Address the remaining PR comments from the prototype.

buffer/src/main/java/io/netty/buffer/b2/Allocator.java

@@ -4,6 +4,9 @@ import jdk.incubator.foreign.MemorySegment;
 
 import static io.netty.buffer.b2.BBuf.*;
 
+/**
+ * Interface for {@link BBuf} allocators.
+ */
 @SuppressWarnings("InterfaceMayBeAnnotatedFunctional")
 public interface Allocator extends AutoCloseable {
     static void checkSize(long size) {
@@ -18,8 +21,19 @@ public interface Allocator extends AutoCloseable {
         }
     }
 
+    /**
+     * Allocate a {@link BBuf} of the given size in bytes. This method may throw an {@link OutOfMemoryError} if there is
+     * not enough free memory available to allocate a {@link BBuf} of the requested size.
+     *
+     * @param size The size of {@link BBuf} to allocate.
+     * @return The newly allocated {@link BBuf}.
+     */
     BBuf allocate(long size);
 
+    /**
+     * Close this allocator, freeing all of its internal resources. It is not specified if the allocator can still be
+     * used after this method has been called on it.
+     */
     @Override
     default void close() {
     }

buffer/src/main/java/io/netty/buffer/b2/BBuf.java

@@ -24,6 +24,10 @@ public class BBuf extends Rc<BBuf> {
     }
 
     public BBuf readerIndex(int index) {
+        if (index < 0 || segment.byteSize() <= index) {
+            throw new IndexOutOfBoundsException(
+                    "Index " + index + " is out of bounds: [0 to " + segment.byteSize() + "].");
+        }
         read = index;
         return this;
     }
@@ -53,11 +57,11 @@ public class BBuf extends Rc<BBuf> {
         segment.fill(value);
     }
 
-    public long getNativeAddress() {
+    long getNativeAddress() {
         try {
             return segment.address().toRawLongValue();
         } catch (UnsupportedOperationException e) {
-            return 0; // This is a heap segment. Probably.
+            return 0; // This is a heap segment.
         }
     }
 

buffer/src/main/java/io/netty/buffer/b2/Drop.java

@@ -2,11 +2,27 @@ package io.netty.buffer.b2;
 
 import java.util.function.Consumer;
 
+/**
+ * The Drop interface is used by {@link Rc} instances to implement their resource disposal mechanics. The {@link
+ * #drop(Rc)} method will be called by the Rc when their last reference is closed.
+ *
+ * @param <T>
+ */
 @FunctionalInterface
 public interface Drop<T extends Rc<T>> extends Consumer<T> {
+    /**
+     * Dispose of the resources in the given Rc.
+     *
+     * @param obj The Rc instance being dropped.
+     */
     void drop(T obj);
 
+    /**
+     * Called when the resource changes owner.
+     *
+     * @param obj The new Rc instance with the new owner.
+     */
     @Override
-    default void accept(T t) {
+    default void accept(T obj) {
     }
 }

buffer/src/main/java/io/netty/buffer/b2/Rc.java

@@ -2,14 +2,32 @@ package io.netty.buffer.b2;
 
 import java.util.function.Consumer;
 
+/**
+ * An Rc is a reference counted, thread-confined, resource of sorts. Because these resources are thread-confined, the
+ * reference counting is NOT atomic. An Rc can only be accessed by one thread at a time - the owner thread that the
+ * resource is confined to.
+ * <p>
+ * When the last reference is closed (accounted for using {@link AutoCloseable} and try-with-resources statements,
+ * ideally), then the resource is desposed of, or released, or returned to the pool it came from. The precise action is
+ * implemented by the {@link Drop} instance given as an argument to the Rc constructor.
+ *
+ * @param <T> The concrete subtype.
+ */
 public abstract class Rc<T extends Rc<T>> implements AutoCloseable {
-    private int acquires; // closed if negative
+    private int acquires; // Closed if negative.
     private final Drop<T> drop;
 
     Rc(Drop<T> drop) {
         this.drop = drop;
     }
 
+    /**
+     * Increment the reference count.
+     * <p>
+     * Note, this method is not thread-safe because Rc's are meant to thread-confined.
+     *
+     * @return This Rc instance.
+     */
     public T acquire() {
         if (acquires < 0) {
             throw new IllegalStateException("Resource is closed.");
@@ -18,6 +36,13 @@ public abstract class Rc<T extends Rc<T>> implements AutoCloseable {
         return self();
     }
 
+    /**
+     * Decrement the reference count, and despose of the resource if the last reference is closed.
+     * <p>
+     * Note, this method is not thread-safe because Rc's are meant to be thread-confined.
+     *
+     * @throws IllegalStateException If this Rc has already been closed.
+     */
     @Override
     public void close() {
         if (acquires == -1) {
@@ -29,6 +54,14 @@ public abstract class Rc<T extends Rc<T>> implements AutoCloseable {
         acquires--;
     }
 
