Ensure the DefaultChannelHandlerContext is unlinked once removed (#9970)

Motivation:

At the moment the next / prev references are not set to "null" in the DefaultChannelHandlerContext once the ChannelHandler is removed. This is bad as it basically let users still use the ChannelHandlerContext of a ChannelHandler after it is removed and may produce very suprising behaviour.

Modifications:

- Fail if someone tries to use the ChannelHandlerContext once the ChannelHandler was removed (for outbound operations fail the promise, for inbound fire the error through the ChannelPipeline)
- Fix some handlers to ensure we not use the ChannelHandlerContext after the handler was removed
- Adjust DefaultChannelPipeline / DefaultChannelHandlerContext to fixes races with removal / replacement of handlers

Result:

Cleanup behaviour and make it more predictable for pipeline modifications

codec-haproxy/src/main/java/io/netty/handler/codec/haproxy/HAProxyMessageDecoder.java

@@ -253,6 +253,14 @@ public class HAProxyMessageDecoder extends ByteToMessageDecoder {
         }
     }
 
+    @Override
+    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
+        ctx.fireExceptionCaught(cause);
+        if (cause instanceof HAProxyProtocolException) {
+            ctx.close(); // drop connection immediately per spec
+        }
+    }
+
     @Override
     protected final void decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
         // determine the specification version
@@ -325,7 +333,6 @@ public class HAProxyMessageDecoder extends ByteToMessageDecoder {
 
     private void fail(final ChannelHandlerContext ctx, String errMsg, Exception e) {
         finished = true;
-        ctx.close(); // drop connection immediately per spec
         HAProxyProtocolException ppex;
         if (errMsg != null && e != null) {
             ppex = new HAProxyProtocolException(errMsg, e);

codec-http/src/main/java/io/netty/handler/codec/http/HttpClientUpgradeHandler.java

@@ -203,8 +203,8 @@ public class HttpClientUpgradeHandler extends HttpObjectAggregator {
                     // NOTE: not releasing the response since we're letting it propagate to the
                     // next handler.
                     ctx.fireUserEventTriggered(UpgradeEvent.UPGRADE_REJECTED);
-                    removeThisHandler(ctx);
                     ctx.fireChannelRead(msg);
+                    removeThisHandler(ctx);
                     return;
                 }
             }

codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectAggregator.java

@@ -266,13 +266,26 @@ public class HttpObjectAggregator
                 });
             }
         } else if (oversized instanceof HttpResponse) {
-            ctx.close();
+            throw new ResponseTooLargeException("Response entity too large: " + oversized);
-            throw new TooLongFrameException("Response entity too large: " + oversized);
         } else {
             throw new IllegalStateException();
         }
     }
 
+    @Override
+    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
+        super.exceptionCaught(ctx, cause);
+        if (cause instanceof ResponseTooLargeException) {
+            ctx.close();
+        }
+    }
+
+    private static final class ResponseTooLargeException extends TooLongFrameException {
+        ResponseTooLargeException(String message) {
+            super(message);
+        }
+    }
+
     private abstract static class AggregatedFullHttpMessage implements FullHttpMessage {
         protected final HttpMessage message;
         private final ByteBuf content;

codec-http/src/main/java/io/netty/handler/codec/http/HttpServerUpgradeHandler.java

@@ -328,13 +328,13 @@ public class HttpServerUpgradeHandler extends HttpObjectAggregator {
             sourceCodec.upgradeFrom(ctx);
             upgradeCodec.upgradeTo(ctx, request);
 
-            // Remove this handler from the pipeline.
-            ctx.pipeline().remove(HttpServerUpgradeHandler.this);
-
             // Notify that the upgrade has occurred. Retain the event to offset
             // the release() in the finally block.
             ctx.fireUserEventTriggered(event.retain());
 
+            // Remove this handler from the pipeline.
+            ctx.pipeline().remove(HttpServerUpgradeHandler.this);
+
             // Add the listener last to avoid firing upgrade logic after
             // the channel is already closed since the listener may fire
             // immediately if the write failed eagerly.

codec-http/src/main/java/io/netty/handler/codec/http/websocketx/WebSocketServerHandshaker.java

@@ -300,9 +300,9 @@ public abstract class WebSocketServerHandshaker {
             @Override
             public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
                 // Remove ourself and fail the handshake promise.
-                ctx.pipeline().remove(this);
                 promise.tryFailure(cause);
                 ctx.fireExceptionCaught(cause);
+                ctx.pipeline().remove(this);
             }
 
             @Override

codec-http/src/main/java/io/netty/handler/codec/http/websocketx/WebSocketServerProtocolHandshakeHandler.java

@@ -45,7 +45,6 @@ import static io.netty.handler.codec.http.HttpVersion.*;
 class WebSocketServerProtocolHandshakeHandler implements ChannelHandler {
 
     private final WebSocketServerProtocolConfig serverConfig;
-    private ChannelHandlerContext ctx;
     private ChannelPromise handshakePromise;
 
     WebSocketServerProtocolHandshakeHandler(WebSocketServerProtocolConfig serverConfig) {
@@ -54,7 +53,6 @@ class WebSocketServerProtocolHandshakeHandler implements ChannelHandler {
 
     @Override
     public void handlerAdded(ChannelHandlerContext ctx) {
-        this.ctx = ctx;
         handshakePromise = ctx.newPromise();
     }
 
@@ -86,7 +84,6 @@ class WebSocketServerProtocolHandshakeHandler implements ChannelHandler {
                 //
                 // See https://github.com/netty/netty/issues/9471.
                 WebSocketServerProtocolHandler.setHandshaker(ctx.channel(), handshaker);
-                ctx.pipeline().remove(this);
 
                 final ChannelFuture handshakeFuture = handshaker.handshake(ctx.channel(), req);
                 handshakeFuture.addListener((ChannelFutureListener) future -> {
@@ -102,8 +99,9 @@ class WebSocketServerProtocolHandshakeHandler implements ChannelHandler {
                                 new WebSocketServerProtocolHandler.HandshakeComplete(
                                         req.uri(), req.headers(), handshaker.selectedSubprotocol()));
                     }
+                    ctx.pipeline().remove(this);
                 });
-                applyHandshakeTimeout();
+                applyHandshakeTimeout(ctx);
             }
         } finally {
             req.release();
@@ -132,7 +130,7 @@ class WebSocketServerProtocolHandshakeHandler implements ChannelHandler {
         return protocol + "://" + host + path;
     }
 
-    private void applyHandshakeTimeout() {
+    private void applyHandshakeTimeout(ChannelHandlerContext ctx) {
         final ChannelPromise localHandshakePromise = handshakePromise;
         final long handshakeTimeoutMillis = serverConfig.handshakeTimeoutMillis();
         if (handshakeTimeoutMillis <= 0 || localHandshakePromise.isDone()) {

codec-http/src/main/java/io/netty/handler/codec/http/websocketx/extensions/WebSocketClientExtensionHandler.java

@@ -120,8 +120,9 @@ public class WebSocketClientExtensionHandler implements ChannelHandler {
                         ctx.pipeline().addAfter(ctx.name(), encoder.getClass().getName(), encoder);
                     }
                 }
-
+                ctx.fireChannelRead(msg);
-                ctx.pipeline().remove(ctx.name());
+                ctx.pipeline().remove(this);
+                return;
             }
         }
 

codec-http/src/test/java/io/netty/handler/codec/http/websocketx/WebSocketServerProtocolHandlerTest.java

