Fix duplicate completion handling in DataBufferUtils

[abrar2309]

Fix duplicate completion handling in DataBufferUtils.readAsynchronousFileChannel().

Problem

On Windows, an AsynchronousFileChannel completion handler may be invoked more than once for the same read operation.

ReadCompletionHandler.completed() currently processes each callback independently. If the same Attachment is delivered to completed() twice, the associated DataBuffer can be released more than once.

Solution

Associate an AtomicBoolean completion guard with each Attachment.

The first invocation of completed() marks the attachment as completed and proceeds normally. Any subsequent invocation for the same attachment returns immediately.

This prevents duplicate completion callbacks from closing the iterator and releasing the same DataBuffer more than once.

Tests

Added a regression test that invokes the completion handler twice with the same attachment and verifies that the PooledDataBuffer is released only once.

Validation

• :spring-core:test --tests "org.springframework.core.io.buffer.DataBufferUtilsTests.readAsynchronousFileChannelDuplicateCompletion"
• :spring-core:compileJava

Both completed successfully.