GH-587: prevent NPE in NettyIoSession

If a write occurs on an already closed channel, an NPE might occur.
Handle this case by setting a ChannelClosedException on the returned
future.

This prevents the NPE, but other code may still issue other exceptions
if a write on a closed channel occurs.

Bug: #587

CHANGES.md

@@ -41,6 +41,7 @@
 * [GH-524](https://github.com/apache/mina-sshd/issues/524) Performance improvements
 * [GH-533](https://github.com/apache/mina-sshd/issues/533) Fix multi-step authentication
 * [GH-582](https://github.com/apache/mina-sshd/issues/582) Fix filtering in `NamedFactory`
+* [GH-587](https://github.com/apache/mina-sshd/issues/587) Prevent `NullPointerException`on closed channel in `NettyIoSession`
 * [GH-590](https://github.com/apache/mina-sshd/issues/590) Better support for FIPS
 
 ## New Features

sshd-netty/src/main/java/org/apache/sshd/netty/NettyIoSession.java

@@ -21,6 +21,7 @@
 
 import java.io.IOException;
 import java.net.SocketAddress;
+import java.nio.channels.ClosedChannelException;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
@@ -134,11 +135,19 @@ public IoWriteFuture writeBuffer(Buffer buffer) {
         ByteBuf buf = Unpooled.buffer(bufLen);
         buf.writeBytes(buffer.array(), buffer.rpos(), bufLen);
         DefaultIoWriteFuture msg = new DefaultIoWriteFuture(getRemoteAddress(), null);
-        ChannelPromise next = context.newPromise();
+        ChannelHandlerContext ctx = context;
+        if (ctx == null) {
+            msg.setValue(new ClosedChannelException());
+            return msg;
+        }
+        ChannelPromise next = ctx.newPromise();
         prev.addListener(whatever -> {
-            ChannelHandlerContext ctx = context;
-            if (ctx != null) {
-                ctx.writeAndFlush(buf, next);
+            ChannelHandlerContext c = context;
+            if (c != null) {
+                c.writeAndFlush(buf, next);
+            } else {
+                msg.setValue(new ClosedChannelException());
+                next.cancel(true);
             }
         });
         prev = next;