apacheGH-524: Performance: avoid buffer copying

In ChannelAsyncOutputStream, do not create a new SSH packet if the
caller already had passed in an SSH packet buffer, and we're writing
it fully. This avoids allocating a new buffer, which gives GC less
to do, and it also avoid needlessly copying the data around. Instead
the prepared SSH packet buffer is encoded and written directly.

Bug: apache#524

sshd-core/src/main/java/org/apache/sshd/common/channel/ChannelAsyncOutputStream.java

@@ -31,6 +31,7 @@
 import org.apache.sshd.common.io.WritePendingException;
 import org.apache.sshd.common.session.Session;
 import org.apache.sshd.common.session.SessionContext;
+import org.apache.sshd.common.session.helpers.PacketBuffer;
 import org.apache.sshd.common.util.buffer.Buffer;
 import org.apache.sshd.common.util.buffer.ByteArrayBuffer;
 import org.apache.sshd.common.util.closeable.AbstractCloseable;
@@ -397,6 +398,20 @@ protected void onWritten(IoWriteFutureImpl future, int total, int length, IoWrit
     protected Buffer createSendBuffer(Buffer buffer, Channel channel, int length) {
         SessionContext.validateSessionPayloadSize(length, "Invalid send buffer length: %d");
 
+        if (buffer instanceof PacketBuffer && buffer.available() == length && cmd == SshConstants.SSH_MSG_CHANNEL_DATA) {
+            // Caller had prepared a packet buffer with SSH header and everything already,
+            // and we're writing it completely.
+            //
+            // Fill in cmd and length, and off we go.
+            int wpos = buffer.wpos();
+            buffer.rpos(SshConstants.SSH_PACKET_HEADER_LEN);
+            buffer.wpos(SshConstants.SSH_PACKET_HEADER_LEN);
+            buffer.putByte(cmd);
+            buffer.putUInt(channel.getRecipient());
+            buffer.putUInt(length);
+            buffer.wpos(wpos);
+            return buffer;
+        }
         Session s = channel.getSession();
         Buffer buf = s.createBuffer(cmd, length + 12);
         buf.putUInt(channel.getRecipient());

sshd-core/src/main/java/org/apache/sshd/common/session/helpers/AbstractSession.java

@@ -1361,7 +1361,7 @@ protected boolean doInvokeUnimplementedMessageHandler(int cmd, Buffer buffer) th
     @Override
     public Buffer createBuffer(byte cmd, int len) {
         if (len <= 0) {
-            return prepareBuffer(cmd, new ByteArrayBuffer());
+            return prepareBuffer(cmd, new PacketBuffer());
         }
 
         // Since the caller claims to know how many bytes they will need
@@ -1376,7 +1376,7 @@ public Buffer createBuffer(byte cmd, int len) {
             len += outMacSize;
         }
 
-        return prepareBuffer(cmd, new ByteArrayBuffer(new byte[len + Byte.SIZE], false));
+        return prepareBuffer(cmd, new PacketBuffer(new byte[len + Byte.SIZE], false));
     }
 
     @Override

sshd-core/src/main/java/org/apache/sshd/common/session/helpers/PacketBuffer.java

@@ -0,0 +1,35 @@
+/*
+ * 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.sshd.common.session.helpers;
+
+import org.apache.sshd.common.util.buffer.ByteArrayBuffer;
+
+/**
+ * A {@link ByteArrayBuffer} that will be prepared with the SSH header (5 bytes).
+ */
+public class PacketBuffer extends ByteArrayBuffer {
+
+    public PacketBuffer() {
+        super();
+    }
+
+    public PacketBuffer(byte[] buf, boolean readOnly) {
+        super(buf, readOnly);
+    }
+}