Fix an infinite loop when writing a zero-length FileRegion

Related: #2964

Motivation:

Writing a zero-length FileRegion to an NIO channel will lead to an
infinite loop.

Modification:

- Do not write a zero-length FileRegion by protecting with proper 'if'.
- Update the testsuite

Result:

Another bug fixed

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

@@ -134,16 +134,23 @@ public class SocketFileRegionTest extends AbstractSocketTest {
         Channel cc = cb.connect().sync().channel();
         FileRegion region = new DefaultFileRegion(
                 new FileInputStream(file).getChannel(), startOffset, data.length - bufferSize);
-        // Do write ByteBuf and FileRegion to be sure that mixed writes work
+        FileRegion emptyRegion = new DefaultFileRegion(new FileInputStream(file).getChannel(), 0, 0);
+
+        // Do write ByteBuf and then FileRegion to ensure that mixed writes work
+        // Also, write an empty FileRegion to test if writing an empty FileRegion does not cause any issues.
         //
         // See https://github.com/netty/netty/issues/2769
+        //     https://github.com/netty/netty/issues/2964
         if (voidPromise) {
             assertEquals(cc.voidPromise(), cc.write(Unpooled.wrappedBuffer(data, 0, bufferSize), cc.voidPromise()));
+            assertEquals(cc.voidPromise(), cc.write(emptyRegion, 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.write(emptyRegion));
             assertNotEquals(cc.voidPromise(), cc.writeAndFlush(region));
         }
+
         while (sh.counter < data.length) {
             if (sh.exception.get() != null) {
                 break;

transport/src/main/java/io/netty/channel/nio/AbstractNioByteChannel.java

@@ -219,12 +219,15 @@ public abstract class AbstractNioByteChannel extends AbstractNioChannel {
                 }
             } else if (msg instanceof FileRegion) {
                 FileRegion region = (FileRegion) msg;
+                boolean done = region.transfered() >= region.count();
                 boolean setOpWrite = false;
-                boolean done = false;
+
+                if (!done) {
                     long flushedAmount = 0;
                     if (writeSpinCount == -1) {
                         writeSpinCount = config().getWriteSpinCount();
                     }
+
                     for (int i = writeSpinCount - 1; i >= 0; i--) {
                         long localFlushedAmount = doWriteFileRegion(region);
                         if (localFlushedAmount == 0) {
@@ -240,6 +243,7 @@ public abstract class AbstractNioByteChannel extends AbstractNioChannel {
                     }
 
                     in.progress(flushedAmount);
+                }
 
                 if (done) {
                     in.remove();