Fix racy ChannelOutboundBuffer.testWriteTaskRejected test. (#8735)

Motivation:

testWriteTaskRejected was racy as we did not ensure we dispatched all events to the executor before shutting it down.

Modifications:

Add a latch to ensure we dispatched everything.

Result:

Fix racy test that failed sometimes before.

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

@@ -383,17 +383,23 @@ public class ChannelOutboundBufferTest {
             }
         };
         final CountDownLatch handlerAddedLatch = new CountDownLatch(1);
+        final CountDownLatch handlerRemovedLatch = new CountDownLatch(1);
         EmbeddedChannel ch = new EmbeddedChannel();
-        ch.pipeline().addLast(executor, new ChannelOutboundHandlerAdapter() {
+        ch.pipeline().addLast(executor, "handler", new ChannelOutboundHandlerAdapter() {
             @Override
             public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
                 promise.setFailure(new AssertionError("Should not be called"));
             }
 
             @Override
-            public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
+            public void handlerAdded(ChannelHandlerContext ctx) {
                 handlerAddedLatch.countDown();
             }
+
+            @Override
+            public void handlerRemoved(ChannelHandlerContext ctx) {
+                handlerRemovedLatch.countDown();
+            }
         });
 
         // Lets wait until we are sure the handler was added.
@@ -436,7 +442,19 @@ public class ChannelOutboundBufferTest {
         assertEquals(0, ch.unsafe().outboundBuffer().totalPendingWriteBytes());
         executeLatch.countDown();
 
+        while (executor.pendingTasks() != 0) {
+            // Wait until there is no more pending task left.
+            Thread.sleep(10);
+        }
+
+        ch.pipeline().remove("handler");
+
+        // Ensure we do not try to shutdown the executor before we handled everything for the Channel. Otherwise
+        // the Executor may reject when the Channel tries to add a task to it.
+        handlerRemovedLatch.await();
+
         safeClose(ch);
+
         executor.shutdownGracefully();
     }