[#2680] ChannelOutboundBuffer.nioBuffers() should always return non-null array as stated in javadocs

Motivation:

At the moment ChannelOutboundBuffer.nioBuffers() returns null if something is contained in the ChannelOutboundBuffer which is not a ByteBuf. This is a problem for two reasons:
 1 - In the javadocs we state that it will never return null
 2 - We may do a not optimal write as there may be things that could be written via gathering writes

Modifications:

Change ChannelOutboundBuffer.nioBuffers() to never return null but have it contain all ByteBuffer that were found before the non ByteBuf. This way we can do a gathering write and also conform to the javadocs.

Result:

Better speed and also correct implementation in terms of the api.

transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollSocketChannel.java

@@ -63,7 +63,7 @@ public final class EpollSocketChannel extends AbstractEpollChannel implements So
     private volatile boolean inputShutdown;
     private volatile boolean outputShutdown;
 
-    EpollSocketChannel(Channel parent, int fd) throws IOException {
+    EpollSocketChannel(Channel parent, int fd) {
         super(parent, fd, Native.EPOLLIN, true);
         config = new EpollSocketChannelConfig(this);
         // Directly cache the remote and local addresses
@@ -135,16 +135,11 @@ public final class EpollSocketChannel extends AbstractEpollChannel implements So
             return done;
         } else {
             ByteBuffer[] nioBuffers = buf.nioBuffers();
-            return writeBytesMultiple0(in, 1, nioBuffers, nioBuffers.length, readableBytes);
+            return writeBytesMultiple(in, 1, nioBuffers, nioBuffers.length, readableBytes);
         }
     }
 
     private boolean writeBytesMultiple(
-            ChannelOutboundBuffer in, int msgCount, ByteBuffer[] nioBuffers) throws IOException {
-        return writeBytesMultiple0(in, msgCount, nioBuffers, in.nioBufferCount(), in.nioBufferSize());
-    }
-
-    private boolean writeBytesMultiple0(
             ChannelOutboundBuffer in, int msgCount, ByteBuffer[] nioBuffers,
             int nioBufferCnt, long expectedWrittenBytes) throws IOException {
         boolean done = false;
@@ -189,7 +184,7 @@ public final class EpollSocketChannel extends AbstractEpollChannel implements So
         return done;
     }
 
-    private void updateOutboundBuffer(ChannelOutboundBuffer in, long writtenBytes, int msgCount,
+    private static void updateOutboundBuffer(ChannelOutboundBuffer in, long writtenBytes, int msgCount,
                                       boolean done) {
         if (done) {
             // Release all buffers
@@ -274,8 +269,9 @@ public final class EpollSocketChannel extends AbstractEpollChannel implements So
             if (msgCount > 1) {
                 // Ensure the pending writes are made of ByteBufs only.
                 ByteBuffer[] nioBuffers = in.nioBuffers();
-                if (nioBuffers != null) {
-                    if (!writeBytesMultiple(in, msgCount, nioBuffers)) {
+                int nioBufferCount = in.nioBufferCount();
+                if (nioBufferCount != 0) {
+                    if (!writeBytesMultiple(in, msgCount, nioBuffers, nioBufferCount, in.nioBufferSize())) {
                         // was not able to write everything so break here we will get notified later again once
                         // the network stack can handle more writes.
                         break;

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

@@ -388,9 +388,9 @@ public final class ChannelOutboundBuffer {
         int i = flushed;
         while (i != unflushed && (m = buffer[i].msg) != null) {
             if (!(m instanceof ByteBuf)) {
-                this.nioBufferCount = 0;
-                this.nioBufferSize = 0;
-                return null;
+                // Just break out of the loop as we can still use gathering writes for the buffers that we
+                // found by now.
+                break;
             }
 
             Entry entry = buffer[i];

transport/src/main/java/io/netty/channel/socket/nio/NioSocketChannel.java

@@ -249,12 +249,14 @@ public class NioSocketChannel extends AbstractNioByteChannel implements io.netty
 
             // Ensure the pending writes are made of ByteBufs only.
             ByteBuffer[] nioBuffers = in.nioBuffers();
-            if (nioBuffers == null) {
+            int nioBufferCnt = in.nioBufferCount();
+
+            if (nioBufferCnt == 0) {
+                // We have something else beside ByteBuffers to write so fallback to normal writes.
                 super.doWrite(in);
-                return;
+                break;
             }
 
-            int nioBufferCnt = in.nioBufferCount();
             long expectedWrittenBytes = in.nioBufferSize();
 
             final SocketChannel ch = javaChannel();