@@ -66,7 +66,7 @@ public class WebSocketServerProtocolHandlerTest {
     }
 
     @Test
-    public void testWebSocketServerProtocolHandshakeHandlerReplacedBeforeHandshake() {
+    public void testWebSocketServerProtocolHandshakeHandlerRemovedAfterHandshake() {
         EmbeddedChannel ch = createChannel(new MockOutboundHandler());
         ChannelHandlerContext handshakerCtx = ch.pipeline().context(WebSocketServerProtocolHandshakeHandler.class);
         ch.pipeline().addLast(new ChannelHandler() {
@@ -74,7 +74,7 @@ public class WebSocketServerProtocolHandlerTest {
             public void userEventTriggered(ChannelHandlerContext ctx, Object evt) {
                 if (evt instanceof WebSocketServerProtocolHandler.HandshakeComplete) {
                     // We should have removed the handler already.
-                    assertNull(ctx.pipeline().context(WebSocketServerProtocolHandshakeHandler.class));
+                    ctx.executor().execute(() -> ctx.pipeline().context(WebSocketServerProtocolHandshakeHandler.class));
                 }
             }
         });

codec-http2/src/main/java/io/netty/handler/codec/http2/CleartextHttp2ServerUpgradeHandler.java

@@ -87,9 +87,9 @@ public final class CleartextHttp2ServerUpgradeHandler extends ByteToMessageDecod
                     .remove(httpServerUpgradeHandler);
 
             ctx.pipeline().addAfter(ctx.name(), null, http2ServerHandler);
-            ctx.pipeline().remove(this);
-
             ctx.fireUserEventTriggered(PriorKnowledgeUpgradeEvent.INSTANCE);
+
+            ctx.pipeline().remove(this);
         }
     }
 

codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java

@@ -154,10 +154,6 @@ public abstract class ByteToMessageDecoder extends ChannelHandlerAdapter {
         }
     };
 
-    private static final byte STATE_INIT = 0;
-    private static final byte STATE_CALLING_CHILD_DECODE = 1;
-    private static final byte STATE_HANDLER_REMOVED_PENDING = 2;
-
     ByteBuf cumulation;
     private Cumulator cumulator = MERGE_CUMULATOR;
     private boolean singleDecode;
@@ -169,15 +165,6 @@ public abstract class ByteToMessageDecoder extends ChannelHandlerAdapter {
      */
     private boolean firedChannelRead;
 
-    /**
-     * A bitmask where the bits are defined as
-     * <ul>
-     *     <li>{@link #STATE_INIT}</li>
-     *     <li>{@link #STATE_CALLING_CHILD_DECODE}</li>
-     *     <li>{@link #STATE_HANDLER_REMOVED_PENDING}</li>
-     * </ul>
-     */
-    private byte decodeState = STATE_INIT;
     private int discardAfterReads = 16;
     private int numReads;
     private ByteToMessageDecoderContext context;
@@ -257,10 +244,6 @@ public abstract class ByteToMessageDecoder extends ChannelHandlerAdapter {
 
     @Override
     public final void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
-        if (decodeState == STATE_CALLING_CHILD_DECODE) {
-            decodeState = STATE_HANDLER_REMOVED_PENDING;
-            return;
-        }
         ByteBuf buf = cumulation;
         if (buf != null) {
             // Directly set this to null so we are sure we not access it in any other method here anymore.
@@ -469,16 +452,7 @@ public abstract class ByteToMessageDecoder extends ChannelHandlerAdapter {
      */
     final void decodeRemovalReentryProtection(ChannelHandlerContext ctx, ByteBuf in)
             throws Exception {
-        decodeState = STATE_CALLING_CHILD_DECODE;
-        try {
         decode(ctx, in);
-        } finally {
-            boolean removePending = decodeState == STATE_HANDLER_REMOVED_PENDING;
-            decodeState = STATE_INIT;
-            if (removePending) {
-                handlerRemoved(ctx);
-            }
-        }
     }
 
     /**

example/src/main/java/io/netty/example/http2/helloworld/frame/server/Http2ServerInitializer.java

@@ -97,8 +97,9 @@ public class Http2ServerInitializer extends ChannelInitializer<SocketChannel> {
                 System.err.println("Directly talking: " + msg.protocolVersion() + " (no upgrade was attempted)");
                 ChannelPipeline pipeline = ctx.pipeline();
                 pipeline.addAfter(ctx.name(), null, new HelloWorldHttp1Handler("Direct. No Upgrade Attempted."));
-                pipeline.replace(this, null, new HttpObjectAggregator(maxHttpContentLength));
+                pipeline.addAfter(ctx.name(), null, new HttpObjectAggregator(maxHttpContentLength));
                 ctx.fireChannelRead(ReferenceCountUtil.retain(msg));
+                pipeline.remove(this);
             }
         });
 

example/src/main/java/io/netty/example/http2/helloworld/multiplex/server/Http2ServerInitializer.java

@@ -99,8 +99,9 @@ public class Http2ServerInitializer extends ChannelInitializer<SocketChannel> {
                 System.err.println("Directly talking: " + msg.protocolVersion() + " (no upgrade was attempted)");
                 ChannelPipeline pipeline = ctx.pipeline();
                 pipeline.addAfter(ctx.name(), null, new HelloWorldHttp1Handler("Direct. No Upgrade Attempted."));
-                pipeline.replace(this, null, new HttpObjectAggregator(maxHttpContentLength));
+                pipeline.addAfter(ctx.name(), null, new HttpObjectAggregator(maxHttpContentLength));
                 ctx.fireChannelRead(ReferenceCountUtil.retain(msg));
+                pipeline.remove(this);
             }
         });
 

example/src/main/java/io/netty/example/http2/helloworld/server/Http2ServerInitializer.java

@@ -98,8 +98,9 @@ public class Http2ServerInitializer extends ChannelInitializer<SocketChannel> {
                 System.err.println("Directly talking: " + msg.protocolVersion() + " (no upgrade was attempted)");
                 ChannelPipeline pipeline = ctx.pipeline();
                 pipeline.addAfter(ctx.name(), null, new HelloWorldHttp1Handler("Direct. No Upgrade Attempted."));
-                pipeline.replace(this, null, new HttpObjectAggregator(maxHttpContentLength));
+                pipeline.addAfter(ctx.name(), null, new HttpObjectAggregator(maxHttpContentLength));
                 ctx.fireChannelRead(ReferenceCountUtil.retain(msg));
+                pipeline.remove(this);
             }
         });
 

example/src/main/java/io/netty/example/socksproxy/SocksServerHandler.java

@@ -45,8 +45,8 @@ public final class SocksServerHandler extends SimpleChannelInboundHandler<SocksM
                 Socks4CommandRequest socksV4CmdRequest = (Socks4CommandRequest) socksRequest;
                 if (socksV4CmdRequest.type() == Socks4CommandType.CONNECT) {
                     ctx.pipeline().addLast(new SocksServerConnectHandler());
-                    ctx.pipeline().remove(this);
                     ctx.fireChannelRead(socksRequest);
+                    ctx.pipeline().remove(this);
                 } else {
                     ctx.close();
                 }
@@ -65,8 +65,8 @@ public final class SocksServerHandler extends SimpleChannelInboundHandler<SocksM
                     Socks5CommandRequest socks5CmdRequest = (Socks5CommandRequest) socksRequest;
                     if (socks5CmdRequest.type() == Socks5CommandType.CONNECT) {
                         ctx.pipeline().addLast(new SocksServerConnectHandler());
-                        ctx.pipeline().remove(this);
                         ctx.fireChannelRead(socksRequest);
+                        ctx.pipeline().remove(this);
                     } else {
                         ctx.close();
                     }

handler/src/main/java/io/netty/handler/ipfilter/AbstractRemoteAddressFilter.java

@@ -39,44 +39,52 @@ public abstract class AbstractRemoteAddressFilter<T extends SocketAddress> imple
 
     @Override
     public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
-        handleNewChannel(ctx);
+        handleNewChannel(ctx, true);
-        ctx.fireChannelRegistered();
     }
 
     @Override
     public void channelActive(ChannelHandlerContext ctx) throws Exception {
-        if (!handleNewChannel(ctx)) {
+        if (!handleNewChannel(ctx, false)) {
             throw new IllegalStateException("cannot determine to accept or reject a channel: " + ctx.channel());
-        } else {
-            ctx.fireChannelActive();
         }
     }
 
