Overall clean-up in EpollSocketChannel

- Extract writev part from doWrite() for simplicity
- Clearer comments

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

@@ -128,6 +128,45 @@ private int doWriteBytes(ByteBuf buf, int readable) throws Exception {
         return localFlushedAmount;
     }
 
+    private void writeBytesMultiple(
+            ChannelOutboundBuffer in, int msgCount, ByteBuffer[] nioBuffers) throws IOException {
+
+        int nioBufferCnt = in.nioBufferCount();
+        long expectedWrittenBytes = in.nioBufferSize();
+
+        long localWrittenBytes = Native.writev(fd, nioBuffers, 0, nioBufferCnt);
+
+        if (localWrittenBytes < expectedWrittenBytes) {
+            setEpollOut();
+
+            // Did not write all buffers completely.
+            // Release the fully written buffers and update the indexes of the partially written buffer.
+            for (int i = msgCount; i > 0; i --) {
+                final ByteBuf buf = (ByteBuf) in.current();
+                final int readerIndex = buf.readerIndex();
+                final int readableBytes = buf.writerIndex() - readerIndex;
+
+                if (readableBytes < localWrittenBytes) {
+                    in.remove();
+                    localWrittenBytes -= readableBytes;
+                } else if (readableBytes > localWrittenBytes) {
+
+                    buf.readerIndex(readerIndex + (int) localWrittenBytes);
+                    in.progress(localWrittenBytes);
+                    break;
+                } else { // readable == writtenBytes
+                    in.remove();
+                    break;
+                }
+            }
+        } else {
+            // Release all buffers
+            for (int i = msgCount; i > 0; i --) {
+                in.remove();
+            }
+        }
+    }
+
     /**
      * Write a {@link DefaultFileRegion}
      *
@@ -148,51 +187,24 @@ protected void doWrite(ChannelOutboundBuffer in) throws Exception {
                 clearEpollOut();
                 break;
             }
-            // Do non-gathering write for a single buffer case.
+
+            // Do gathering write if:
+            // * the outbound buffer contains more than one messages and
+            // * they are all buffers rather than a file region.
             if (msgCount > 1) {
                 // Ensure the pending writes are made of ByteBufs only.
                 ByteBuffer[] nioBuffers = in.nioBuffers();
                 if (nioBuffers != null) {
+                    writeBytesMultiple(in, msgCount, nioBuffers);
 
-                    int nioBufferCnt = in.nioBufferCount();
-                    long expectedWrittenBytes = in.nioBufferSize();
-
-                    long localWrittenBytes = Native.writev(fd, nioBuffers, 0, nioBufferCnt);
-
-                    if (localWrittenBytes < expectedWrittenBytes) {
-                        setEpollOut();
-
-                        // Did not write all buffers completely.
-                        // Release the fully written buffers and update the indexes of the partially written buffer.
-                        for (int i = msgCount; i > 0; i --) {
-                            final ByteBuf buf = (ByteBuf) in.current();
-                            final int readerIndex = buf.readerIndex();
-                            final int readableBytes = buf.writerIndex() - readerIndex;
-
-                            if (readableBytes < localWrittenBytes) {
-                                in.remove();
-                                localWrittenBytes -= readableBytes;
-                            } else if (readableBytes > localWrittenBytes) {
-
-                                buf.readerIndex(readerIndex + (int) localWrittenBytes);
-                                in.progress(localWrittenBytes);
-                                break;
-                            } else { // readable == writtenBytes
-                                in.remove();
-                                break;
-                            }
-                        }
-                    } else {
-                        // Release all buffers
-                        for (int i = msgCount; i > 0; i --) {
-                            in.remove();
-                        }
-                    }
-                    // try again as a ChannelFuture may be notified in the meantime and triggered another flush
+                    // We do not break the loop here even if the outbound buffer was flushed completely,
+                    // because a user might have triggered another write and flush when we notify his or her
+                    // listeners.
                     continue;
                 }
             }
 
+            // The outbound buffer contains only one message or it contains a file region.
             Object msg = in.current();
             if (msg instanceof ByteBuf) {
                 ByteBuf buf = (ByteBuf) msg;