FlushConsolidationHandler may suppress flushes by mistake (#9931)

Motivation:

When `consolidatedWhenNoReadInProgress` is true, `channel.writeAndFlush (data) .addListener (f-> channel.writeAndFlush (data2))` Will cause data2 to never be flushed. 

Because the flush operation will synchronously execute the `channel.writeAndFlush (data2))` in the `listener`, and at this time, since the current execution thread is still an `eventloop`(`executor.inEventLoop()` was true), all handlers will be executed synchronously. At this time, since `nextScheduledFlush` is still not null, the `flush` operation of `data2` will be ignored in `FlushConsolidationHandler#scheduleFlush`.

Modification:

 - reset `nextScheduledFlush` before `ctx.flush`
 - use `ObjectUtil` to polish code

Result:

Fixes https://github.com/netty/netty/issues/9923

handler/src/main/java/io/netty/handler/flush/FlushConsolidationHandler.java

@@ -23,6 +23,7 @@ import io.netty.channel.ChannelOutboundHandler;
 import io.netty.channel.ChannelOutboundInvoker;
 import io.netty.channel.ChannelPipeline;
 import io.netty.channel.ChannelPromise;
+import io.netty.util.internal.ObjectUtil;
 
 import java.util.concurrent.Future;
 
@@ -95,20 +96,17 @@ public class FlushConsolidationHandler extends ChannelDuplexHandler {
      *                                        ongoing.
      */
     public FlushConsolidationHandler(int explicitFlushAfterFlushes, boolean consolidateWhenNoReadInProgress) {
-        if (explicitFlushAfterFlushes <= 0) {
-            throw new IllegalArgumentException("explicitFlushAfterFlushes: "
-                    + explicitFlushAfterFlushes + " (expected: > 0)");
-        }
-        this.explicitFlushAfterFlushes = explicitFlushAfterFlushes;
+        this.explicitFlushAfterFlushes =
+                ObjectUtil.checkPositive(explicitFlushAfterFlushes, "explicitFlushAfterFlushes");
         this.consolidateWhenNoReadInProgress = consolidateWhenNoReadInProgress;
-        flushTask = consolidateWhenNoReadInProgress ?
+        this.flushTask = consolidateWhenNoReadInProgress ?
                 new Runnable() {
                     @Override
                     public void run() {
                         if (flushPendingCount > 0 && !readInProgress) {
                             flushPendingCount = 0;
-                            ctx.flush();
                             nextScheduledFlush = null;
+                            ctx.flush();
                         } // else we'll flush when the read completes
                     }
                 }

handler/src/test/java/io/netty/handler/flush/FlushConsolidationHandlerTest.java

@@ -19,6 +19,8 @@ import io.netty.channel.ChannelHandlerContext;
 import io.netty.channel.ChannelInboundHandlerAdapter;
 import io.netty.channel.ChannelOutboundHandlerAdapter;
 import io.netty.channel.embedded.EmbeddedChannel;
+import io.netty.util.concurrent.Future;
+import io.netty.util.concurrent.GenericFutureListener;
 import org.junit.Test;
 
 import java.util.concurrent.atomic.AtomicInteger;
@@ -152,6 +154,26 @@ public class FlushConsolidationHandlerTest {
         assertFalse(channel.finish());
     }
 
+    /**
+     * See https://github.com/netty/netty/issues/9923
+     */
+    @Test
+    public void testResend() throws Exception {
+        final AtomicInteger flushCount = new AtomicInteger();
+        final EmbeddedChannel channel = newChannel(flushCount, true);
+        channel.writeAndFlush(1L).addListener(new GenericFutureListener<Future<? super Void>>() {
+            @Override
+            public void operationComplete(Future<? super Void> future) throws Exception {
+                channel.writeAndFlush(1L);
+            }
+        });
+        channel.flushOutbound();
+        assertEquals(1L, channel.readOutbound());
+        assertEquals(1L, channel.readOutbound());
+        assertNull(channel.readOutbound());
+        assertFalse(channel.finish());
+    }
+
     private static EmbeddedChannel newChannel(final AtomicInteger flushCount, boolean consolidateWhenNoReadInProgress) {
         return new EmbeddedChannel(
                 new ChannelOutboundHandlerAdapter() {