-    private boolean handleNewChannel(ChannelHandlerContext ctx) throws Exception {
+    private boolean handleNewChannel(ChannelHandlerContext ctx, boolean register) throws Exception {
         @SuppressWarnings("unchecked")
         T remoteAddress = (T) ctx.channel().remoteAddress();
-
+        boolean remove = false;
+        try {
             // If the remote address is not available yet, defer the decision.
             if (remoteAddress == null) {
                 return false;
             }
 
-        // No need to keep this handler in the pipeline anymore because the decision is going to be made now.
-        // Also, this will prevent the subsequent events from being handled by this handler.
-        ctx.pipeline().remove(this);
-
             if (accept(ctx, remoteAddress)) {
                 channelAccepted(ctx, remoteAddress);
+                remove = true;
             } else {
                 ChannelFuture rejectedFuture = channelRejected(ctx, remoteAddress);
-            if (rejectedFuture != null) {
+                if (rejectedFuture != null && !rejectedFuture.isDone()) {
                     rejectedFuture.addListener(ChannelFutureListener.CLOSE);
                 } else {
                     ctx.close();
                 }
             }
-
             return true;
+        } finally {
+            if (!ctx.isRemoved()) {
+                if (register) {
+                    ctx.fireChannelRegistered();
+                } else {
+                    ctx.fireChannelActive();
+                }
+                if (remove) {
+                    // No need to keep this handler in the pipeline anymore because the decision is going to be made
+                    // now. Also, this will prevent the subsequent events from being handled by this handler.
+                    ctx.pipeline().remove(this);
+                }
+            }
+        }
     }
 
     /**

handler/src/main/java/io/netty/handler/ssl/ApplicationProtocolNegotiationHandler.java

@@ -97,14 +97,17 @@ public abstract class ApplicationProtocolNegotiationHandler implements ChannelHa
             } catch (Throwable cause) {
                 exceptionCaught(ctx, cause);
             } finally {
+                ctx.fireUserEventTriggered(evt);
+
                 ChannelPipeline pipeline = ctx.pipeline();
                 if (pipeline.context(this) != null) {
                     pipeline.remove(this);
                 }
             }
-        }
+        } else {
             ctx.fireUserEventTriggered(evt);
         }
+    }
 
     /**
      * Invoked on successful initial SSL/TLS handshake. Implement this method to configure your pipeline

handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java

@@ -63,7 +63,7 @@ public abstract class SslClientHelloHandler<T> extends ByteToMessageDecoder {
                                         "not an SSL/TLS record: " + ByteBufUtil.hexDump(in));
                                 in.skipBytes(in.readableBytes());
                                 ctx.fireUserEventTriggered(new SniCompletionEvent(e));
-                                SslUtils.handleHandshakeFailure(ctx, e, true);
+                                ctx.fireUserEventTriggered(new SslHandshakeCompletionEvent(e));
                                 throw e;
                             }
                             if (len == SslUtils.NOT_ENOUGH_DATA) {

handler/src/main/java/io/netty/handler/ssl/SslHandler.java

@@ -1091,7 +1091,8 @@ public class SslHandler extends ByteToMessageDecoder {
             if (logger.isDebugEnabled()) {
                 logger.debug(
                         "{} Swallowing a harmless 'connection reset by peer / broken pipe' error that occurred " +
-                        "while writing close_notify in response to the peer's close_notify", ctx.channel(), cause);
+                                "while writing close_notify in response to the peer's close_notify",
+                        ctx.channel(), cause);
             }
 
             // Close the connection explicitly just in case the transport
@@ -1101,6 +1102,11 @@ public class SslHandler extends ByteToMessageDecoder {
             }
         } else {
             ctx.fireExceptionCaught(cause);
+
+            if (cause instanceof SSLException ||
+                    ((cause instanceof DecoderException) && cause.getCause() instanceof SSLException)) {
+                ctx.close();
+            }
         }
     }
 
@@ -2057,9 +2063,13 @@ public class SslHandler extends ByteToMessageDecoder {
             }
             final long closeNotifyReadTimeout = closeNotifyReadTimeoutMillis;
             if (closeNotifyReadTimeout <= 0) {
+                if (ctx.channel().isActive()) {
                     // Trigger the close in all cases to make sure the promise is notified
                     // See https://github.com/netty/netty/issues/2358
                     addCloseListener(ctx.close(ctx.newPromise()), promise);
+                } else {
+                    promise.trySuccess();
+                }
             } else {
                 final ScheduledFuture<?> closeNotifyReadTimeoutFuture;
 
@@ -2083,7 +2093,11 @@ public class SslHandler extends ByteToMessageDecoder {
                     if (closeNotifyReadTimeoutFuture != null) {
                         closeNotifyReadTimeoutFuture.cancel(false);
                     }
+                    if (ctx.channel().isActive()) {
                         addCloseListener(ctx.close(ctx.newPromise()), promise);
+                    } else {
+                        promise.trySuccess();
+                    }
                 });
             }
         });

handler/src/main/java/io/netty/handler/ssl/SslUtils.java

@@ -346,7 +346,6 @@ final class SslUtils {
         if (notify) {
             ctx.fireUserEventTriggered(new SslHandshakeCompletionEvent(cause));
         }
-        ctx.close();
     }
 
     /**

handler/src/main/java/io/netty/handler/ssl/ocsp/OcspClientHandler.java

@@ -48,15 +48,13 @@ public abstract class OcspClientHandler implements ChannelHandler {
 
     @Override
     public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
+        ctx.fireUserEventTriggered(evt);
         if (evt instanceof SslHandshakeCompletionEvent) {
-            ctx.pipeline().remove(this);
-
             SslHandshakeCompletionEvent event = (SslHandshakeCompletionEvent) evt;
             if (event.isSuccess() && !verify(ctx, engine)) {
                 throw new SSLHandshakeException("Bad OCSP response");
             }
-        }
+            ctx.pipeline().remove(this);
-
+        }
-        ctx.fireUserEventTriggered(evt);
     }
 }

handler/src/test/java/io/netty/handler/ssl/SslHandlerTest.java

@@ -442,11 +442,13 @@ public class SslHandlerTest {
                 sslHandler.setHandshakeTimeoutMillis(1000);
                 ch.pipeline().addFirst(sslHandler);
                 sslHandler.handshakeFuture().addListener((FutureListener<Channel>) future -> {
+                    ch.eventLoop().execute(() -> {
                         ch.pipeline().remove(sslHandler);
 
                         // Schedule the close so removal has time to propagate exception if any.
                         ch.eventLoop().execute(ch::close);
                     });
+                });
 
                 ch.pipeline().addLast(new ChannelHandler() {
                     @Override
@@ -457,6 +459,7 @@ public class SslHandlerTest {
                         if (cause instanceof IllegalReferenceCountException) {
                             promise.setFailure(cause);
                         }
+                        cause.printStackTrace();
                     }
 
                     @Override

testsuite-http2/src/main/java/io/netty/testsuite/http2/Http2ServerInitializer.java

@@ -85,8 +85,9 @@ public class Http2ServerInitializer extends ChannelInitializer<SocketChannel> {
                 ChannelPipeline pipeline = ctx.pipeline();
                 ChannelHandlerContext thisCtx = pipeline.context(this);
                 pipeline.addAfter(thisCtx.name(), null, new HelloWorldHttp1Handler("Direct. No Upgrade Attempted."));
-                pipeline.replace(this, null, new HttpObjectAggregator(maxHttpContentLength));
+                pipeline.addAfter(thisCtx.name(), null, new HttpObjectAggregator(maxHttpContentLength));
                 ctx.fireChannelRead(ReferenceCountUtil.retain(msg));
+                pipeline.remove(this);
             }
         });
 

transport/src/main/java/io/netty/channel/AbstractChannel.java

@@ -36,6 +36,7 @@ import java.net.SocketAddress;
 import java.net.SocketException;
 import java.nio.channels.ClosedChannelException;
 import java.nio.channels.NotYetConnectedException;
+import java.util.NoSuchElementException;
 import java.util.concurrent.Executor;
 import java.util.concurrent.RejectedExecutionException;
 
