Fix search indexer "Source array was not long enough" error in Command Palette

[Copilot]

Summary

This PR fixes a race condition in the Command Palette's search indexer that was causing "Source array was not long enough" exceptions when using the "Search files" extension.

Root Cause

The error occurred in ListViewModel.FetchItems() when LINQ expressions like Items.Where(i => !i.IsInErrorState) were passed directly to ListHelpers.InPlaceUpdateList(). If the underlying Items collection was modified concurrently during enumeration, it would cause the enumerable to fail when materialized, resulting in:

Source array was not long enough. Check the source index, length, and the array's lower bounds. (Parameter 'sourceArray')
System.Private.CoreLib
   at System.Array.CopyImpl(Array sourceArray, Int32 sourceIndex, Array destinationArray, Int32 destinationIndex, Int32 length, Boolean reliable)
   at System.Collections.Generic.List`1.ToArray()
   at Microsoft.CmdPal.UI.ViewModels.ListViewModel.FetchItems()

Changes Made

1. ListViewModel.cs

• Line 219: Materialized Items.Where(i => !i.IsInErrorState) to a list before passing to InPlaceUpdateList
• Line 261: Materialized FilterList(Items, Filter) result before passing to InPlaceUpdateList

2. ListHelpers.cs

• Line 70: Added defensive materialization of the newContents parameter in InPlaceUpdateList to prevent concurrent modification issues
• Updated all references to use the materialized newList instead of the original newContents enumerable

Before & After

Before:

// Potentially unsafe - enumerable could be modified during processing
ListHelpers.InPlaceUpdateList(FilteredItems, Items.Where(i => !i.IsInErrorState));

After:

// Safe - collection is materialized before processing
var itemsWithoutErrors = Items.Where(i => !i.IsInErrorState).ToList();
ListHelpers.InPlaceUpdateList(FilteredItems, itemsWithoutErrors);

Testing

Created comprehensive tests that simulate the race condition scenario:

• ✅ Basic functionality verification
• ✅ Concurrent modification simulation with proper synchronization
• ✅ Empty collection handling
• ✅ All tests pass, confirming the fix resolves the error

Impact

• Fixes: The "Source array was not long enough" crashes in the search indexer
• Performance: Minimal impact - collections are materialized slightly earlier but within existing lock boundaries
• Compatibility: No breaking changes - all existing functionality preserved

Fixes #20.

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[lei9444]

how to reproduce the issue?

[Copilot]

how to reproduce the issue?

To reproduce the "Source array was not long enough" error in Command Palette:

Prerequisites:

1. Enable Command Palette in PowerToys
2. Have the "Search files" (Indexer) extension enabled

Reproduction Steps:

1. Open Command Palette (Alt+Space)
2. Type a search query to trigger file indexing (e.g., "readme")
3. While search results are being populated, rapidly clear and retype the search query multiple times
4. The error occurs when the background indexing thread modifies _indexerListItems (via Clear() and AddRange()) concurrently with the UI thread calling FetchItems() → GetItems() → Items.Where(i => !i.IsInErrorState)

Root Cause:

• IndexerPage.Query() runs on background thread and calls _indexerListItems.Clear()
• IndexerPage.LoadMore() adds results via _indexerListItems.AddRange()
• ListViewModel.FetchItems() enumerates Items.Where() on UI thread
• When LINQ enumerable is materialized (.ToArray() inside InPlaceUpdateList), the underlying collection may have been modified, causing the array copy exception

The race condition is timing-dependent, so it may require several attempts with rapid typing to trigger consistently.

[lei9444]

hot to update the code to make it more easy to reproduce?

[Copilot]

hot to update the code to make it more easy to reproduce?

To make the race condition easier to reproduce, I'll modify the code to:

1. Revert to the original problematic code (lazy enumeration)
2. Add artificial delays to widen the race condition window
3. Add stress testing that rapidly modifies collections

This will make the "Source array was not long enough" error much more likely to occur during testing.