memory-leak-audit

SKILL.md

Memory Leak Audit

The #1 bug category in VS Code. This skill encodes the patterns that prevent and fix leaks.

When to Use

Reviewing code that registers event listeners or DOM handlers

Fixing reported memory leaks (listener counts growing over time)

Creating objects in methods that are called repeatedly

Working with model lifecycle events (onWillDispose, onDidClose)

Adding event subscriptions in constructors or setup methods

Audit Checklist

Work through each check in order. A single missed pattern can cause thousands of leaked objects.

Step 1: DOM Event Listeners

Rule: Never use raw .onload, .onclick, or addEventListener() directly. Always use addDisposableListener().
// BAD — leaks a listener every call
this.iconElement.onload = () => { ... };

// GOOD — tracked and disposable
this._register(addDisposableListener(this.iconElement, 'load', () => { ... }));

Validated by: PR #280566 — Extension icon widget leaked 185 listeners after 37 toggles.

Step 2: One-Time Events

Rule: Use Event.once() for events that should only fire once (lifecycle events, close events, first-change events).

// BAD — listener stays registered forever after first fire
model.onDidDispose(() => store.dispose());

// GOOD — auto-removes after first invocation
Event.once(model.onDidDispose)(() => store.dispose());

Validated by: PRs #285657, #285661 — Terminal lifecycle hacks replaced with Event.once().

Step 3: Repeated Method Calls

Rule: Objects created in methods called multiple times must NOT be registered to the class this._register(). Use MutableDisposable or return IDisposable to the caller.

// BAD — every call adds another listener to the class store
startSearch() {
    this._register(this.model.onResults(() => { ... }));
}

// GOOD — MutableDisposable ensures max 1 listener
private readonly _searchListener = this._register(new MutableDisposable());

startSearch() {
    this._searchListener.value = this.model.onResults(() => { ... });
}

When the event should only fire once per method call, combine Event.once() with MutableDisposable — this auto-removes the listener after the first invocation while still guarding against repeated calls:

private readonly _searchListener = this._register(new MutableDisposable());

startSearch() {
    this._searchListener.value = Event.once(this.model.onResults)(() => { ... });
}

Validated by: PR #283466 — Terminal find widget leaked 1 listener per search.

Step 4: Model-Tied DisposableStores

Rule: When creating a DisposableStore tied to a model's lifetime, register model.onWillDispose(() => store.dispose()) to the store itself.

const store = new DisposableStore();
store.add(model.onWillDispose(() => store.dispose()));
store.add(model.onDidChange(() => { ... }));

Validated by: Pattern used in chatEditingSession.ts, fileBasedRecommendations.ts, testingContentProvider.ts.

Step 5: Resource Pool Patterns

Rule: When using factory methods that create pooled objects (lists, trees), disposables must be registered to the individual item, not the pool class.

// BAD — registers to pool, never cleaned per item
createItem() {
    const item = new Item();
    this._register(item.onEvent(() => { ... }));
    return item;
}

// GOOD — wrap with item-scoped disposal
createItem(): IDisposable & Item {
    const store = new DisposableStore();
    const item = new Item();
    store.add(item.onEvent(() => { ... }));
    return { ...item, dispose: () => store.dispose() };
}

Validated by: PR #290505 — Chat content parts CollapsibleListPool and TreePool leaked disposables.

Step 6: Test Validation

Rule: Every test suite that creates disposable objects must call ensureNoDisposablesAreLeakedInTestSuite().

import { ensureNoDisposablesAreLeakedInTestSuite } from '../../../../base/test/common/utils.js';

suite('MyFeature', () => {
    ensureNoDisposablesAreLeakedInTestSuite();

    test('does something', () => {
        // test disposables are tracked automatically
    });
});

Quick Reference

Scenario
Pattern
Anti-Pattern

DOM events
addDisposableListener()
.onclick =, addEventListener()

One-time events
Event.once(event)(handler)
event(handler) for lifecycle

Repeated methods
MutableDisposable or return IDisposable
this._register() in non-constructor

Model lifecycle
store.add(model.onWillDispose(...))
Forgetting cleanup

Pooled objects
Item-scoped DisposableStore
Pool-scoped this._register()

Tests
ensureNoDisposablesAreLeakedInTestSuite()
No leak checking

Verification

After fixing leaks, verify by:

Checking listener counts before/after repeated operations

Running ensureNoDisposablesAreLeakedInTestSuite() in tests

Confirming object counts stabilize (don't grow linearly with usage)

For chat-specific leaks: Run the chat memory leak checker via npm run perf:chat-leak (see the chat-perf skill). It sends N messages in a single session, forces GC between each, and uses linear regression on heap/DOM samples to detect per-message growth. A slope above 2 MB/msg indicates a leak. Use --messages 20 --verbose for more accurate results.