DataBuffer should offer restricted access to underlying ByteBuffer

[poutsma]

Netty 4's ByteBuf allowed for direct, unrestricted access of the underlying ByteBuffer(s) through the nioBuffers method. This allowed for integration with ByteBuffer-based APIs (including java.nio), but also opened the door for memory leaks. Spring 5 exposed nioBuffer through DataBuffer::asByteBuffer.

Netty 5's Buffer does not allow unrestricted access to the underlying ByteBuffer. As a consequence, in Spring 6 DataBuffer::toByteBuffer was introduced. This method makes a copy, and is implemented in Netty5DataBuffer::toByteBuffer using Buffer::copyInto.

In various places throughout Spring Framework 6, including frequently invoked code such as codecs and transports, DataBuffer::toByteBuffer is used to integrate with ByteBuffer-based APIs. This means that buffers are copied more frequently than in Spring 5, even in other server environments than Netty. See #29889.

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Netty 5's Buffer does allow restricted access to the underlying ByteBuffer, through Buffer::forEachComponent. Memory leaks are negated by only allowing access to the raw memory for the scope of the returned iterator, like so:

  try (var iteration = buffer.forEachReadable()) {
          for (var c = iteration.first(); c != null; c = c.next()) {
              ByteBuffer componentBuffer = c.readableBuffer();
              // ...
          }
      }

We should expose Netty 5's Buffer::forEachComponent through a method in DataBuffer, implement that method in Netty 4 using ByteBuf::nioBuffers, and write an implementations for DefaultDataBuffer.