fix a potential bug

paddle/fluid/operators/reader/create_custom_reader_op.cc

@@ -21,25 +21,22 @@ namespace reader {
 
 class CustomReader : public framework::DecoratedReader {
  public:
-  CustomReader(ReaderBase* reader, const framework::BlockDesc* sub_block,
+  CustomReader(ReaderBase* reader, const framework::BlockDesc& sub_block,
                const platform::Place& dev_place,
                const std::vector<std::string>& source_var_names,
                const std::vector<std::string>& sink_var_names)
       : DecoratedReader(reader),
-        sub_block_(sub_block),
+        program_(*sub_block.Program()),
+        sub_block_id_(sub_block.ID()),
         exe_(framework::Executor(dev_place)),
         source_var_names_(source_var_names),
         sink_var_names_(sink_var_names) {}
 
   void ReadNext(std::vector<framework::LoDTensor>* out) override;
 
-  void UpdateBlockAndScope(const framework::BlockDesc* sub_block,
-                           const framework::Scope* scope) {
-    sub_block_ = sub_block;
-  }
-
  private:
-  const framework::BlockDesc* sub_block_;
+  const framework::ProgramDesc program_;
+  int sub_block_id_;
   framework::Executor exe_;
 
   std::vector<std::string> source_var_names_;
@@ -57,14 +54,12 @@ class CreateCustomReaderOp : public framework::OperatorBase {
                     ->template GetMutable<framework::ReaderHolder>();
     auto* sub_block = Attr<framework::BlockDesc*>("sub_block");
     if (out->Get() != nullptr) {
-      auto* custom_reader = reinterpret_cast<CustomReader*>(out->Get());
-      custom_reader->UpdateBlockAndScope(sub_block, &scope);
       return;
     }
     const auto& underlying_reader = scope.FindVar(Input("UnderlyingReader"))
                                         ->Get<framework::ReaderHolder>();
     out->Reset(
-        new CustomReader(underlying_reader.Get(), sub_block, dev_place,
+        new CustomReader(underlying_reader.Get(), *sub_block, dev_place,
                          Attr<std::vector<std::string>>("source_var_names"),
                          Attr<std::vector<std::string>>("sink_var_names")));
   }
@@ -159,8 +154,7 @@ void CustomReader::ReadNext(std::vector<framework::LoDTensor>* out) {
     tensor->set_lod(underlying_outs[i].lod());
   }
   // 2. Run the sub-block.
-  framework::ProgramDesc* program = sub_block_->Program();
-  exe_.Run(*program, scope, sub_block_->ID(), false, true);
+  exe_.Run(program_, scope, sub_block_id_, false, true);
   // 3. Copy LoDTensors from sink variables to out.
   out->resize(sink_var_names_.size());
   for (size_t i = 0; i < sink_var_names_.size(); ++i) {