fix: normalize extension-provided CancellationToken in _findFilesBase (fixes #318999)

[vs-code-engineering]

Summary

Error: TypeError: i.onCancellationRequested is not a function in raceCancellationError when the SearchService.doSearch method passes an invalid token to raceCancellationError.

Root Cause: Extensions that bundle their own CancellationToken module (e.g., GitHub Copilot Chat) create tokens from a separate MutableToken class. These tokens fail the instanceof MutableToken check in the RPC protocol's isCancellationToken(). While they normally pass the duck-type fallback check, under certain bundling/runtime conditions the token is not recognized, causing it to be serialized over the wire as a plain object. Since functions and getters don't survive serialization, the main thread receives { _isCancelled: false, _emitter: null } — truthy but without onCancellationRequested.

Fixes #318999
Recommended reviewer: @roblourens

Culprit Commit

Most likely b02737df (May 14, 2026) by @osortega — "Add experimental opt-in to honor .gitignore in findFiles extension API (#316349)". This commit restructured _findFilesImpl and extracted _findFilesBase, creating the code path where extension-provided tokens are passed directly to the RPC proxy without validation.

Code Flow

sequenceDiagram
    participant Ext as Copilot Extension
    participant EH as ExtHostWorkspace
    participant RPC as RPC Protocol
    participant MT as MainThreadWorkspace
    participant SS as SearchService

    Ext->>EH: findFiles2(patterns, opts, bundledToken)
    EH->>EH: _findFilesImpl -> _findFilesBase(queryOptions, token)
    EH->>RPC: proxy.$startFileSearch(folder, options, bundledToken)
    Note over RPC: isCancellationToken(bundledToken) fails
    RPC->>RPC: Serializes token as plain object
    RPC->>MT: $startFileSearch(folder, options, brokenToken)
    MT->>SS: fileSearch(query, brokenToken)
    SS->>SS: doSearch -> raceCancellationError(promise, brokenToken)
    Note over SS: brokenToken.onCancellationRequested is undefined
    SS-->>SS: TypeError thrown

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Affected Files

File	Role
src/vs/workbench/api/common/extHostWorkspace.ts	Producer — passes unvalidated token to RPC
src/vs/workbench/services/search/common/searchService.ts	Consumer — calls raceCancellationError(promise, token)
src/vs/base/common/async.ts	Crash site — token.onCancellationRequested()
src/vs/workbench/api/browser/mainThreadWorkspace.ts	Intermediary — receives broken token from RPC

Repro Steps

1. Use VS Code Insiders on Linux with Copilot Chat extension v0.50.0
2. Trigger a file search operation via Copilot Chat (e.g., ask it to find files)
3. The Copilot extension's bundled CancellationTokenSource creates a token
4. Under certain conditions (likely bundler optimization or module duplication), the token fails isCancellationToken() in the RPC layer
5. The serialized token arrives on the main thread without onCancellationRequested
6. SearchService.doSearch crashes when calling raceCancellationError

How the Fix Works

Chosen approach (src/vs/workbench/api/common/extHostWorkspace.ts):

Added a validation guard in _findFilesBase that checks if the incoming token is recognized by CancellationToken.isCancellationToken() before passing it to the RPC proxy. If the token is not recognized (fails the instanceof and duck-type checks), a new CancellationTokenSource is created and linked to the original token's onCancellationRequested event. The linked token — which IS recognized by the RPC — is used instead. This fixes the problem at the data producer boundary (where the unrecognized token enters the RPC layer), not at the crash site.

Alternatives considered:

• Adding a guard in mainThreadWorkspace.$startFileSearch to validate the received token — this would be fixing at the crash site rather than preventing bad data from being produced.
• Modifying raceCancellationError to handle missing onCancellationRequested — this silences the error instead of fixing the cause.

Recommended Owner

@roblourens — owns the search/workspace extension API area where the fix is applied.

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