Address review comments on bifurcate PR

src/main/java/io/netty/buffer/api/CompositeBuf.java

@@ -130,9 +130,11 @@ final class CompositeBuf extends RcSupport<Buf, CompositeBuf> implements Buf {
 
     @Override
     public Buf order(ByteOrder order) {
-        this.order = order;
-        for (Buf buf : bufs) {
-            buf.order(order);
+        if (this.order != order) {
+            this.order = order;
+            for (Buf buf : bufs) {
+                buf.order(order);
+            }
         }
         return this;
     }
@@ -891,7 +893,7 @@ final class CompositeBuf extends RcSupport<Buf, CompositeBuf> implements Buf {
                     received[i] = sends[i].receive();
                 }
                 var composite = new CompositeBuf(allocator, true, received, drop);
-                drop.reconnect(composite);
+                drop.attach(composite);
                 return composite;
             }
         };

src/main/java/io/netty/buffer/api/Drop.java

@@ -21,6 +21,7 @@ package io.netty.buffer.api;
  *
  * @param <T>
  */
+@FunctionalInterface
 public interface Drop<T> {
     /**
      * Dispose of the resources in the given Rc.
@@ -34,6 +35,6 @@ public interface Drop<T> {
      *
      * @param obj The new Rc instance with the new owner.
      */
-    default void reconnect(T obj) {
+    default void attach(T obj) {
     }
 }

src/main/java/io/netty/buffer/api/NativeMemoryCleanerDrop.java

@@ -49,7 +49,7 @@ class NativeMemoryCleanerDrop implements Drop<Buf> {
     }
 
     @Override
-    public void reconnect(Buf buf) {
+    public void attach(Buf buf) {
         // Unregister old cleanable, if any, to avoid uncontrolled build-up.
         GatedCleanable c = (GatedCleanable) CLEANABLE.getAndSet(this, null);
         if (c != null) {

src/main/java/io/netty/buffer/api/RcSupport.java

@@ -118,8 +118,7 @@ public abstract class RcSupport<I extends Rc<I>, T extends RcSupport<I, T>> impl
     }
 
     protected Drop<T> unsafeExchangeDrop(Drop<T> replacement) {
-        Objects.requireNonNull(replacement, "Replacement drop cannot be null.");
-        drop = replacement;
+        drop = Objects.requireNonNull(replacement, "Replacement drop cannot be null.");
         return replacement;
     }
 

src/main/java/io/netty/buffer/api/SizeClassedMemoryPool.java

@@ -54,7 +54,7 @@ class SizeClassedMemoryPool implements Allocator, AllocatorControl, Drop<Buf> {
 
     protected Buf createBuf(int size, Drop<Buf> drop) {
         var buf = manager.allocateShared(this, size, drop, null);
-        drop.reconnect(buf);
+        drop.attach(buf);
         return buf;
     }
 
@@ -119,7 +119,7 @@ class SizeClassedMemoryPool implements Allocator, AllocatorControl, Drop<Buf> {
     public void recoverMemory(Object memory) {
         var drop = getDrop();
         var buf = manager.recoverMemory(memory, drop);
-        drop.reconnect(buf);
+        drop.attach(buf);
         buf.close();
     }
 

src/main/java/io/netty/buffer/api/TransferSend.java

@@ -37,7 +37,7 @@ class TransferSend<I extends Rc<I>, T extends Rc<I>> implements Send<I> {
     public I receive() {
         gateReception();
         var copy = outgoing.transferOwnership(drop);
-        drop.reconnect(copy);
+        drop.attach(copy);
         return (I) copy;
     }
 

src/main/java/io/netty/buffer/api/memseg/BifurcatedDrop.java

@@ -17,45 +17,65 @@ package io.netty.buffer.api.memseg;
 
 import io.netty.buffer.api.Drop;
 
-public class BifurcatedDrop<T> implements Drop<T> {
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.VarHandle;
+
+class BifurcatedDrop<T> implements Drop<T> {
+    private static final VarHandle COUNT;
+    static {
+        try {
+            COUNT = MethodHandles.lookup().findVarHandle(BifurcatedDrop.class, "count", int.class);
+        } catch (Exception e) {
+            throw new ExceptionInInitializerError(e);
+        }
+    }
+
     private final T originalBuf;
     private final Drop<T> delegate;
-    private int count;
-    private Exception closeTrace;
+    @SuppressWarnings("FieldMayBeFinal")
+    private volatile int count;
 
-    public BifurcatedDrop(T originalBuf, Drop<T> delegate) {
+    BifurcatedDrop(T originalBuf, Drop<T> delegate) {
         this.originalBuf = originalBuf;
         this.delegate = delegate;
         count = 2; // These are created by buffer bifurcation, so we initially have 2 references to this drop.
     }
 
-    public synchronized void increment() {
-        checkValidState();
-        count++;
+    void increment() {
+        int c;
+        do {
+            c = count;
+            checkValidState(c);
+        } while (!COUNT.compareAndSet(this, c, c + 1));
     }
 
     @Override
     public synchronized void drop(T obj) {
-        checkValidState();
-        if (--count == 0) {
-            closeTrace = new Exception("close: " + delegate);
-            delegate.reconnect(originalBuf);
+        int c;
+        int n;
+        do {
+            c = count;
+            n = c - 1;
+            checkValidState(c);
+        } while (!COUNT.compareAndSet(this, c, n));
+        if (n == 0) {
+            delegate.attach(originalBuf);
             delegate.drop(originalBuf);
         }
     }
 
     @Override
-    public void reconnect(T obj) {
-        delegate.reconnect(obj);
+    public void attach(T obj) {
+        delegate.attach(obj);
     }
 
     Drop<T> unwrap() {
         return delegate;
     }
 
-    private void checkValidState() {
+    private static void checkValidState(int count) {
         if (count == 0) {
-            throw new IllegalStateException("Underlying resources have already been freed.", closeTrace);
+            throw new IllegalStateException("Underlying resources have already been freed.");
         }
     }
 }

src/main/java/io/netty/buffer/api/memseg/MemSegBuf.java

@@ -328,7 +328,7 @@ class MemSegBuf extends RcSupport<Buf, MemSegBuf> implements Buf {
             }
 
             seg = newSegment;
-            drop.reconnect(this);
+            drop.attach(this);
         }
     }
 
@@ -340,7 +340,7 @@ class MemSegBuf extends RcSupport<Buf, MemSegBuf> implements Buf {
         var drop = unsafeGetDrop();
         if (seg.ownerThread() != null) {
             seg = seg.share();
-            drop.reconnect(this);
+            drop.attach(this);
         }
         if (drop instanceof BifurcatedDrop) {
             ((BifurcatedDrop<?>) drop).increment();