@@ -781,6 +782,19 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                     if (registered) {
                         registered = false;
                         pipeline.fireChannelUnregistered();
+
+                        if (!isOpen()) {
+                            // Remove all handlers from the ChannelPipeline. This is needed to ensure
+                            // handlerRemoved(...) is called and so resources are released.
+                            while (!pipeline.isEmpty()) {
+                                try {
+                                    pipeline.removeLast();
+                                } catch (NoSuchElementException ignore) {
+                                    // try again as there may be a race when someone outside the EventLoop removes
+                                    // handlers concurrently as well.
+                                }
+                            }
+                        }
                     }
                     safeSetSuccess(promise);
                 }

transport/src/main/java/io/netty/channel/ChannelPipeline.java

@@ -437,6 +437,14 @@ public interface ChannelPipeline
      */
     ChannelHandlerContext lastContext();
 
+    /**
+     * Returns {@code true} if this {@link ChannelPipeline} is empty, which means no {@link ChannelHandler} is
+     * present.
+     */
+    default boolean isEmpty() {
+        return lastContext() == null;
+    }
+
     /**
      * Returns the {@link ChannelHandler} with the specified name in this
      * pipeline.

transport/src/main/java/io/netty/channel/DefaultChannelHandlerContext.java

@@ -50,10 +50,15 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
      */
     private static final int ADD_COMPLETE = 1;
 
+    /**
+     * {@link ChannelHandler#handlerRemoved(ChannelHandlerContext)} is about to be called.
+     */
+    private static final int REMOVE_STARTED = 2;
+
     /**
      * {@link ChannelHandler#handlerRemoved(ChannelHandlerContext)} was called.
      */
-    private static final int REMOVE_COMPLETE = 2;
+    private static final int REMOVE_COMPLETE = 3;
 
     private final int executionMask;
     private final DefaultChannelPipeline pipeline;
@@ -63,10 +68,10 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     // Lazily instantiated tasks used to trigger events to a handler with different executor.
     // There is no need to make this volatile as at worse it will just create a few more instances then needed.
     private Tasks invokeTasks;
+    private int handlerState = INIT;
 
     DefaultChannelHandlerContext next;
     DefaultChannelHandlerContext prev;
-    private int handlerState = INIT;
 
     DefaultChannelHandlerContext(DefaultChannelPipeline pipeline, String name,
                                  ChannelHandler handler) {
@@ -76,6 +81,22 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
         this.handler = handler;
     }
 
+    private static void failRemoved(DefaultChannelHandlerContext ctx, ChannelPromise promise) {
+        promise.setFailure(newRemovedException(ctx, null));
+    }
+
+    private void notifyHandlerRemovedAlready() {
+        notifyHandlerRemovedAlready(null);
+    }
+
+    private void notifyHandlerRemovedAlready(Throwable cause) {
+        pipeline().fireExceptionCaught(newRemovedException(this, cause));
+    }
+
+    private static ChannelPipelineException newRemovedException(ChannelHandlerContext ctx, Throwable cause) {
+        return new ChannelPipelineException("Context " + ctx + " already removed", cause);
+    }
+
     private Tasks invokeTasks() {
         Tasks tasks = invokeTasks;
         if (tasks == null) {
@@ -126,7 +147,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeChannelRegistered() {
-        findContextInbound(MASK_CHANNEL_REGISTERED).invokeChannelRegistered();
+        DefaultChannelHandlerContext ctx = findContextInbound(MASK_CHANNEL_REGISTERED);
+        if (ctx == null) {
+            notifyHandlerRemovedAlready();
+            return;
+        }
+        ctx.invokeChannelRegistered();
     }
 
     void invokeChannelRegistered() {
@@ -149,7 +175,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeChannelUnregistered() {
-        findContextInbound(MASK_CHANNEL_UNREGISTERED).invokeChannelUnregistered();
+        DefaultChannelHandlerContext ctx = findContextInbound(MASK_CHANNEL_UNREGISTERED);
+        if (ctx == null) {
+            notifyHandlerRemovedAlready();
+            return;
+        }
+        ctx.invokeChannelUnregistered();
     }
 
     void invokeChannelUnregistered() {
@@ -172,7 +203,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeChannelActive() {
-        findContextInbound(MASK_CHANNEL_ACTIVE).invokeChannelActive();
+        DefaultChannelHandlerContext ctx = findContextInbound(MASK_CHANNEL_ACTIVE);
+        if (ctx == null) {
+            notifyHandlerRemovedAlready();
+            return;
+        }
+        ctx.invokeChannelActive();
     }
 
     void invokeChannelActive() {
@@ -195,7 +231,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeChannelInactive() {
-        findContextInbound(MASK_CHANNEL_INACTIVE).invokeChannelInactive();
+        DefaultChannelHandlerContext ctx = findContextInbound(MASK_CHANNEL_INACTIVE);
+        if (ctx == null) {
+            notifyHandlerRemovedAlready();
+            return;
+        }
+        ctx.invokeChannelInactive();
     }
 
     void invokeChannelInactive() {
@@ -226,7 +267,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeExceptionCaught(Throwable cause) {
-        findContextInbound(MASK_EXCEPTION_CAUGHT).invokeExceptionCaught(cause);
+        DefaultChannelHandlerContext ctx = findContextInbound(MASK_EXCEPTION_CAUGHT);
+        if (ctx == null) {
+            notifyHandlerRemovedAlready(cause);
+            return;
+        }
+        ctx.invokeExceptionCaught(cause);
     }
 
     void invokeExceptionCaught(final Throwable cause) {
@@ -261,7 +307,13 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeUserEventTriggered(Object event) {
-        findContextInbound(MASK_USER_EVENT_TRIGGERED).invokeUserEventTriggered(event);
+        DefaultChannelHandlerContext ctx = findContextInbound(MASK_USER_EVENT_TRIGGERED);
+        if (ctx == null) {
+            ReferenceCountUtil.release(event);
+            notifyHandlerRemovedAlready();
+            return;
+        }
+        ctx.invokeUserEventTriggered(event);
     }
 
     void invokeUserEventTriggered(Object event) {
@@ -290,7 +342,13 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeChannelRead(Object msg) {
-        findContextInbound(MASK_CHANNEL_READ).invokeChannelRead(msg);
+        DefaultChannelHandlerContext ctx = findContextInbound(MASK_CHANNEL_READ);
+        if (ctx == null) {
+            ReferenceCountUtil.release(msg);
+            notifyHandlerRemovedAlready();
+            return;
+        }
+        ctx.invokeChannelRead(msg);
     }
 
     void invokeChannelRead(Object msg) {
@@ -315,7 +373,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeChannelReadComplete() {
-        findContextInbound(MASK_CHANNEL_READ_COMPLETE).invokeChannelReadComplete();
+        DefaultChannelHandlerContext ctx = findContextInbound(MASK_CHANNEL_READ_COMPLETE);
+        if (ctx == null) {
+            notifyHandlerRemovedAlready();
+            return;
+        }
+        ctx.invokeChannelReadComplete();
     }
 
     void invokeChannelReadComplete() {
@@ -339,7 +402,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeChannelWritabilityChanged() {
-        findContextInbound(MASK_CHANNEL_WRITABILITY_CHANGED).invokeChannelWritabilityChanged();
+        DefaultChannelHandlerContext ctx = findContextInbound(MASK_CHANNEL_WRITABILITY_CHANGED);
+        if (ctx == null) {
+            notifyHandlerRemovedAlready();
+            return;
+        }
+        ctx.invokeChannelWritabilityChanged();
     }
 
