSampled;

client.on('spanStart', span => {
if (!this._sessionSampled) {
DEBUG_BUILD && debug.log('[Profiling] Session not sampled because of negative sampling decision.');
return;
}
if (span !== getRootSpan(span)) {
return;
}
// Only count sampled root spans
if (!span.isRecording()) {
DEBUG_BUILD && debug.log('[Profiling] Discarding profile because root span was not sampled.');
return;
}

// Matching root spans with profiles
getGlobalScope().setContext('profile', {
profiler_id: this._profilerId,
});

const spanId = span.spanContext().spanId;
if (!spanId) {
return;
}
if (this._activeRootSpanIds.has(spanId)) {
return;
}

this._activeRootSpanIds.add(spanId);
const rootSpanCount = this._activeRootSpanIds.size;

const timeout = setTimeout(() => {
this._onRootSpanTimeout(spanId);
}, MAX_ROOT_SPAN_PROFILE_MS);
this._rootSpanTimeouts.set(spanId, timeout);

if (rootSpanCount === 1) {
DEBUG_BUILD &&
debug.log(
`[Profiling] Root span with ID ${spanId} started. Will continue profiling for as long as there are active root spans (currently: ${rootSpanCount}).`,
);

this.start();
}
});

client.on('spanEnd', span => {
if (!this._sessionSampled) {
return;
}

const spanId = span.spanContext().spanId;
if (!spanId || !this._activeRootSpanIds.has(spanId)) {
return;
}

this._activeRootSpanIds.delete(spanId);
const rootSpanCount = this._activeRootSpanIds.size;

DEBUG_BUILD &&
debug.log(
`[Profiling] Root span with ID ${spanId} ended. Will continue profiling for as long as there are active root spans (currently: ${rootSpanCount}).`,
);
if (rootSpanCount === 0) {
this._collectCurrentChunk().catch(e => {
DEBUG_BUILD && debug.error('[Profiling] Failed to collect current profile chunk on `spanEnd`:', e);
});

this.stop();
}
});
this._setupTraceLifecycleListeners(client);
}

/**
Comment on lines 62 to 68
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@sentry sentry bot Nov 17, 2025

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Bug: notifyRootSpanActive() fails to register a safeguard timeout for already-active root spans, leading to indefinite profiling if spanEnd is not called.
Severity: CRITICAL | Confidence: 0.95

🔍 Detailed Analysis

The notifyRootSpanActive() method, used for already-active root spans during profiler initialization, adds the span ID to _activeRootSpanIds but fails to call _registerTraceRootSpan(). This omission means no safeguard timeout is registered for these spans. If such a pre-existing root span never fires its spanEnd event, the profiler will continue running indefinitely, wasting resources by continuously collecting and sending profile chunks every 60 seconds, contradicting the intended design of MAX_ROOT_SPAN_PROFILE_MS.

💡 Suggested Fix

Modify notifyRootSpanActive() to call _registerTraceRootSpan(spanId) instead of directly adding spanId to _activeRootSpanIds. This ensures that all active root spans, regardless of how they are initialized, have the necessary safeguard timeout registered.

🤖 Prompt for AI Agent 
Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: packages/browser/src/profiling/UIProfiler.ts#L62-L68

Potential issue: The `notifyRootSpanActive()` method, used for already-active root spans
during profiler initialization, adds the span ID to `_activeRootSpanIds` but fails to
call `_registerTraceRootSpan()`. This omission means no safeguard timeout is registered
for these spans. If such a pre-existing root span never fires its `spanEnd` event, the
profiler will continue running indefinitely, wasting resources by continuously
collecting and sending profile chunks every 60 seconds, contradicting the intended
design of `MAX_ROOT_SPAN_PROFILE_MS`.

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

Reference_id: 2726774

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@s1gr1d s1gr1d Nov 17, 2025 •
edited
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This is done in another function - no problem here

It's kept track of it in _rootSpanTimeouts

Expand Down Expand Up @@ -170,6 +101,9 @@ export class UIProfiler {

DEBUG_BUILD && debug.log('[Profiling] Started profiling with profile ID:', this._profilerId);

// Expose profiler_id to match root spans with profiles
getGlobalScope().setContext('profile', { profiler_id: this._profilerId });
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@cursor cursor bot Nov 17, 2025

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Bug: Profiler Context: Spans Adrift

Moving the global scope context setting from the spanStart listener to start() breaks profile-span association when the profiler fails to initialize. If the first root span fails to start the profiler, _resetProfilerInfo() clears the context and sets _isRunning to false. Subsequent root spans won't call start() (since rootSpanCount !== 1), leaving them without the profiler_id context needed to match spans with profiles.

Fix in Cursor Fix in Web

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@s1gr1d s1gr1d Nov 17, 2025

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Works as intended, this is still attached at the correct time.


this._startProfilerInstance();

if (!this._profiler) {
Expand Down Expand Up @@ -203,6 +137,63 @@ export class UIProfiler {
});
}

/** Trace-mode: attach spanStart/spanEnd listeners. */
private _setupTraceLifecycleListeners(client: Client): void {
client.on('spanStart', span => {
if (!this._sessionSampled) {
DEBUG_BUILD && debug.log('[Profiling] Session not sampled because of negative sampling decision.');
return;
}
if (span !== getRootSpan(span)) {
return; // only care about root spans
}
// Only count sampled root spans
if (!span.isRecording()) {
DEBUG_BUILD && debug.log('[Profiling] Discarding profile because root span was not sampled.');
return;
}

const spanId = span.spanContext().spanId;
if (!spanId || this._activeRootSpanIds.has(spanId)) {
return;
}

this._registerTraceRootSpan(spanId);

const rootSpanCount = this._activeRootSpanIds.size;
if (rootSpanCount === 1) {
DEBUG_BUILD &&
debug.log(
`[Profiling] Root span ${spanId} started. Profiling active while there are active root spans (count=${rootSpanCount}).`,
);
this.start();
}
});

client.on('spanEnd', span => {
if (!this._sessionSampled) {
return;
}
const spanId = span.spanContext().spanId;
if (!spanId || !this._activeRootSpanIds.has(spanId)) {
return;
}
this._activeRootSpanIds.delete(spanId);
const rootSpanCount = this._activeRootSpanIds.size;

DEBUG_BUILD &&
debug.log(
`[Profiling] Root span with ID ${spanId} ended. Will continue profiling for as long as there are active root spans (currently: ${rootSpanCount}).`,
);
if (rootSpanCount === 0) {
this._collectCurrentChunk().catch(e => {
DEBUG_BUILD && debug.error('[Profiling] Failed to collect current profile chunk on last `spanEnd`:', e);
});
this.stop();
}
});
}

/**
* Resets profiling information from scope and resets running state
*/
Expand All @@ -219,6 +210,13 @@ export class UIProfiler {
this._rootSpanTimeouts.clear();
}

/** Register root span and schedule safeguard timeout (trace mode). */
private _registerTraceRootSpan(spanId: string): void {
this._activeRootSpanIds.add(spanId);
const timeout = setTimeout(() => this._onRootSpanTimeout(spanId), MAX_ROOT_SPAN_PROFILE_MS);
this._rootSpanTimeouts.set(spanId, timeout);
}

/**
* Start a profiler instance if needed.
*/
Expand Down
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