add comment in the PerMessageDeflateExtension IncomingFlusher

Signed-off-by: Lachlan Roberts <lachlan@webtide.com>

jetty-websocket/websocket-core-common/src/main/java/org/eclipse/jetty/websocket/core/internal/PerMessageDeflateExtension.java

@@ -464,6 +464,11 @@ public class PerMessageDeflateExtension extends AbstractExtension implements Dem
             chunk.setPayload(payload);
             chunk.setFin(frame.isFin() && complete);
 
+            // If we are complete we return true, then DemandingFlusher.process() will null out the Frame and Callback.
+            // The application may decide to hold onto the buffer and delay completing the callback, so we need to capture
+            // references to these in the payloadCallback and not rely on state of the flusher which may have moved on.
+            // This flusher could be failed while the application already has the payloadCallback, so we need protection against
+            // the flusher failing and the application completing the callback, that's why we use the payload AtomicReference.
             boolean completeCallback = complete;
             AtomicReference<ByteBuffer> payloadRef = _payloadRef;
             Callback payloadCallback = Callback.from(() ->