Race condition in InMemoryTaskStore causes flaky test failures

[halter73]

Summary

The in-memory test data store InMemoryTaskStore used by McpServerTaskTests is not thread-safe, resulting in occasional test failures due to field mutation races between test background tasks and server polling.

Symptoms:

• Flaky failures in CallToolAsync_AsyncTool_FailedTask_ThrowsMcpException (and similar tests)
• Stack trace shows Nullable object must have a value thrown from GetTask when Status == Failed but Error is null.
• The client receives a McpProtocolException (internal error) instead of the expected McpException for failed tasks.

How to Reproduce:

• Run the test suite repeatedly (e.g., on CI, especially macOS)
• Failing run shows error:

  Assert.Throws() Failure: Exception type was not an exact match
  Expected: typeof(ModelContextProtocol.McpException)
  Actual:   typeof(ModelContextProtocol.McpProtocolException)
  System.InvalidOperationException: Nullable object must have a value.
    at InMemoryTaskStore.GetTask(...)
  

Root Cause

• The test's FailTask sets entry.Status = Failed before entry.Error, so a concurrent reader (GetTask, called by the server handler) can see a failed status with Error == null.
• GetTask then dereferences entry.Error!.Value, which throws.
• Similar races exist in other methods mutating/testing TaskEntry fields.

Solution

• Make InMemoryTaskStore atomic for each operation: lock around mutations and reads, or replace ConcurrentDictionary + mutable TaskEntry fields with an immutable snapshot approach.
• For a minimal fix: add a private object _gate lock to synchronize all mutations and reads, and always lock the critical section.
• Also reorder assignments in FailTask etc. so all fields are valid before status is set.

Code suggestion

private readonly object _gate = new();
// ...
public GetTaskResult GetTask(string taskId)
{
    lock (_gate)
    {
        // ... see full solution in assistant's comment ...
    }
}
// ... lock everywhere else ...

Impact

• This causes rare but real test failures on CI, especially with concurrent test scheduling.

References

• Failing job: Build and Test matrix in .github/workflows/ci-build-test.yml
• Example failure: CallToolAsync_AsyncTool_FailedTask_ThrowsMcpException in McpServerTaskTests.cs