Fix for Issue #3308 related to slice missing retain

Motivations:
It seems that slicing a buffer and using this slice to write to CTX will
decrease the initial refCnt to 0, while the original buffer is not yet
fully used (not empty).

Modifications:
As suggested in the ticket and tested, when the currentBuffer is sliced
since it will still be used later on, the currentBuffer is retained.

Add a test case for this issue.

Result:
The currentBuffer still has its correct refCnt when reaching the last
write (not sliced) of 1 and therefore will be released correctly.
The exception does no more occur.

This fix should be applied to all branches >= 4.0.

codec-http/src/main/java/io/netty/handler/codec/http/multipart/HttpPostRequestEncoder.java

@@ -828,6 +828,7 @@ public class HttpPostRequestEncoder implements ChunkedInput<HttpContent> {
         int length = currentBuffer.readableBytes();
         if (length > HttpPostBodyUtil.chunkSize) {
             ByteBuf slice = currentBuffer.slice(currentBuffer.readerIndex(), HttpPostBodyUtil.chunkSize);
+            currentBuffer.retain();
             currentBuffer.skipBytes(HttpPostBodyUtil.chunkSize);
             return slice;
         } else {

codec-http/src/test/java/io/netty/handler/codec/http/multipart/HttpPostRequestEncoderTest.java

@@ -16,8 +16,10 @@
 package io.netty.handler.codec.http.multipart;
 
 import io.netty.buffer.ByteBuf;
+import io.netty.buffer.SlicedByteBuf;
 import io.netty.buffer.Unpooled;
 import io.netty.handler.codec.http.DefaultFullHttpRequest;
+import io.netty.handler.codec.http.HttpContent;
 import io.netty.handler.codec.http.HttpMethod;
 import io.netty.handler.codec.http.HttpVersion;
 import io.netty.handler.codec.http.multipart.HttpPostRequestEncoder.EncoderMode;
@@ -26,6 +28,7 @@ import io.netty.util.internal.StringUtil;
 import org.junit.Test;
 
 import java.io.File;
+import java.util.Arrays;
 import java.util.List;
 
 import static io.netty.handler.codec.http.HttpHeaderNames.*;
@@ -187,6 +190,38 @@ public class HttpPostRequestEncoderTest {
         assertEquals(expected, content);
     }
 
+    @Test
+    public void testHttpPostRequestEncoderSlicedBuffer() throws Exception {
+        DefaultFullHttpRequest request = new DefaultFullHttpRequest(HttpVersion.HTTP_1_1,
+                HttpMethod.POST, "http://localhost");
+
+        HttpPostRequestEncoder encoder = new HttpPostRequestEncoder(request, true);
+        // add Form attribute
+        encoder.addBodyAttribute("getform", "POST");
+        encoder.addBodyAttribute("info", "first value");
+        encoder.addBodyAttribute("secondinfo", "secondvalue a&");
+        encoder.addBodyAttribute("thirdinfo", "short text");
+        int length = 100000;
+        char[] array = new char[length];
+        Arrays.fill(array, 'a');
+        String longText = new String(array);
+        encoder.addBodyAttribute("fourthinfo", longText.substring(0, 7470));
+        File file1 = new File(getClass().getResource("/file-01.txt").toURI());
+        encoder.addBodyFileUpload("myfile", file1, "application/x-zip-compressed", false);
+        encoder.finalizeRequest();
+        while (! encoder.isEndOfInput()) {
+            HttpContent httpContent = encoder.readChunk(null);
+            if (httpContent.content() instanceof SlicedByteBuf) {
+                assertEquals(2, httpContent.content().refCnt());
+            } else {
+                assertEquals(1, httpContent.content().refCnt());
+            }
+            httpContent.release();
+        }
+        encoder.cleanFiles();
+        encoder.close();
+    }
+
     private static String getRequestBody(HttpPostRequestEncoder encoder) throws Exception {
         encoder.finalizeRequest();