better handle subclasses, relates still to #4 and #6

fixture/realclasses/AbstractAllocator.java

@@ -0,0 +1,76 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.cassandra.utils.memory;
+
+import java.nio.ByteBuffer;
+
+import org.apache.cassandra.db.Clustering;
+import org.apache.cassandra.db.Columns;
+import org.apache.cassandra.db.rows.BTreeRow;
+import org.apache.cassandra.db.rows.Cell;
+import org.apache.cassandra.db.rows.Row;
+import org.apache.cassandra.utils.ByteBufferUtil;
+
+public abstract class AbstractAllocator
+{
+    /**
+     * Allocate a slice of the given length.
+     */
+    public ByteBuffer clone(ByteBuffer buffer)
+    {
+        assert buffer != null;
+        if (buffer.remaining() == 0)
+            return ByteBufferUtil.EMPTY_BYTE_BUFFER;
+        ByteBuffer cloned = allocate(buffer.remaining());
+
+        cloned.mark();
+        cloned.put(buffer.duplicate());
+        cloned.reset();
+        return cloned;
+    }
+
+    public abstract ByteBuffer allocate(int size);
+
+    public Row.Builder cloningBTreeRowBuilder()
+    {
+        return new CloningBTreeRowBuilder(this);
+    }
+
+    private static class CloningBTreeRowBuilder extends BTreeRow.Builder
+    {
+        private final AbstractAllocator allocator;
+
+        private CloningBTreeRowBuilder(AbstractAllocator allocator)
+        {
+            super(true);
+            this.allocator = allocator;
+        }
+
+        @Override
+        public void newRow(Clustering clustering)
+        {
+            super.newRow(clustering.copy(allocator));
+        }
+
+        @Override
+        public void addCell(Cell cell)
+        {
+            super.addCell(cell.copy(allocator));
+        }
+    }
+}