Memory Leak Audit

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

Verification

After fixing leaks, verify by: 1. Checking listener counts before/after repeated operations 2. Running ensureNoDisposablesAreLeakedInTestSuite() in tests 3. Confirming object counts stabilize (don't grow linearly with usage) 4. 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.