     void invokeChannelWritabilityChanged() {
@@ -403,7 +471,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeBind(SocketAddress localAddress, ChannelPromise promise) {
-        findContextOutbound(MASK_BIND).invokeBind(localAddress, promise);
+        DefaultChannelHandlerContext ctx = findContextOutbound(MASK_BIND);
+        if (ctx == null) {
+            failRemoved(this, promise);
+            return;
+        }
+        ctx.invokeBind(localAddress, promise);
     }
 
     private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {
@@ -438,7 +511,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeConnect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise) {
-        findContextOutbound(MASK_CONNECT).invokeConnect(remoteAddress, localAddress, promise);
+        DefaultChannelHandlerContext ctx = findContextOutbound(MASK_CONNECT);
+        if (ctx == null) {
+            failRemoved(this, promise);
+            return;
+        }
+        ctx.invokeConnect(remoteAddress, localAddress, promise);
     }
 
     private void invokeConnect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise) {
@@ -472,7 +550,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeDisconnect(ChannelPromise promise) {
-        findContextOutbound(MASK_DISCONNECT).invokeDisconnect(promise);
+        DefaultChannelHandlerContext ctx = findContextOutbound(MASK_DISCONNECT);
+        if (ctx == null) {
+            failRemoved(this, promise);
+            return;
+        }
+        ctx.invokeDisconnect(promise);
     }
 
     private void invokeDisconnect(ChannelPromise promise) {
@@ -500,7 +583,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeClose(ChannelPromise promise) {
-        findContextOutbound(MASK_CLOSE).invokeClose(promise);
+        DefaultChannelHandlerContext ctx = findContextOutbound(MASK_CLOSE);
+        if (ctx == null) {
+            failRemoved(this, promise);
+            return;
+        }
+        ctx.invokeClose(promise);
     }
 
     private void invokeClose(ChannelPromise promise) {
@@ -528,7 +616,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeRegister(ChannelPromise promise) {
-        findContextOutbound(MASK_REGISTER).invokeRegister(promise);
+        DefaultChannelHandlerContext ctx = findContextOutbound(MASK_REGISTER);
+        if (ctx == null) {
+            failRemoved(this, promise);
+            return;
+        }
+        ctx.invokeRegister(promise);
     }
 
     private void invokeRegister(ChannelPromise promise) {
@@ -556,7 +649,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeDeregister(ChannelPromise promise) {
-        findContextOutbound(MASK_DEREGISTER).invokeDeregister(promise);
+        DefaultChannelHandlerContext ctx = findContextOutbound(MASK_DEREGISTER);
+        if (ctx == null) {
+            failRemoved(this, promise);
+            return;
+        }
+        ctx.invokeDeregister(promise);
     }
 
     private void invokeDeregister(ChannelPromise promise) {
@@ -580,7 +678,10 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeRead() {
-        findContextOutbound(MASK_READ).invokeRead();
+        DefaultChannelHandlerContext ctx = findContextOutbound(MASK_READ);
+        if (ctx != null) {
+            ctx.invokeRead();
+        }
     }
 
     private void invokeRead() {
@@ -595,7 +696,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
         if ((executionMask & MASK_EXCEPTION_CAUGHT) != 0) {
             notifyHandlerException(t);
         } else {
-            findContextInbound(MASK_EXCEPTION_CAUGHT).notifyHandlerException(t);
+            DefaultChannelHandlerContext ctx = findContextInbound(MASK_EXCEPTION_CAUGHT);
+            if (ctx == null) {
+                notifyHandlerRemovedAlready();
+                return;
+            }
+            ctx.invokeExceptionCaught(t);
         }
     }
 
@@ -634,7 +740,10 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
     }
 
     private void findAndInvokeFlush() {
-        findContextOutbound(MASK_FLUSH).invokeFlush();
+        DefaultChannelHandlerContext ctx = findContextOutbound(MASK_FLUSH);
+        if (ctx != null) {
+            ctx.invokeFlush();
+        }
     }
 
     private void invokeFlush() {
@@ -673,6 +782,11 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
         if (executor.inEventLoop()) {
             final DefaultChannelHandlerContext next = findContextOutbound(flush ?
                     (MASK_WRITE | MASK_FLUSH) : MASK_WRITE);
+            if (next == null) {
+                ReferenceCountUtil.release(msg);
+                failRemoved(this, promise);
+                return;
+            }
             if (flush) {
                 next.invokeWriteAndFlush(msg, promise);
             } else {
@@ -796,17 +910,23 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
 
     private DefaultChannelHandlerContext findContextInbound(int mask) {
         DefaultChannelHandlerContext ctx = this;
+        if (ctx.next == null) {
+            return null;
+        }
         do {
             ctx = ctx.next;
-        } while ((ctx.executionMask & mask) == 0);
+        } while ((ctx.executionMask & mask) == 0 || ctx.handlerState == REMOVE_STARTED);
         return ctx;
     }
 
     private DefaultChannelHandlerContext findContextOutbound(int mask) {
         DefaultChannelHandlerContext ctx = this;
+        if (ctx.prev == null) {
+            return null;
+        }
         do {
             ctx = ctx.prev;
-        } while ((ctx.executionMask & mask) == 0);
+        } while ((ctx.executionMask & mask) == 0 || ctx.handlerState == REMOVE_STARTED);
         return ctx;
     }
 
@@ -815,15 +935,11 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
         return channel().voidPromise();
     }
 
-    private void setRemoved() {
-        handlerState = REMOVE_COMPLETE;
-    }
-
     boolean setAddComplete() {
         // Ensure we never update when the handlerState is REMOVE_COMPLETE already.
         // oldState is usually ADD_PENDING but can also be REMOVE_COMPLETE when an EventExecutor is used that is not
         // exposing ordering guarantees.
-        if (handlerState != REMOVE_COMPLETE) {
+        if (handlerState == INIT) {
             handlerState = ADD_COMPLETE;
             return true;
         }
@@ -842,11 +958,12 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
         try {
             // Only call handlerRemoved(...) if we called handlerAdded(...) before.
             if (handlerState == ADD_COMPLETE) {
+                handlerState = REMOVE_STARTED;
                 handler().handlerRemoved(this);
             }
         } finally {
             // Mark the handler as removed in any case.
-            setRemoved();
+            handlerState = REMOVE_COMPLETE;
         }
     }
 
@@ -855,6 +972,24 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
         return handlerState == REMOVE_COMPLETE;
     }
 
+    void remove(boolean relink) {
+        assert handlerState == REMOVE_COMPLETE;
+        if (relink) {
+            DefaultChannelHandlerContext prev = this.prev;
+            DefaultChannelHandlerContext next = this.next;
+            // prev and next may be null if the handler was never really added to the pipeline
+            if (prev != null) {
+                prev.next = next;
+            }
+            if (next != null) {
+                next.prev = prev;
+            }
+        }
+
+        this.prev = null;
+        this.next = null;
+    }
+
     @Override
     public <T> Attribute<T> attr(AttributeKey<T> key) {
         return channel().attr(key);
@@ -931,6 +1066,11 @@ final class DefaultChannelHandlerContext implements ChannelHandlerContext, Resou
             try {
                 decrementPendingOutboundBytes();
                 DefaultChannelHandlerContext next = findContext(ctx);
+                if (next == null) {
+                    ReferenceCountUtil.release(msg);
+                    failRemoved(ctx, promise);
+                    return;
+                }
                 write(next, msg, promise);
             } finally {
                 recycle();

transport/src/main/java/io/netty/channel/DefaultChannelPipeline.java

@@ -48,6 +48,7 @@ public class DefaultChannelPipeline implements ChannelPipeline {
     private static final InternalLogger logger = InternalLoggerFactory.getInstance(DefaultChannelPipeline.class);
     private static final String HEAD_NAME = generateName0(HeadHandler.class);
     private static final String TAIL_NAME = generateName0(TailHandler.class);
+
     private static final ChannelHandler HEAD_HANDLER = new HeadHandler();
     private static final ChannelHandler TAIL_HANDLER = new TailHandler();
 
