ByteToMessageDecoder.handlerRemoved(...) should only call fireChannel…

…ReadComplete() if fireChannelRead(...) was called before (#9211)

Motivation:

At the moment ByteToMessageDecoder always calls fireChannelReadComplete() when the handler is removed from the pipeline and the cumulation buffer is not null. We should only call it when we also call fireChannelRead(...), which only happens if the cumulation buffer is not null and readable.

Modifications:

Only call fireChannelReadComplete() if fireChannelRead(...) is called before during removal of the handler.

Result:

More correct semantics

codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java

@@ -235,18 +235,16 @@ public final void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
         if (buf != null) {
             // Directly set this to null so we are sure we not access it in any other method here anymore.
             cumulation = null;
-
+            numReads = 0;
             int readable = buf.readableBytes();
             if (readable > 0) {
                 ByteBuf bytes = buf.readBytes(readable);
                 buf.release();
                 ctx.fireChannelRead(bytes);
+                ctx.fireChannelReadComplete();
             } else {
                 buf.release();
             }
-
-            numReads = 0;
-            ctx.fireChannelReadComplete();
         }
         handlerRemoved0(ctx);
     }