Fix race condition in ActivationCollector.DeactivateInDueTimeOrder

[Copilot]

Problem

The DeactivateInDueTimeOrder method throws an ArgumentException when multiple threads concurrently modify the activation buckets during enumeration:

System.ArgumentException: The index is equal to or greater than the length of the array, 
or the number of elements in the dictionary is greater than the available space from index 
to the end of the destination array.
   at System.Linq.Enumerable.OrderedIterator`2.MoveNext()
   at Orleans.Runtime.ActivationCollector.DeactivateInDueTimeOrder(Int32 count, CancellationToken cancellationToken)

Root Cause

The method was calling buckets.OrderBy(b => b.Key) directly on a ConcurrentDictionary<DateTime, Bucket>. When LINQ's OrderBy internally materializes the collection to an array for sorting, concurrent modifications to the dictionary cause the array size calculation to become invalid, resulting in the exception.

While ConcurrentDictionary is thread-safe for individual operations, LINQ operations that materialize collections are not safe during concurrent modifications.

Solution

Changed line 365 in src/Orleans.Runtime/Catalog/ActivationCollector.cs:

- foreach (var bucket in buckets.OrderBy(b => b.Key))
+ foreach (var bucket in buckets.ToList().OrderBy(b => b.Key))

This creates a point-in-time snapshot of the dictionary before sorting, eliminating the race condition. This pattern is already used elsewhere in the same class (in the ToString() method at line 219), making the codebase consistent.

Testing

Added DeactivateInDueTimeOrder_HandlesRaceDuringEnumeration test that:

• Creates 100 activations
• Starts concurrent deactivation (attempting to deactivate 50)
• Simultaneously adds 50 new activations from another thread
• Verifies no exceptions are thrown and state remains consistent

This test would fail with the original code due to the race condition.

Impact

• Minimal change: Only 9 characters added (.ToList())
• No breaking changes: Same functionality, just thread-safe
• Performance: Negligible overhead from snapshot creation
• Consistency: Follows existing pattern in the same class

Fixes #9702

Warning

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>ActivationCollector.DeactivateInDueTimeOrder throws exception</issue_title>
<issue_description>Orleans version 9.1.2

System.ArgumentException: The index is equal to or greater than the length of the array, or the number of elements in the dictionary is greater than the available space from index to the end of the destination array.
   at System.Collections.Concurrent.ConcurrentDictionary`2.System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey,TValue>>.CopyTo(KeyValuePair`2[] array, Int32 index)
   at System.Linq.Enumerable.ICollectionToArray[TSource](ICollection`1 collection)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at System.Linq.Enumerable.OrderedIterator`2.MoveNext()
   at Orleans.Runtime.ActivationCollector.DeactivateInDueTimeOrder(Int32 count, CancellationToken cancellationToken) in D:\vrecluse\orleans\src\Orleans.Runtime\Catalog\ActivationCollector.cs:line 375
   at Orleans.Runtime.ActivationCollector.RunMemoryBasedDeactivationLoop() in D:\vrecluse\orleans\src\Orleans.Runtime\Catalog\ActivationCollector.cs:line 604
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@ReubenBond</author><body>
@vrecluse does this repro on 9.2.1?</body></comment_new>
</comments>

Fixes #9686

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Microsoft Reviewers: Open in CodeFlow

[DeagleGross]

I would close it in favor of #9740, which has a more consistent test reproducing the issue.