Zero length data frames should apply flow control.

Motivation:
A downstream consumer of Netty failed as emitting zero-length http2 data frames in a unit test resulted in assertion errors in Http2LocalFlowController. Since zero-length frames are valid, an assertion that http2 data frame length must be positive is invalid.

Modifications:

Assertions of data length in Http2LocalFlowController now permit zero.

Result:

Those running netty with assertions on can now emit zero length http2 data frames.

codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2LocalFlowController.java

@@ -309,7 +309,7 @@ public class DefaultHttp2LocalFlowController implements Http2LocalFlowController
          * @throws Http2Exception If too much data is used relative to how much is available.
          */
         void receiveFlowControlledFrame(int dataLength) throws Http2Exception {
-            assert dataLength > 0;
+            assert dataLength >= 0;
 
             // Apply the delta. Even if we throw an exception we want to have taken this delta into account.
             window -= dataLength;

codec-http2/src/main/java/io/netty/handler/codec/http2/Http2LocalFlowController.java

@@ -53,7 +53,7 @@ public interface Http2LocalFlowController extends Http2FlowController {
      * @param stream the stream for which window space should be freed. The connection stream object
      *            must not be used.
      * @param numBytes the number of bytes to be returned to the flow control window.
-     * @throws Http2Exception if the number of bytes returned exceeds the {@link #unconsumedBytes()}
+     * @throws Http2Exception if the number of bytes returned exceeds the {@link #unconsumedBytes}
      *             for the stream.
      */
     void consumeBytes(ChannelHandlerContext ctx, Http2Stream stream, int numBytes) throws Http2Exception;

codec-http2/src/test/java/io/netty/handler/codec/http2/DefaultHttp2ConnectionDecoderTest.java

@@ -174,17 +174,37 @@ public class DefaultHttp2ConnectionDecoderTest {
         int processedBytes = data.readableBytes() + padding;
         mockFlowControl(processedBytes);
         try {
-            decode().onDataRead(ctx, STREAM_ID, data, 10, true);
+            decode().onDataRead(ctx, STREAM_ID, data, padding, true);
             verify(localFlow).receiveFlowControlledFrame(eq(ctx), eq(stream), eq(data), eq(padding), eq(true));
             verify(localFlow).consumeBytes(eq(ctx), eq(stream), eq(processedBytes));
 
-            // Verify that the event was absorbed and not propagated to the oberver.
+            // Verify that the event was absorbed and not propagated to the observer.
             verify(listener, never()).onDataRead(eq(ctx), anyInt(), any(ByteBuf.class), anyInt(), anyBoolean());
         } finally {
             data.release();
         }
     }
 
+    @Test
+    public void emptyDataFrameShouldApplyFlowControl() throws Exception {
+        final ByteBuf data = EMPTY_BUFFER;
+        int padding = 0;
+        int processedBytes = data.readableBytes() + padding;
+        mockFlowControl(processedBytes);
+        try {
+            decode().onDataRead(ctx, STREAM_ID, data, padding, true);
+            verify(localFlow).receiveFlowControlledFrame(eq(ctx), eq(stream), eq(data), eq(padding), eq(true));
+
+            // No bytes were consumed, so there's no window update needed.
+            verify(localFlow, never()).consumeBytes(eq(ctx), eq(stream), eq(processedBytes));
+
+            // Verify that the empty data event was propagated to the observer.
+            verify(listener).onDataRead(eq(ctx), eq(STREAM_ID), eq(data), eq(padding), eq(true));
+        } finally {
+            data.release();
+        }
+    }
+
     @Test(expected = Http2Exception.class)
     public void dataReadForStreamInInvalidStateShouldThrow() throws Exception {
         // Throw an exception when checking stream state.