Issue #6566 - Create BufferCallbackAccumulator to accumulate buffers with their callbacks

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

jetty-io/src/main/java/org/eclipse/jetty/io/BufferCallbackAccumulator.java

@@ -0,0 +1,106 @@
+//
+// ========================================================================
+// Copyright (c) 1995-2021 Mort Bay Consulting Pty Ltd and others.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Eclipse Public License v. 2.0 which is available at
+// https://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
+// which is available at https://www.apache.org/licenses/LICENSE-2.0.
+//
+// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
+// ========================================================================
+//
+
+package org.eclipse.jetty.io;
+
+import java.nio.ByteBuffer;
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+
+import org.eclipse.jetty.util.BufferUtil;
+import org.eclipse.jetty.util.Callback;
+
+public class BufferCallbackAccumulator
+{
+    private final List<Entry> _entries = new ArrayList<>();
+    private final ByteBufferPool _bufferPool;
+    private final boolean _direct;
+
+    private static class Entry
+    {
+        Entry(ByteBuffer buffer, Callback callback)
+        {
+            this.buffer = buffer;
+            this.callback = callback;
+        }
+
+        ByteBuffer buffer;
+        Callback callback;
+    }
+
+    public BufferCallbackAccumulator()
+    {
+        this(null, false);
+    }
+
+    BufferCallbackAccumulator(ByteBufferPool bufferPool, boolean direct)
+    {
+        _bufferPool = (bufferPool == null) ? new NullByteBufferPool() : bufferPool;
+        _direct = direct;
+    }
+
+    public void addEntry(ByteBuffer buffer, Callback callback)
+    {
+        _entries.add(new Entry(buffer, callback));
+    }
+
+    /**
+     * Get the amount of bytes which have been accumulated.
+     * This will add up the remaining of each buffer in the accumulator.
+     * @return the total length of the content in the accumulator.
+     */
+    public int getLength()
+    {
+        int length = 0;
+        for (Entry entry : _entries)
+            length = Math.addExact(length, entry.buffer.remaining());
+        return length;
+    }
+
+    /**
+     * @return a newly allocated byte array containing all content written into the accumulator.
+     */
+    public byte[] toByteArray()
+    {
+        int length = getLength();
+        if (length == 0)
+            return new byte[0];
+
+        byte[] bytes = new byte[length];
+        ByteBuffer buffer = BufferUtil.toBuffer(bytes);
+        BufferUtil.clearToFill(buffer);
+        writeTo(buffer);
+        return bytes;
+    }
+
+    public void writeTo(ByteBuffer buffer)
+    {
+        for (Iterator<Entry> iterator = _entries.iterator(); iterator.hasNext(); )
+        {
+            Entry entry = iterator.next();
+            buffer.put(entry.buffer);
+            iterator.remove();
+            entry.callback.succeeded();
+        }
+    }
+
+    public void fail(Throwable t)
+    {
+        for (Entry entry : _entries)
+        {
+            entry.callback.failed(t);
+        }
+        _entries.clear();
+    }
+}