@@ -64,6 +65,7 @@ public class DefaultChannelPipeline implements ChannelPipeline {
                     DefaultChannelPipeline.class, MessageSizeEstimator.Handle.class, "estimatorHandle");
     private final DefaultChannelHandlerContext head;
     private final DefaultChannelHandlerContext tail;
+
     private final Channel channel;
     private final ChannelFuture succeededFuture;
     private final VoidChannelPromise voidPromise;
@@ -102,9 +104,26 @@ public class DefaultChannelPipeline implements ChannelPipeline {
     }
 
     private DefaultChannelHandlerContext newContext(String name, ChannelHandler handler) {
+        checkMultiplicity(handler);
+        if (name == null) {
+            name = generateName(handler);
+        }
         return new DefaultChannelHandlerContext(this, name, handler);
     }
 
+    private void checkDuplicateName(String name) {
+        if (context(name) != null) {
+            throw new IllegalArgumentException("Duplicate handler name: " + name);
+        }
+    }
+
+    private int checkNoSuchElement(int idx, String name) {
+        if (idx == -1) {
+            throw new NoSuchElementException(name);
+        }
+        return idx;
+    }
+
     @Override
     public final Channel channel() {
         return channel;
@@ -117,18 +136,12 @@ public class DefaultChannelPipeline implements ChannelPipeline {
 
     @Override
     public final ChannelPipeline addFirst(String name, ChannelHandler handler) {
-        checkMultiplicity(handler);
-        if (name == null) {
-            name = generateName(handler);
-        }
-
         DefaultChannelHandlerContext newCtx = newContext(name, handler);
         EventExecutor executor = executor();
         boolean inEventLoop = executor.inEventLoop();
         synchronized (handlers) {
-            if (context(name) != null) {
+            checkDuplicateName(newCtx.name());
-                throw new IllegalArgumentException("Duplicate handler name: " + name);
+
-            }
             handlers.add(0, newCtx);
             if (!inEventLoop) {
                 try {
@@ -156,18 +169,12 @@ public class DefaultChannelPipeline implements ChannelPipeline {
 
     @Override
     public final ChannelPipeline addLast(String name, ChannelHandler handler) {
-        checkMultiplicity(handler);
-        if (name == null) {
-            name = generateName(handler);
-        }
-
         DefaultChannelHandlerContext newCtx = newContext(name, handler);
         EventExecutor executor = executor();
         boolean inEventLoop = executor.inEventLoop();
         synchronized (handlers) {
-            if (context(name) != null) {
+            checkDuplicateName(newCtx.name());
-                throw new IllegalArgumentException("Duplicate handler name: " + name);
+
-            }
             handlers.add(newCtx);
             if (!inEventLoop) {
                 try {
@@ -197,24 +204,13 @@ public class DefaultChannelPipeline implements ChannelPipeline {
     public final ChannelPipeline addBefore(String baseName, String name, ChannelHandler handler) {
         final DefaultChannelHandlerContext ctx;
 
-        checkMultiplicity(handler);
-        if (name == null) {
-            name = generateName(handler);
-        }
-
         DefaultChannelHandlerContext newCtx = newContext(name, handler);
         EventExecutor executor = executor();
         boolean inEventLoop = executor.inEventLoop();
         synchronized (handlers) {
-            int i = findCtxIdx(context -> context.name().equals(baseName));
+            int i = checkNoSuchElement(findCtxIdx(context -> context.name().equals(baseName)), baseName);
+            checkDuplicateName(newCtx.name());
 
-            if (i == -1) {
-                throw new NoSuchElementException(baseName);
-            }
-
-            if (context(name) != null) {
-                throw new IllegalArgumentException("Duplicate handler name: " + name);
-            }
             ctx = handlers.get(i);
             handlers.add(i, newCtx);
             if (!inEventLoop) {
@@ -244,7 +240,6 @@ public class DefaultChannelPipeline implements ChannelPipeline {
     public final ChannelPipeline addAfter(String baseName, String name, ChannelHandler handler) {
         final DefaultChannelHandlerContext ctx;
 
-        checkMultiplicity(handler);
         if (name == null) {
             name = generateName(handler);
         }
@@ -253,15 +248,10 @@ public class DefaultChannelPipeline implements ChannelPipeline {
         EventExecutor executor = executor();
         boolean inEventLoop = executor.inEventLoop();
         synchronized (handlers) {
-            int i = findCtxIdx(context -> context.name().equals(baseName));
+            int i = checkNoSuchElement(findCtxIdx(context -> context.name().equals(baseName)), baseName);
 
-            if (i == -1) {
+            checkDuplicateName(newCtx.name());
-                throw new NoSuchElementException(baseName);
-            }
 
-            if (context(name) != null) {
-                throw new IllegalArgumentException("Duplicate handler name: " + name);
-            }
             ctx = handlers.get(i);
             handlers.add(i + 1, newCtx);
             if (!inEventLoop) {
@@ -377,21 +367,13 @@ public class DefaultChannelPipeline implements ChannelPipeline {
         EventExecutor executor = executor();
         boolean inEventLoop = executor.inEventLoop();
         synchronized (handlers) {
-            int idx = findCtxIdx(context -> context.handler() == handler);
+            int idx = checkNoSuchElement(findCtxIdx(context -> context.handler() == handler), null);
-            if (idx == -1) {
-                throw new NoSuchElementException();
-            }
             ctx = handlers.remove(idx);
             assert ctx != null;
 
             if (!inEventLoop) {
-                try {
+                scheduleRemove(idx, ctx);
-                    executor.execute(() -> remove0(ctx));
                 return this;
-                } catch (Throwable cause) {
-                    handlers.add(idx, ctx);
-                    throw cause;
-                }
             }
         }
 
@@ -405,21 +387,12 @@ public class DefaultChannelPipeline implements ChannelPipeline {
         EventExecutor executor = executor();
         boolean inEventLoop = executor.inEventLoop();
         synchronized (handlers) {
-            int idx = findCtxIdx(context -> context.name().equals(name));
+            int idx = checkNoSuchElement(findCtxIdx(context -> context.name().equals(name)), name);
-            if (idx == -1) {
-                throw new NoSuchElementException();
-            }
             ctx = handlers.remove(idx);
             assert ctx != null;
 
             if (!inEventLoop) {
-                try {
+                return scheduleRemove(idx, ctx);
-                    executor.execute(() -> remove0(ctx));
-                    return ctx.handler();
-                } catch (Throwable cause) {
-                    handlers.add(idx, ctx);
-                    throw cause;
-                }
             }
         }
 
@@ -434,21 +407,13 @@ public class DefaultChannelPipeline implements ChannelPipeline {
         EventExecutor executor = executor();
         boolean inEventLoop = executor.inEventLoop();
         synchronized (handlers) {
-            int idx = findCtxIdx(context -> handlerType.isAssignableFrom(context.handler().getClass()));
+            int idx = checkNoSuchElement(findCtxIdx(
-            if (idx == -1) {
+                    context -> handlerType.isAssignableFrom(context.handler().getClass())), null);
-                throw new NoSuchElementException();
-            }
             ctx = handlers.remove(idx);
             assert ctx != null;
 
             if (!inEventLoop) {
-                try {
+                return scheduleRemove(idx, ctx);
-                    executor.execute(() -> remove0(ctx));
-                    return (T) ctx.handler();
-                } catch (Throwable cause) {
-                    handlers.add(idx, ctx);
-                    throw cause;
-                }
             }
         }
 
@@ -483,30 +448,19 @@ public class DefaultChannelPipeline implements ChannelPipeline {
             assert ctx != null;
 
             if (!inEventLoop) {
-                try {
+                return scheduleRemove(idx, ctx);
-                    executor.execute(() -> remove0(ctx));
-                    return (T) ctx.handler();
-                } catch (Throwable cause) {
-                    handlers.add(idx, ctx);
-                    throw cause;
-                }
             }
         }
         remove0(ctx);
         return (T) ctx.handler();
     }
 
