Fix a bug where HttpContentDecoder emits duplicate HttpMessage or raises NPE

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

@@ -56,7 +56,7 @@ public abstract class HttpContentDecoder extends MessageToMessageDecoder<HttpObj
         if (msg instanceof HttpMessage) {
             assert message == null;
             message = (HttpMessage) msg;
-
+            decodeStarted = false;
             cleanup();
         }
 
@@ -100,11 +100,15 @@ public abstract class HttpContentDecoder extends MessageToMessageDecoder<HttpObj
                 }
                 return new Object[] { message, c };
             }
-            return decodeContent(null, c);
+
+            if (decoder != null) {
+                return decodeContent(null, c);
+            } else {
+                return c;
+            }
         }
 
-        // Because FullHttpMessage and HttpChunk is a mutable object, we can simply forward it.
-        return msg;
+        return null;
     }
 
     @Override

example/src/main/java/io/netty/example/http/snoop/HttpSnoopClientInitializer.java

@@ -56,7 +56,7 @@ public class HttpSnoopClientInitializer extends ChannelInitializer<SocketChannel
         p.addLast("inflater", new HttpContentDecompressor());
 
         // Uncomment the following line if you don't want to handle HttpChunks.
-        //pipeline.addLast("aggregator", new HttpChunkAggregator(1048576));
+        //p.addLast("aggregator", new HttpObjectAggregator(1048576));
 
         p.addLast("handler", new HttpSnoopClientHandler());
     }