[#2144] Fix NPE in Local transport caused by a race

Motivation:
At the moment it is possible to see a NPE when the LocalSocketChannels doRegister() method is called and the LocalSocketChannels doClose() method is called before the registration was completed.

Modifications:
Make sure we delay the actual close until the registration task was executed.

Result:
No more NPE

transport/src/main/java/io/netty/channel/local/LocalChannel.java

@@ -85,6 +85,7 @@ public class LocalChannel extends AbstractChannel {
     private volatile LocalAddress remoteAddress;
     private volatile ChannelPromise connectPromise;
     private volatile boolean readInProgress;
+    private volatile boolean registerInProgress;
 
     public LocalChannel(EventLoop eventLoop) {
         super(null, eventLoop);
@@ -155,6 +156,14 @@ public class LocalChannel extends AbstractChannel {
     @Override
     protected void doRegister() throws Exception {
         if (peer != null) {
+            // Store the peer in a local variable as it may be set to null if doClose() is called.
+            // Because of this we also set registerInProgress to true as we check for this in doClose() and make sure
+            // we delay the fireChannelInactive() to be fired after the fireChannelActive() and so keep the correct
+            // order of events.
+            //
+            // See https://github.com/netty/netty/issues/2144
+            final LocalChannel peer = this.peer;
+            registerInProgress = true;
             state = State.CONNECTED;
 
             peer.remoteAddress = parent() == null ? null : parent().localAddress();
@@ -167,6 +176,7 @@ public class LocalChannel extends AbstractChannel {
             peer.eventLoop().execute(new Runnable() {
                 @Override
                 public void run() {
+                    registerInProgress = false;
                     peer.pipeline().fireChannelActive();
                     peer.connectPromise.setSuccess();
                 }
@@ -206,7 +216,12 @@ public class LocalChannel extends AbstractChannel {
             // Need to execute the close in the correct EventLoop
             // See https://github.com/netty/netty/issues/1777
             EventLoop eventLoop = peer.eventLoop();
-            if (eventLoop.inEventLoop()) {
+
+            // Also check if the registration was not done yet. In this case we submit the close to the EventLoop
+            // to make sure it is run after the registration completes.
+            //
+            // See https://github.com/netty/netty/issues/2144
+            if (eventLoop.inEventLoop() && !registerInProgress) {
                 peer.unsafe().close(unsafe().voidPromise());
             } else {
                 peer.eventLoop().execute(new Runnable() {

transport/src/test/java/io/netty/channel/local/LocalChannelTest.java

@@ -20,17 +20,22 @@ import io.netty.bootstrap.ServerBootstrap;
 import io.netty.channel.AbstractChannel;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelHandlerAdapter;
+import io.netty.channel.ChannelFuture;
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.channel.ChannelInitializer;
 import io.netty.channel.DefaultEventLoopGroup;
+import io.netty.channel.EventLoop;
 import io.netty.channel.EventLoopGroup;
 import io.netty.channel.SimpleChannelInboundHandler;
+import io.netty.channel.SingleThreadEventLoop;
 import io.netty.util.internal.logging.InternalLogger;
 import io.netty.util.internal.logging.InternalLoggerFactory;
 import org.junit.Test;
 
 import java.nio.channels.ClosedChannelException;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Executor;
+import java.util.concurrent.TimeUnit;
 
 import static org.hamcrest.CoreMatchers.*;
 import static org.junit.Assert.*;
@@ -178,7 +183,73 @@ public class LocalChannelTest {
         group.terminationFuture().sync();
     }
 
+    @Test
+    public void localChannelRaceCondition() throws Exception {
+        final LocalAddress address = new LocalAddress("test");
+        final CountDownLatch closeLatch = new CountDownLatch(1);
+        final EventLoopGroup serverGroup = new DefaultEventLoopGroup(1);
+        final EventLoopGroup clientGroup = new DefaultEventLoopGroup(1) {
+            @Override
+            protected EventLoop newChild(Executor threadFactory, Object... args)
+                    throws Exception {
+                return new SingleThreadEventLoop(this, threadFactory, true) {
+                    @Override
+                    protected void run() {
+                        for (;;) {
+                            Runnable task = takeTask();
+                            if (task != null) {
+                                /* Only slow down the anonymous class in LocalChannel#doRegister() */
+                                if (task.getClass().getEnclosingClass() == LocalChannel.class) {
+                                    try {
+                                        closeLatch.await();
+                                    } catch (InterruptedException e) {
+                                        throw new Error(e);
+                                    }
+                                }
+                                task.run();
+                                updateLastExecutionTime();
+                            }
+
+                            if (confirmShutdown()) {
+                                break;
+                            }
+                        }
+                    }
+                };
+            }
+        };
+        try {
+            ServerBootstrap sb = new ServerBootstrap();
+            sb.group(serverGroup).
+                    channel(LocalServerChannel.class).
+                    childHandler(new ChannelInitializer<Channel>() {
+                        @Override
+                        protected void initChannel(Channel ch) throws Exception {
+                            ch.close();
+                            closeLatch.countDown();
+                        }
+                    }).
+                    bind(address).
+                    sync();
+            Bootstrap bootstrap = new Bootstrap();
+            bootstrap.group(clientGroup).
+                    channel(LocalChannel.class).
+                    handler(new ChannelInitializer<Channel>() {
+                        @Override
+                        protected void initChannel(Channel ch) throws Exception {
+                            /* Do nothing */
+                        }
+                    });
+            ChannelFuture future = bootstrap.connect(address);
+            assertTrue("Connection should finish, not time out", future.await(200));
+        } finally {
+            serverGroup.shutdownGracefully(0, 0, TimeUnit.SECONDS).await();
+            clientGroup.shutdownGracefully(0, 0, TimeUnit.SECONDS).await();
+        }
+    }
+
     static class TestHandler extends ChannelHandlerAdapter {
+
         @Override
         public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
             logger.info(String.format("Received mesage: %s", msg));