[#4241] Ensure NioEventLoopGroup.shutdownGracefully(...) with no quiet period shutdown as fast as expected.

Motivation:

If the user uses 0 as quiet period we should shutdown without any delay if possible.

Modifications:

Ensure we not introduce extra delay when a shutdown quit period of 0 is used.

Result:

EventLoop shutdown as fast as expected.

common/src/main/java/io/netty/util/concurrent/SingleThreadEventExecutor.java

@@ -696,7 +696,12 @@ public abstract class SingleThreadEventExecutor extends AbstractScheduledEventEx
                 return true;
             }
 
-            // There were tasks in the queue. Wait a little bit more until no tasks are queued for the quiet period.
+            // There were tasks in the queue. Wait a little bit more until no tasks are queued for the quiet period or
+            // terminate if the quiet period is 0.
+            // See https://github.com/netty/netty/issues/4241
+            if (gracefulShutdownQuietPeriod == 0) {
+                return true;
+            }
             wakeup(true);
             return false;
         }

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

@@ -21,8 +21,11 @@ import io.netty.channel.socket.SocketChannel;
 import io.netty.util.concurrent.DefaultEventExecutorGroup;
 import io.netty.util.concurrent.EventExecutor;
 import io.netty.util.concurrent.EventExecutorGroup;
+import io.netty.util.concurrent.Future;
 import org.junit.Test;
 
+import java.util.concurrent.TimeUnit;
+
 import static org.junit.Assert.*;
 
 public abstract class AbstractEventLoopTest {
@@ -60,6 +63,24 @@ public abstract class AbstractEventLoopTest {
         assertSame(executor, future.channel().pipeline().context(TestChannelHandler2.class).executor());
     }
 
+    @Test(timeout = 5000)
+    public void testShutdownGracefullyNoQuietPeriod() throws Exception {
+        EventLoopGroup loop = newEventLoopGroup();
+        ServerBootstrap b = new ServerBootstrap();
+        b.group(loop)
+                .channel(newChannel())
+                .childHandler(new ChannelInboundHandlerAdapter());
+
+        // Not close the Channel to ensure the EventLoop is still shutdown in time.
+        b.bind(0).sync().channel();
+
+        Future<?> f = loop.shutdownGracefully(0, 1, TimeUnit.MINUTES);
+        assertTrue(loop.awaitTermination(2, TimeUnit.SECONDS));
+        assertTrue(f.syncUninterruptibly().isSuccess());
+        assertTrue(loop.isShutdown());
+        assertTrue(loop.isTerminated());
+    }
+
     private static final class TestChannelHandler extends ChannelDuplexHandler { }
 
     private static final class TestChannelHandler2 extends ChannelDuplexHandler {