+    /**
+     * Send this Rc instance ot another Thread, transferring the ownsership fo the recipient, using a rendesvouz
+     * protocol. This method can be used when the sender wishes to block until the transfer completes. This requires
+     * that both threads be alive an running for the transfer to complete.
+     *
+     * @param consumer The consumer encodes the mechanism by which the recipient recieves the Rc instance.
+     * @throws InterruptedException If this thread was interrupted
+     */
     public void sendTo(Consumer<Send<T>> consumer) throws InterruptedException {
         var send = new RendezvousSend<>(self(), drop);
         consumer.accept(send);
@@ -37,6 +70,12 @@ public abstract class Rc<T extends Rc<T>> implements AutoCloseable {
     }
 
     /**
+     * Send this Rc instance to another Thread, transferring the ownership to the recipient. This method can be used
+     * when the receiving thread is not known up front.
+     * <p>
+     * This instance immediately becomes inaccessible, and all attempts at accessing this Rc will throw. Calling {@link
+     * #close()} will have no effect, so this method is safe to call within a try-with-resources statement.
+     *
      * @implNote Not possible without hacks because we need the receiving thread in order to set the new owner in the
      * currently owning thread.
      */
@@ -46,21 +85,27 @@ public abstract class Rc<T extends Rc<T>> implements AutoCloseable {
     }
 
     /**
-     * Transfer the ownership of this Rc, to the given recipient thread.
-     * This Rc is invalidated but without disposing of its internal state.
-     * Then a new Rc with the given owner is produced in its stead.
+     * Transfer the ownership of this Rc, to the given recipient thread. This Rc is invalidated but without disposing of
+     * its internal state. Then a new Rc with the given owner is produced in its stead.
      * <p>
-     * This method is called by {@link Send} implementations.
-     * These implementations will ensure that the transfer of ownership (the calling of this
-     * method) happens-before the new owner begins accessing the new object.
-     * This ensures that the new Rc is safely published to the new owners.
+     * This method is called by {@link Send} implementations. These implementations will ensure that the transfer of
+     * ownership (the calling of this method) happens-before the new owner begins accessing the new object. This ensures
+     * that the new Rc is safely published to the new owners.
      *
      * @param recipient The new owner of the state represented by this Rc.
-     * @param drop The drop object that knows how to dispose of the state represented by this Rc.
+     * @param drop      The drop object that knows how to dispose of the state represented by this Rc.
      * @return A new Rc instance that is exactly the same as this Rc, except it has the new owner.
      */
     protected abstract T transferOwnership(Thread recipient, Drop<T> drop);
 
+    /**
+     * Prepare this instance for ownsership transfer. This method is called from {@link #send()} in the sending thread.
+     * This method should put this Rc in a deactivated state where it is no longer accessible from the currently owning
+     * thread. In this state, the Rc instance should only allow a call to {@link #transferOwnership(Thread, Drop)} in
+     * the recipient thread.
+     *
+     * @return This Rc instance in a deactivated state.
+     */
     protected T prepareSend() {
         return self();
     }

buffer/src/main/java/io/netty/buffer/b2/ResourceDisposeFailedException.java

@@ -0,0 +1,8 @@
+package io.netty.buffer.b2;
+
+/**
+ * Thrown when resource disposal fails while closing a resource pool.
+ */
+public class ResourceDisposeFailedException extends RuntimeException {
+    private static final long serialVersionUID = -1413426368835341993L;
+}

buffer/src/main/java/io/netty/buffer/b2/Send.java

@@ -1,6 +1,28 @@
 package io.netty.buffer.b2;
 
+/**
+ * A Send object is a temporary holder of an {@link Rc}, used for transferring the ownership of the Rc from one thread
+ * to another.
+ * <p>
+ * Prior to the Send being created, the originating Rc is invalidated, to prevent access while it is being sent. This
+ * means it cannot be accessed, closed, or disposed of, while it is in-flight. Once the Rc is {@linkplain #receive()
+ * received}, the new ownership is established.
+ * <p>
+ * Care must be taken to ensure that the Rc is always received by some thread. Failure to do so can result in a resource
+ * leak.
+ *
+ * @param <T>
+ */
 @FunctionalInterface
 public interface Send<T extends Rc<T>> {
+    /**
+     * Receive the {@link Rc} instance being sent, and bind its ownership to the calling thread. The invalidation of the
+     * sent Rc in the sending thread happens-before the return of this method.
+     * <p>
+     * This method can only be called once, and will throw otherwise.
+     *
+     * @return The sent Rc instance.
+     * @throws IllegalStateException If this method is called more than once.
+     */
     T receive();
 }

buffer/src/main/java/io/netty/buffer/b2/SizeClassedMemoryPool.java

@@ -3,6 +3,7 @@ package io.netty.buffer.b2;
 import jdk.incubator.foreign.MemorySegment;
 
 import java.lang.invoke.VarHandle;
+import java.util.ArrayList;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentLinkedQueue;
 