-    private void unlink(DefaultChannelHandlerContext ctx) {
-        assert ctx != head && ctx != tail;
-        DefaultChannelHandlerContext prev = ctx.prev;
-        DefaultChannelHandlerContext next = ctx.next;
-        prev.next = next;
-        next.prev = prev;
-    }
-
     private void remove0(DefaultChannelHandlerContext ctx) {
-        unlink(ctx);
+        try {
             callHandlerRemoved0(ctx);
+        } finally {
+            ctx.remove(true);
+        }
     }
 
     @Override
@@ -529,27 +483,17 @@ public class DefaultChannelPipeline implements ChannelPipeline {
 
     private ChannelHandler replace(
             Predicate<DefaultChannelHandlerContext> predicate, String newName, ChannelHandler newHandler) {
-        checkMultiplicity(newHandler);
-
-        if (newName == null) {
-            newName = generateName(newHandler);
-        }
         DefaultChannelHandlerContext oldCtx;
         DefaultChannelHandlerContext newCtx = newContext(newName, newHandler);
         EventExecutor executor = executor();
         boolean inEventLoop = executor.inEventLoop();
         synchronized (handlers) {
-            int idx = findCtxIdx(predicate);
+            int idx = checkNoSuchElement(findCtxIdx(predicate), null);
-            if (idx == -1) {
-                throw new NoSuchElementException();
-            }
             oldCtx = handlers.get(idx);
             assert oldCtx != head && oldCtx != tail && oldCtx != null;
 
-            if (!oldCtx.name().equals(newName)) {
+            if (!oldCtx.name().equals(newCtx.name())) {
-                if (context(newName) != null) {
+                checkDuplicateName(newCtx.name());
-                    throw new IllegalArgumentException("Duplicate handler name: " + newName);
-                }
             }
             DefaultChannelHandlerContext removed = handlers.set(idx, newCtx);
             assert removed != null;
@@ -586,11 +530,15 @@ public class DefaultChannelPipeline implements ChannelPipeline {
         oldCtx.prev = newCtx;
         oldCtx.next = newCtx;
 
+        try {
             // Invoke newHandler.handlerAdded() first (i.e. before oldHandler.handlerRemoved() is invoked)
             // because callHandlerRemoved() will trigger channelRead() or flush() on newHandler and those
             // event handlers must be called after handlerAdded().
             callHandlerAdded0(newCtx);
             callHandlerRemoved0(oldCtx);
+        } finally {
+            oldCtx.remove(false);
+        }
     }
 
     private static void checkMultiplicity(ChannelHandler handler) {
@@ -615,7 +563,6 @@ public class DefaultChannelPipeline implements ChannelPipeline {
                     handlers.remove(ctx);
                 }
 
-                unlink(ctx);
                 ctx.callHandlerRemoved();
 
                 removed = true;
@@ -623,6 +570,8 @@ public class DefaultChannelPipeline implements ChannelPipeline {
                 if (logger.isWarnEnabled()) {
                     logger.warn("Failed to remove a handler: " + ctx.name(), t2);
                 }
+            } finally {
+                ctx.remove(true);
             }
 
             if (removed) {
@@ -749,13 +698,7 @@ public class DefaultChannelPipeline implements ChannelPipeline {
             assert ctx != null;
 
             if (!inEventLoop) {
-                try {
+                return scheduleRemove(idx, ctx);
-                    executor.execute(() -> remove0(ctx));
-                    return ctx.handler();
-                } catch (Throwable cause) {
-                    handlers.add(idx, ctx);
-                    throw cause;
-                }
             }
         }
         remove0(ctx);
@@ -777,19 +720,24 @@ public class DefaultChannelPipeline implements ChannelPipeline {
             assert ctx != null;
 
             if (!inEventLoop) {
-                try {
+                return scheduleRemove(idx, ctx);
-                    executor.execute(() -> remove0(ctx));
-                    return ctx.handler();
-                } catch (Throwable cause) {
-                    handlers.add(idx, ctx);
-                    throw cause;
-                }
             }
         }
         remove0(ctx);
         return ctx.handler();
     }
 
+    @SuppressWarnings("unchecked")
+    private <T extends ChannelHandler> T scheduleRemove(int idx, DefaultChannelHandlerContext ctx) {
+        try {
+            ctx.executor().execute(() -> remove0(ctx));
+            return (T) ctx.handler();
+        } catch (Throwable cause) {
+            handlers.add(idx, ctx);
+            throw cause;
+        }
+    }
+
     @Override
     public ChannelHandler first() {
         ChannelHandlerContext ctx = firstContext();
@@ -849,32 +797,6 @@ public class DefaultChannelPipeline implements ChannelPipeline {
         return this;
     }
 
-    /**
-     * Removes all handlers from the pipeline one by one from tail (exclusive) to head (exclusive) to trigger
-     * handlerRemoved().
-     */
-    private void destroy() {
-        EventExecutor executor = executor();
-        if (executor.inEventLoop()) {
-            destroy0();
-        } else {
-            executor.execute(this::destroy0);
-        }
-    }
-
-    private void destroy0() {
-        assert executor().inEventLoop();
-        DefaultChannelHandlerContext ctx = this.tail.prev;
-        while (ctx != head) {
-            synchronized (handlers) {
-                handlers.remove(ctx);
-            }
-            remove0(ctx);
-
-            ctx = ctx.prev;
-        }
-    }
-
     @Override
     public final ChannelPipeline fireChannelActive() {
         head.invokeChannelActive();
@@ -980,7 +902,7 @@ public class DefaultChannelPipeline implements ChannelPipeline {
     }
 
     @Override
-    public final ChannelFuture close(ChannelPromise promise) {
+    public ChannelFuture close(ChannelPromise promise) {
         return tail.close(promise);
     }
 
@@ -1135,10 +1057,14 @@ public class DefaultChannelPipeline implements ChannelPipeline {
     private static final class TailHandler implements ChannelHandler {
 
         @Override
-        public void channelRegistered(ChannelHandlerContext ctx) { }
+        public void channelRegistered(ChannelHandlerContext ctx) {
+            // Just swallow event
+        }
 
         @Override
-        public void channelUnregistered(ChannelHandlerContext ctx) { }
+        public void channelUnregistered(ChannelHandlerContext ctx) {
+            // Just swallow event
+        }
 
         @Override
         public void channelActive(ChannelHandlerContext ctx) {
@@ -1155,12 +1081,6 @@ public class DefaultChannelPipeline implements ChannelPipeline {
             ((DefaultChannelPipeline) ctx.pipeline()).onUnhandledChannelWritabilityChanged();
         }
 
-        @Override
-        public void handlerAdded(ChannelHandlerContext ctx) { }
-
-        @Override
-        public void handlerRemoved(ChannelHandlerContext ctx) { }
-
         @Override
         public void userEventTriggered(ChannelHandlerContext ctx, Object evt) {
             ((DefaultChannelPipeline) ctx.pipeline()).onUnhandledInboundUserEventTriggered(evt);
@@ -1232,15 +1152,5 @@ public class DefaultChannelPipeline implements ChannelPipeline {
         public void flush(ChannelHandlerContext ctx) {
             ctx.channel().unsafe().flush();
         }
-
-        @Override
-        public void channelUnregistered(ChannelHandlerContext ctx) {
-            ctx.fireChannelUnregistered();
-
-            // Remove all handlers sequentially if channel is closed and unregistered.
-            if (!ctx.channel().isOpen()) {
-                ((DefaultChannelPipeline) ctx.pipeline()).destroy();
-            }
-        }
     }
 }

transport/src/main/java/io/netty/channel/embedded/EmbeddedChannel.java

@@ -782,64 +782,70 @@ public class EmbeddedChannel extends AbstractChannel {
                 return EmbeddedUnsafe.this.remoteAddress();
             }
 
