Force thread group on threads created by Netty's DefaultThreadFactory.

As a workaround for https://github.com/netty/netty/issues/5084

netty/src/main/java/io/grpc/netty/Utils.java

@@ -50,6 +50,7 @@ import io.netty.handler.codec.http2.Http2Headers;
 import io.netty.util.AsciiString;
 import io.netty.util.AttributeKey;
 import io.netty.util.concurrent.DefaultThreadFactory;
+import io.netty.util.concurrent.FastThreadLocalThread;
 
 import java.io.IOException;
 import java.nio.channels.ClosedChannelException;
@@ -201,7 +202,8 @@ class Utils {
     public EventLoopGroup create() {
       // Use Netty's DefaultThreadFactory in order to get the benefit of FastThreadLocal.
       boolean useDaemonThreads = true;
-      ThreadFactory threadFactory = new DefaultThreadFactory(name, useDaemonThreads);
+      ThreadFactory threadFactory = new ThreadGroupSavingDefaultThreadFactory(
+          name, useDaemonThreads);
       int parallelism = numEventLoops == 0
           ? Runtime.getRuntime().availableProcessors() * 2 : numEventLoops;
       return new NioEventLoopGroup(parallelism, threadFactory);
@@ -218,6 +220,22 @@ class Utils {
     }
   }
 
+  // A workaround for https://github.com/netty/netty/issues/5084
+  // TODO(zhangkun83): remove it once the issue has been resolved in netty.
+  private static class ThreadGroupSavingDefaultThreadFactory extends DefaultThreadFactory {
+    final ThreadGroup threadGroup;
+
+    ThreadGroupSavingDefaultThreadFactory(String name, boolean daemon) {
+      super(name, daemon);
+      threadGroup = Thread.currentThread().getThreadGroup();
+    }
+
+    @Override
+    protected Thread newThread(Runnable r, String name) {
+      return new FastThreadLocalThread(threadGroup, r, name);
+    }
+  }
+
   private Utils() {
     // Prevents instantiation
   }