Allow and skip null handlers when adding a vararg list of handlers (#10751)

Motivation:

Allowing null handlers allows for more convenient idioms in
conditionally adding handlers, e.g.,

ch.pipeline().addLast(
        new FooHandler(),
        condition ? new BarHandler() : null,
        new BazHandler()
);

Modifications:

* Change addFirst(..) and addLast(..) to skip null handlers, rather than
break or short-circuit.
* Add new unit tests.

Result:

* Makes addFirst(..) and addLast(..) behavior more consistent
* Resolves https://github.com/netty/netty/issues/10728

transport/src/main/java/io/netty/channel/DefaultChannelPipeline.java

@@ -342,20 +342,12 @@ public class DefaultChannelPipeline implements ChannelPipeline {
     @Override
     public final ChannelPipeline addFirst(EventExecutorGroup executor, ChannelHandler... handlers) {
         ObjectUtil.checkNotNull(handlers, "handlers");
-        if (handlers.length == 0 || handlers[0] == null) {
-            return this;
-        }
 
-        int size;
+        for (int i = handlers.length - 1; i >= 0; i--) {
-        for (size = 1; size < handlers.length; size ++) {
-            if (handlers[size] == null) {
-                break;
-            }
-        }
-
-        for (int i = size - 1; i >= 0; i --) {
             ChannelHandler h = handlers[i];
-            addFirst(executor, null, h);
+            if (h != null) {
+                addFirst(executor, null, h);
+            }
         }
 
         return this;
@@ -374,11 +366,10 @@ public class DefaultChannelPipeline implements ChannelPipeline {
     public final ChannelPipeline addLast(EventExecutorGroup executor, ChannelHandler... handlers) {
         ObjectUtil.checkNotNull(handlers, "handlers");
 
-        for (ChannelHandler h: handlers) {
+        for (ChannelHandler h : handlers) {
-            if (h == null) {
+            if (h != null) {
-                break;
+                addLast(executor, null, h);
             }
-            addLast(executor, null, h);
         }
 
         return this;

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

@@ -171,6 +171,44 @@ public class DefaultChannelPipelineTest {
         }
     }
 
+    @Test
+    public void testAddLastVarArgsSkipsNull() {
+        ChannelPipeline pipeline = new LocalChannel().pipeline();
+        assertEquals(1, pipeline.names().size());
+
+        pipeline.addLast((ChannelHandler) null, newHandler(), null);
+        assertEquals(2, pipeline.names().size());
+        assertEquals("DefaultChannelPipelineTest$TestHandler#0", pipeline.names().get(0));
+
+        pipeline.addLast(newHandler(), null, newHandler());
+        assertEquals(4, pipeline.names().size());
+        assertEquals("DefaultChannelPipelineTest$TestHandler#0", pipeline.names().get(0));
+        assertEquals("DefaultChannelPipelineTest$TestHandler#1", pipeline.names().get(1));
+        assertEquals("DefaultChannelPipelineTest$TestHandler#2", pipeline.names().get(2));
+
+        pipeline.addLast((ChannelHandler) null);
+        assertEquals(4, pipeline.names().size());
+    }
+
+    @Test
+    public void testAddFirstVarArgsSkipsNull() {
+        ChannelPipeline pipeline = new LocalChannel().pipeline();
+        assertEquals(1, pipeline.names().size());
+
+        pipeline.addFirst((ChannelHandler) null, newHandler(), null);
+        assertEquals(2, pipeline.names().size());
+        assertEquals("DefaultChannelPipelineTest$TestHandler#0", pipeline.names().get(0));
+
+        pipeline.addFirst(newHandler(), null, newHandler());
+        assertEquals(4, pipeline.names().size());
+        assertEquals("DefaultChannelPipelineTest$TestHandler#2", pipeline.names().get(0));
+        assertEquals("DefaultChannelPipelineTest$TestHandler#1", pipeline.names().get(1));
+        assertEquals("DefaultChannelPipelineTest$TestHandler#0", pipeline.names().get(2));
+
+        pipeline.addFirst((ChannelHandler) null);
+        assertEquals(4, pipeline.names().size());
+    }
+
     @Test
     public void testRemoveChannelHandler() {
         ChannelPipeline pipeline = new LocalChannel().pipeline();