+            private void mayRunPendingTasks() {
+                if (!((EmbeddedEventLoop) eventLoop()).running) {
+                    runPendingTasks();
+                }
+            }
+
             @Override
             public void register(ChannelPromise promise) {
                 EmbeddedUnsafe.this.register(promise);
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override
             public void bind(SocketAddress localAddress, ChannelPromise promise) {
                 EmbeddedUnsafe.this.bind(localAddress, promise);
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override
             public void connect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise) {
                 EmbeddedUnsafe.this.connect(remoteAddress, localAddress, promise);
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override
             public void disconnect(ChannelPromise promise) {
                 EmbeddedUnsafe.this.disconnect(promise);
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override
             public void close(ChannelPromise promise) {
                 EmbeddedUnsafe.this.close(promise);
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override
             public void closeForcibly() {
                 EmbeddedUnsafe.this.closeForcibly();
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override
             public void deregister(ChannelPromise promise) {
                 EmbeddedUnsafe.this.deregister(promise);
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override
             public void beginRead() {
                 EmbeddedUnsafe.this.beginRead();
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override
             public void write(Object msg, ChannelPromise promise) {
                 EmbeddedUnsafe.this.write(msg, promise);
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override
             public void flush() {
                 EmbeddedUnsafe.this.flush();
-                runPendingTasks();
+                mayRunPendingTasks();
             }
 
             @Override

transport/src/main/java/io/netty/channel/embedded/EmbeddedEventLoop.java

@@ -30,7 +30,7 @@ import java.util.concurrent.TimeUnit;
 final class EmbeddedEventLoop extends AbstractScheduledEventExecutor implements EventLoop {
 
     private final Queue<Runnable> tasks = new ArrayDeque<>(2);
-    private boolean running;
+    boolean running;
 
     private static EmbeddedChannel cast(Channel channel) {
         if (channel instanceof EmbeddedChannel) {

transport/src/test/java/io/netty/channel/DefaultChannelPipelineTest.java

@@ -38,6 +38,7 @@ import io.netty.util.concurrent.Future;
 import io.netty.util.concurrent.ImmediateEventExecutor;
 import io.netty.util.concurrent.Promise;
 import io.netty.util.concurrent.ScheduledFuture;
+import org.hamcrest.Matchers;
 import org.junit.After;
 import org.junit.AfterClass;
 import org.junit.Test;
@@ -50,6 +51,7 @@ import java.util.List;
 import java.util.NoSuchElementException;
 import java.util.Queue;
 import java.util.concurrent.Callable;
+import java.util.concurrent.CompletionException;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -62,6 +64,7 @@ import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
@@ -761,7 +764,7 @@ public class DefaultChannelPipelineTest {
         pipeline.channel().close().syncUninterruptibly();
     }
 
-    @Test(timeout = 2000)
+    @Test(timeout = 5000)
     public void testAddRemoveHandlerCalled() throws Throwable {
         ChannelPipeline pipeline = newLocalChannel().pipeline();
 
@@ -815,6 +818,110 @@ public class DefaultChannelPipelineTest {
         }
     }
 
+    @Test(timeout = 5000)
+    public void testOperationsFailWhenRemoved() {
+        ChannelPipeline pipeline = newLocalChannel().pipeline();
+        try {
+            pipeline.channel().register().syncUninterruptibly();
+
+            ChannelHandler handler = new ChannelHandler() { };
+            pipeline.addFirst(handler);
+            ChannelHandlerContext ctx = pipeline.context(handler);
+            pipeline.remove(handler);
+
+            testOperationsFailsOnContext(ctx);
+        } finally {
+            pipeline.channel().close().syncUninterruptibly();
+        }
+    }
+
+    @Test(timeout = 5000)
+    public void testOperationsFailWhenReplaced() {
+        ChannelPipeline pipeline = newLocalChannel().pipeline();
+        try {
+            pipeline.channel().register().syncUninterruptibly();
+
+            ChannelHandler handler = new ChannelHandler() { };
+            pipeline.addFirst(handler);
+            ChannelHandlerContext ctx = pipeline.context(handler);
+            pipeline.replace(handler, null, new ChannelHandler() { });
+
+            testOperationsFailsOnContext(ctx);
+        } finally {
+            pipeline.channel().close().syncUninterruptibly();
+        }
+    }
+
+    private static void testOperationsFailsOnContext(ChannelHandlerContext ctx) {
+        assertChannelPipelineException(ctx.writeAndFlush(""));
+        assertChannelPipelineException(ctx.write(""));
+        assertChannelPipelineException(ctx.bind(new SocketAddress() { }));
+        assertChannelPipelineException(ctx.close());
+        assertChannelPipelineException(ctx.connect(new SocketAddress() { }));
+        assertChannelPipelineException(ctx.connect(new SocketAddress() { }, new SocketAddress() { }));
+        assertChannelPipelineException(ctx.deregister());
+        assertChannelPipelineException(ctx.disconnect());
+
+        class ChannelPipelineExceptionValidator implements ChannelHandler {
+
+            private Promise<Void> validationPromise = ImmediateEventExecutor.INSTANCE.newPromise();
+
+            @Override
+            public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
+                try {
+                    assertThat(cause, Matchers.instanceOf(ChannelPipelineException.class));
+                } catch (Throwable error) {
+                    validationPromise.setFailure(error);
+                    return;
+                }
+                validationPromise.setSuccess(null);
+            }
+
+            void validate() {
+                validationPromise.syncUninterruptibly();
+                validationPromise = ImmediateEventExecutor.INSTANCE.newPromise();
+            }
+        }
+
+        ChannelPipelineExceptionValidator validator = new ChannelPipelineExceptionValidator();
+        ctx.pipeline().addLast(validator);
+
+        ctx.fireChannelRead("");
+        validator.validate();
+
+        ctx.fireUserEventTriggered("");
+        validator.validate();
+
+        ctx.fireChannelReadComplete();
+        validator.validate();
+
+        ctx.fireExceptionCaught(new Exception());
+        validator.validate();
+
+        ctx.fireChannelActive();
+        validator.validate();
+
+        ctx.fireChannelRegistered();
+        validator.validate();
+
+        ctx.fireChannelInactive();
+        validator.validate();
+
+        ctx.fireChannelUnregistered();
+        validator.validate();
+
+        ctx.fireChannelWritabilityChanged();
+        validator.validate();
+    }
+
+    private static void assertChannelPipelineException(ChannelFuture f) {
+        try {
+            f.syncUninterruptibly();
+        } catch (CompletionException e) {
+            assertThat(e.getCause(), Matchers.instanceOf(ChannelPipelineException.class));
+        }
+    }
+
     @Test(timeout = 3000)
     public void testAddReplaceHandlerCalled() throws Throwable {
         ChannelPipeline pipeline = newLocalChannel().pipeline();

transport/src/test/java/io/netty/channel/PendingWriteQueueTest.java

@@ -232,7 +232,9 @@ public class PendingWriteQueueTest {
 
         ChannelPromise promise = channel.newPromise();
         final ChannelPromise promise3 = channel.newPromise();
-        promise.addListener((ChannelFutureListener) future -> queue.add(3L, promise3));
+        promise.addListener((ChannelFutureListener) future -> {
+            queue.add(3L, promise3);
+        });
         ChannelPromise promise2 = channel.newPromise();
 
         channel.eventLoop().execute(() -> {
@@ -245,8 +247,13 @@ public class PendingWriteQueueTest {
         assertTrue(promise.isSuccess());
         assertTrue(promise2.isDone());
         assertTrue(promise2.isSuccess());
+        assertFalse(promise3.isDone());
+        assertFalse(promise3.isSuccess());
+
+        channel.eventLoop().execute(queue::removeAndWriteAll);
         assertTrue(promise3.isDone());
         assertTrue(promise3.isSuccess());
+        channel.runPendingTasks();
         assertTrue(channel.finish());
         assertEquals(1L, (long) channel.readOutbound());
         assertEquals(2L, (long) channel.readOutbound());