CSHARP-5798: Implement test to track if any UnobservedTaskExceptions were raised while test run

[sanych-sun]

No description provided.

[Copilot]

Pull request overview

This PR implements tracking for unobserved task exceptions during test execution to help identify tasks that complete with exceptions without proper handling. The implementation uses xUnit's extensibility to register a global handler that captures unobserved exceptions and verifies none occurred after all tests complete.

Key Changes

• Added custom xUnit test case and discoverer infrastructure to track UnobservedTaskException events globally
• Implemented a test that runs after all other tests to verify no unobserved exceptions were raised
• Fixed a race condition in CSharp3302Tests where an unhandled task was causing unobserved exceptions

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
UnobservedExceptionTrackingTestCase.cs	New test case class that registers/unregisters the UnobservedTaskException handler and stores exceptions in a static list
UnobservedExceptionTestDiscoverer.cs	New xUnit test discoverer and fact attribute to enable unobserved exception tracking
TimeoutEnforcingXunitTestAssemblyRunner.cs	Modified to run the unobserved exception tracking test after all other tests complete
UnobservedTaskExceptionTracking.cs	New test class that verifies no unobserved exceptions were captured during test execution
CSharp3302Tests.cs	Fixed to properly handle task cancellation to prevent unobserved exceptions
Cluster.cs	Added disposal check to prevent ObjectDisposedException from timer callback

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[Copilot]

The field name __unobservedExceptions uses double underscore prefix which is unconventional for C# public fields. Consider using standard PascalCase naming like UnobservedExceptions or if it needs to remain internal/special, a single underscore prefix like _unobservedExceptions with appropriate access modifier.

[Copilot]

The static __unobservedExceptions list is not thread-safe. Since xUnit can run tests in parallel, multiple threads may attempt to add to this list simultaneously, causing potential race conditions. Consider using ConcurrentBag<string> or ConcurrentQueue<string> instead of List<string>.

[Copilot]

The UnobservedTaskException handler is registered in the constructor of each test case instance, potentially leading to multiple registrations of the same handler. Since the handler writes to a static list, this creates duplicate tracking. The handler should be registered once globally, not per test case instance.

Suggested change

	TaskScheduler.UnobservedTaskException += UnobservedTaskExceptionEventHandler;
	}
	public override void Dispose()
	{
	base.Dispose();
	TaskScheduler.UnobservedTaskException -= UnobservedTaskExceptionEventHandler;
	}
	static UnobservedExceptionTrackingTestCase()
	{
	TaskScheduler.UnobservedTaskException += UnobservedTaskExceptionEventHandler;
	}
	public override void Dispose()
	{
	base.Dispose();

[Copilot]

[nitpick] The formatting change converts this to a multi-line method but the actual logic is still a single return statement. Consider keeping the expression-bodied member syntax => for consistency with the codebase style, or add meaningful logic to justify the block body.

Suggested change

	{
	return new TimeoutEnforcingXunitTestCollectionRunner(testCollection, testCases, DiagnosticMessageSink, messageBus, TestCaseOrderer, new ExceptionAggregator(Aggregator), cancellationTokenSource).RunAsync();
	}
	=> new TimeoutEnforcingXunitTestCollectionRunner(testCollection, testCases, DiagnosticMessageSink, messageBus, TestCaseOrderer, new ExceptionAggregator(Aggregator), cancellationTokenSource).RunAsync();

[BorisDog]

Is it better to have try-catch with catching ObjectDisposed ?
Checking for _disposed is still racy.

[BorisDog]

Does this need to be threadsafe?
Also consider removing UnobservedExceptionTrackingTestCase.

[sanych-sun]

We do not run tests in parallel, at least for now. But probably it makes sense to use something from thread-safe collections.

[BorisDog]

I was wondering whether exceptions can be thrown for different threads?
Or is UnobservedTaskExceptionEventHandler guaranteed to be called from a single thread?

[sanych-sun]

Nope, it will be executed on different threads. Sure will use ConcurrentBag instead.