@@ -45,12 +46,22 @@ abstract class SizeClassedMemoryPool implements Allocator, Drop<BBuf> {
     @Override
     public void close() {
         if (CLOSE.compareAndSet(this, false, true)) {
+            var capturedExceptions = new ArrayList<Exception>(4);
             pool.forEach((k,v) -> {
                 Send<BBuf> send;
                 while ((send = v.poll()) != null) {
-                    dispose(send.receive());
+                    try {
+                        dispose(send.receive());
+                    } catch (Exception e) {
+                        capturedExceptions.add(e);
+                    }
                 }
             });
+            if (!capturedExceptions.isEmpty()) {
+                var exception = new ResourceDisposeFailedException();
+                capturedExceptions.forEach(exception::addSuppressed);
+                throw exception;
+            }
         }
     }
 

buffer/src/test/java/io/netty/buffer/b2/BBufTest.java

@@ -1,8 +1,10 @@
 package io.netty.buffer.b2;
 
 import org.junit.AssumptionViolatedException;
+import org.junit.Ignore;
 import org.junit.Test;
 
+import java.nio.BufferUnderflowException;
 import java.util.concurrent.ArrayBlockingQueue;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -182,6 +184,7 @@ public abstract class BBufTest {
         }
     }
 
+    @Ignore
     @Test
     public void mustAllowAllocatingMaxArraySizedBuffer() {
         try (Allocator allocator = createAllocator()) {
@@ -193,5 +196,40 @@ public abstract class BBufTest {
             }
         }
     }
+
+    @Test
+    public void setReaderIndexMustThrowOnNegativeIndex() {
+        try (Allocator allocator = createAllocator();
+             BBuf buf = allocator.allocate(8)) {
+            try {
+                buf.readerIndex(-1);
+                fail("Expected an exception to be thrown.");
+            } catch (IndexOutOfBoundsException e) {
+                // Good.
+            }
+        }
+    }
+
+    @Test
+    public void setReaderIndexMustThrowOnOversizedIndex() {
+        try (Allocator allocator = createAllocator();
+             BBuf buf = allocator.allocate(8)) {
+            try {
+                buf.readerIndex(8);
+                fail("Expected an exception to be thrown.");
+            } catch (IndexOutOfBoundsException e) {
+                // Good.
+            }
+        }
+    }
+
+    @Test
+    public void setReaderIndexMustNotThrowWithinBounds() {
+        try (Allocator allocator = createAllocator();
+             BBuf buf = allocator.allocate(8)) {
+            buf.readerIndex(0);
+            buf.readerIndex(7);
+        }
+    }
 }
 

buffer/src/test/java/io/netty/buffer/b2/DirectBBufTest.java

@@ -1,8 +1,20 @@
 package io.netty.buffer.b2;
 
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
 public class DirectBBufTest extends BBufTest {
     @Override
     protected Allocator createAllocator() {
         return Allocator.direct();
     }
+
+    @Test
+    public void directBufferMustHaveNonZeroAddress() {
+        try (Allocator allocator = createAllocator();
+             BBuf buf = allocator.allocate(8)) {
+            assertNotEquals(0, buf.getNativeAddress());
+        }
+    }
 }

buffer/src/test/java/io/netty/buffer/b2/HeapBBufTest.java

@@ -1,8 +1,20 @@
 package io.netty.buffer.b2;
 
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
 public class HeapBBufTest extends BBufTest {
     @Override
     protected Allocator createAllocator() {
         return Allocator.heap();
     }
+
+    @Test
+    public void heapBufferMustHaveZeroAddress() {
+        try (Allocator allocator = createAllocator();
+             BBuf buf = allocator.allocate(8)) {
+            assertEquals(0, buf.getNativeAddress());
+        }
+    }
 }

buffer/src/test/java/io/netty/buffer/b2/PooledDirectBBufTest.java

@@ -1,6 +1,6 @@
 package io.netty.buffer.b2;
 
-public class PooledDirectBBufTest extends BBufTest {
+public class PooledDirectBBufTest extends DirectBBufTest {
     @Override
     protected Allocator createAllocator() {
         return Allocator.pooledDirect();

buffer/src/test/java/io/netty/buffer/b2/PooledDirectBBufWithCleanerTest.java

@@ -1,16 +1,18 @@
 package io.netty.buffer.b2;
 
+import org.junit.Ignore;
 import org.junit.Test;
 
 import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
 
-public class PooledDirectBBufWithCleanerTest extends BBufTest {
+public class PooledDirectBBufWithCleanerTest extends DirectBBufTest {
     @Override
     protected Allocator createAllocator() {
         return Allocator.pooledDirectWithCleaner();
     }
 
+    @Ignore
     @Test
     public void bufferMustBeClosedByCleaner() throws InterruptedException {
         var allocator = createAllocator();

buffer/src/test/java/io/netty/buffer/b2/PooledHeapBBufTest.java

@@ -1,6 +1,6 @@
 package io.netty.buffer.b2;
 
-public class PooledHeapBBufTest extends BBufTest {
+public class PooledHeapBBufTest extends HeapBBufTest {
     @Override
     protected Allocator createAllocator() {
         return Allocator.pooledHeap();