Make sure CompositeChanneBuffer does not throw a UnsupportedOperationException if discardReadBytes() discard the whole content of the buffer. See #325

buffer/src/main/java/io/netty/buffer/CompositeChannelBuffer.java

@@ -655,6 +655,15 @@ public class CompositeChannelBuffer extends AbstractChannelBuffer {
         final int bytesToMove = capacity() - localReaderIndex;
         List<ChannelBuffer> list = decompose(localReaderIndex, bytesToMove);
 
+
+        // If the list is empty we need to assign a new one because
+        // we get a List that is immutable. 
+        //
+        // See https://github.com/netty/netty/issues/325
+        if (list.isEmpty()) {
+            list = new ArrayList<ChannelBuffer>(1);
+        }
+        
         // Add a new buffer so that the capacity of this composite buffer does
         // not decrease due to the discarded components.
         // XXX Might create too many components if discarded by small amount.

buffer/src/test/java/io/netty/buffer/AbstractChannelBufferTest.java

@@ -1641,4 +1641,12 @@ public abstract class AbstractChannelBufferTest {
         assertFalse(set.contains(elemB));
         assertEquals(0, set.size());
     }
+    
+    // Test case for https://github.com/netty/netty/issues/325
+    @Test
+    public void testDiscardAllReadBytes() {
+        buffer.writerIndex(buffer.capacity());
+        buffer.readerIndex(buffer.writerIndex());
+        buffer.discardReadBytes();
+    }
 }