Track the actual item count in BlockingCollection

[aw0lid]

Description

BlockingCollection.IsCompleted currently uses _occupiedNodes.CurrentCount to determine whether the collection is empty.

However, _occupiedNodes is also used to synchronize consumer operations. This can cause IsCompleted to temporarily report true even though the collection still contains an item.

For example:

1. A BlockingCollection contains a single item.
2. CompleteAdding() is called, so IsCompleted is initially false.
3. A consumer starts a Take operation and successfully waits on _occupiedNodes.
4. _occupiedNodes.CurrentCount becomes 0, causing IsCompleted to return true even though the item has not yet been removed from the collection.
5. The consumer operation is canceled before the item is removed.
6. _occupiedNodes.CurrentCount is restored to 1.
7. IsCompleted then returns to false.

As a result, IsCompleted can transition from false to true and back to false even though the item was never removed from the collection.

Fix

Introduce a separate _actualCount field to track the number of items successfully added to and removed from the collection.

_actualCount is initialized from the existing collection count and updated atomically after each successful add or remove operation, specifically in TryAddWithNoTimeValidation and TryTakeWithNoTimeValidation.

Count and IsCompleted now use _actualCount instead of _occupiedNodes.CurrentCount.

This separates the synchronization state of _occupiedNodes from the actual item count and prevents consumer cancellation from changing the state reported by IsCompleted.

Tests

Added a regression test that verifies that IsCompleted remains false when:

• the collection contains a single item,
• CompleteAdding() has been called,
• a consumer waits to take the item using a cancellation token,
• the operation is canceled before the item is removed.

The test then removes the remaining item and verifies that IsCompleted becomes true.

Closes #109217

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[gfoidl]

Interlocked.Add can't be used due to build for older targets?

[gfoidl]

You can drop the volatile read, because below the interlocked operation provides the barrier.