Fix race condition in TaskRegistry/TypeLoader when building with /mt /m mode

[Copilot]

Fixes: #12735 #12645

Context

A race condition in the TypeLoader class caused System.ObjectDisposedException crashes when building projects with /mt /m (multi-threaded mode). The error occurred when multiple threads simultaneously loaded task types using MetadataLoadContext:

MSBUILD : error : System.ObjectDisposedException: This object is no longer valid because the MetadataLoadContext that created it has been disposed.
   at System.Reflection.TypeLoading.RoProperty.get_Name()
   at Microsoft.Build.Execution.ReflectableTaskPropertyInfo..ctor(PropertyInfo propertyInfo, ...)
   at Microsoft.Build.Shared.LoadedType..ctor(Type type, ...)

Root Cause

The MetadataLoadContext _context field in TypeLoader.cs (line 46) was declared as static, causing it to be shared across all threads:

1. Thread A creates a MetadataLoadContext and stores it in the static _context field
2. Thread B overwrites the static _context with its own MetadataLoadContext
3. Thread A attempts to access properties on types loaded from its (now-replaced) context
4. Thread B disposes the _context
5. Thread A crashes with ObjectDisposedException when accessing propertyInfo.Name

Changes Made

Made the MetadataLoadContext instance-local instead of static to eliminate the shared state:

1. Removed the static _context field - Eliminated the race condition source
2. Renamed and refactored CreateMetadataLoadContext - Changed from LoadAssemblyUsingMetadataLoadContext to return only MetadataLoadContext to enable idiomatic C# resource management with using statement. The function name now accurately reflects that it only creates the context without loading assemblies.
3. Updated GetLoadedTypeFromTypeNameUsingMetadataLoadContext - Uses using statement for automatic disposal instead of manual disposal in finally block. Assembly loading now happens separately after context creation.

Why This Works

The MetadataLoadContext is only used to extract metadata (property names, types, attributes) which is stored as strings/primitives during LoadedType construction. All metadata extraction completes before context disposal via the using statement. The actual task execution happens in the TaskHost process where assemblies are loaded normally, so no access to disposed contexts occurs during runtime.

Testing

• ✅ Full build completes successfully
• ✅ All TypeLoader unit tests pass
• ✅ All 59 TaskRegistry unit tests pass
• ✅ Sample projects build correctly

Notes

The use of C#'s using statement for resource disposal follows .NET best practices and makes the code cleaner and more maintainable compared to manual disposal in a finally block. The function naming has been updated to accurately reflect its purpose (creating a context rather than loading an assembly).

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[Copilot]

Pull Request Overview

This PR fixes a race condition in the TypeLoader class that caused System.ObjectDisposedException crashes when building with /mt /m (multi-threaded mode). The root cause was a static MetadataLoadContext _context field being shared across threads, leading to one thread disposing the context while another thread was still using types loaded from it.

Key Changes

• Removed the static _context field to eliminate shared state between threads
• Modified LoadAssemblyUsingMetadataLoadContext to return a tuple containing both the assembly and its associated context
• Wrapped type-finding logic in a try-finally block to ensure proper disposal of each thread's local context after metadata extraction

[JanProvaznik]

I think this fixes the issue appropriately. There was no reason for the context to be in a field, and the disposal pattern is corrected