Improve default block fallback logic in Dubbo 2.6.x adapter to avoid serialization problem (#1794)

- convert BlockException to a simple RuntimeException (with necessary message)

sentinel-adapter/sentinel-dubbo-adapter/src/main/java/com/alibaba/csp/sentinel/adapter/dubbo/fallback/DefaultDubboFallback.java

@@ -29,7 +29,9 @@ public class DefaultDubboFallback implements DubboFallback {
 
     @Override
     public Result handle(Invoker<?> invoker, Invocation invocation, BlockException ex) {
-        // Just wrap the exception.
-        return new RpcResult(new SentinelRpcException(ex));
+        // Just wrap the exception. edit by wzg923 2020/9/23
+        RpcResult result = new RpcResult();
+        result.setException(new SentinelRpcException(ex.toRuntimeException()));
+        return result;
     }
 }

sentinel-adapter/sentinel-dubbo-adapter/src/test/java/com/alibaba/csp/sentinel/adapter/dubbo/fallback/DubboFallbackRegistryTest.java

@@ -23,7 +23,6 @@ import com.alibaba.dubbo.rpc.Invocation;
 import com.alibaba.dubbo.rpc.Invoker;
 import com.alibaba.dubbo.rpc.Result;
 import com.alibaba.dubbo.rpc.RpcResult;
-
 import org.junit.Assert;
 import org.junit.Test;