[#2769] Fix regression when writing different message types

Motivation:

Due a regression NioSocketChannel.doWrite(...) will throw a ClassCastException if you do something like:

channel.write(bytebuf);
channel.write(fileregion);
channel.flush();

Modifications:

Correctly handle writing of different message types by using the correct message count while loop over them.

Result:

No more ClassCastException

testsuite/src/test/java/io/netty/testsuite/transport/socket/SocketFileRegionTest.java

@@ -18,6 +18,7 @@ package io.netty.testsuite.transport.socket;
 import io.netty.bootstrap.Bootstrap;
 import io.netty.bootstrap.ServerBootstrap;
 import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelHandler;
 import io.netty.channel.ChannelHandlerContext;
@@ -83,6 +84,7 @@ public class SocketFileRegionTest extends AbstractSocketTest {
 
     private static void testFileRegion0(
             ServerBootstrap sb, Bootstrap cb, boolean voidPromise, final boolean autoRead) throws Throwable {
+        final int bufferSize = 1024;
         final File file = File.createTempFile("netty-", ".tmp");
         file.deleteOnExit();
 
@@ -96,7 +98,7 @@ public class SocketFileRegionTest extends AbstractSocketTest {
         }
 
         // .. and here comes the real data to transfer.
-        out.write(data);
+        out.write(data, bufferSize, data.length - bufferSize);
 
         // .. and then some extra data which is not supposed to be transferred.
         for (int i = random.nextInt(8192); i > 0; i --) {
@@ -130,10 +132,16 @@ public class SocketFileRegionTest extends AbstractSocketTest {
         Channel sc = sb.bind().sync().channel();
 
         Channel cc = cb.connect().sync().channel();
-        FileRegion region = new DefaultFileRegion(new FileInputStream(file).getChannel(), startOffset, data.length);
+        FileRegion region = new DefaultFileRegion(
+                new FileInputStream(file).getChannel(), startOffset, data.length - bufferSize);
+        // Do write ByteBuf and FileRegion to be sure that mixed writes work
+        //
+        // See https://github.com/netty/netty/issues/2769
         if (voidPromise) {
+            assertEquals(cc.voidPromise(), cc.write(Unpooled.wrappedBuffer(data, 0, bufferSize), cc.voidPromise()));
             assertEquals(cc.voidPromise(), cc.writeAndFlush(region, cc.voidPromise()));
         } else {
+            assertNotEquals(cc.voidPromise(), cc.write(Unpooled.wrappedBuffer(data, 0, bufferSize)));
             assertNotEquals(cc.voidPromise(), cc.writeAndFlush(region));
         }
         while (sh.counter < data.length) {

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

@@ -297,18 +297,17 @@ public class NioSocketChannel extends AbstractNioByteChannel implements io.netty
             }
 
             if (done) {
-                // Release all buffers
+                // Release all written buffers.
-                for (int i = size; i > 0; i --) {
+                //
+                // It's important to loop only over nioBufferCnt as there may be other messages in the
+                // ChannelOutboundBuffer that are not of type ByteBuf and so was not included for gathering-write.
+                //
+                // See https://github.com/netty/netty/issues/2769
+                for (int i = nioBufferCnt; i > 0; i --) {
                     final ByteBuf buf = (ByteBuf) in.current();
                     in.progress(buf.readableBytes());
                     in.remove();
                 }
-
-                // Finish the write loop if no new messages were flushed by in.remove().
-                if (in.isEmpty()) {
-                    clearOpWrite();
-                    break;
-                }
             } else {
                 // Did not write all buffers completely.
                 // Release the fully written buffers and update the indexes of the partially written buffer.