Use more reliable logic for specifying overridable dataset config

Copied from HDF5 backend

Author: franzpoeschel

include/openPMD/IO/ADIOS/ADIOS2IOHandler.hpp

@@ -270,9 +270,12 @@ class ADIOS2IOHandlerImpl
         adios2::Params params;
     };
 
-    std::vector<ParameterizedOperator> defaultOperators;
+    // read operators can (currently) not be specified per dataset, so parse
+    // them once and then buffer them
+    std::vector<ParameterizedOperator> readOperators;
 
     json::TracingJSON m_config;
+    std::optional<nlohmann::json> m_buffered_dataset_config;
     static json::TracingJSON nullvalue;
 
     template <typename Callback>

src/IO/ADIOS/ADIOS2IOHandler.cpp

@@ -208,6 +208,19 @@ void ADIOS2IOHandlerImpl::init(
             groupTableViaEnv == 0 ? UseGroupTable::No : UseGroupTable::Yes;
     }
 
+    {
+        constexpr char const *const init_json_shadow_str = R"(
+        {
+          "adios2": {
+            "dataset": {
+              "operators": null
+            }
+          }
+        })";
+        auto init_json_shadow = nlohmann::json::parse(init_json_shadow_str);
+        json::merge(cfg.getShadow(), init_json_shadow);
+    }
+
     if (cfg.json().contains("adios2"))
     {
         m_config = cfg["adios2"];
@@ -266,7 +279,7 @@ void ADIOS2IOHandlerImpl::init(
         auto operators = getOperators();
         if (operators)
         {
-            defaultOperators = std::move(operators.value());
+            readOperators = std::move(operators.value());
         }
     }
 #if !openPMD_HAS_ADIOS_2_9
@@ -753,23 +766,59 @@ void ADIOS2IOHandlerImpl::createDataset(
         filePos->gd = GroupOrDataset::DATASET;
         auto const varName = nameOfVariable(writable);
 
+        json::TracingJSON config = [&]() {
+            if (!m_buffered_dataset_config.has_value())
+            {
+                // we are only interested in these values from the global config
+                constexpr char const *const mask_for_global_conf = R"(
+                {
+                "dataset": {
+                    "operators": null,
+                    "shape": null
+                }
+                })";
+                m_buffered_dataset_config = m_config.json();
+                json::filterByTemplate(
+                    *m_buffered_dataset_config,
+                    nlohmann::json::parse(mask_for_global_conf));
+            }
+            auto parsed_config = json::parseOptions(
+                parameters.options, /* considerFiles = */ false);
+            if (auto adios2_config_it = parsed_config.config.find("adios2");
+                adios2_config_it != parsed_config.config.end())
+            {
+                adios2_config_it.value() = json::merge(
+                    *m_buffered_dataset_config, adios2_config_it.value());
+            }
+            else
+            {
+                parsed_config.config["adios2"] = *m_buffered_dataset_config;
+            }
+            return parsed_config;
+        }();
+
         std::vector<ParameterizedOperator> operators;
-        json::TracingJSON options =
-            json::parseOptions(parameters.options, /* considerFiles = */ false);
-        if (options.json().contains("adios2"))
+        if (config.json().contains("adios2"))
         {
-            json::TracingJSON datasetConfig(options["adios2"]);
+            json::TracingJSON datasetConfig(config["adios2"]);
             auto datasetOperators = getOperators(datasetConfig);
-
-            operators = datasetOperators ? std::move(datasetOperators.value())
-                                         : defaultOperators;
+            if (datasetOperators.has_value())
+            {
+                operators = std::move(*datasetOperators);
+            }
         }
-        else
+
+#if 0
+        std::cout << "Operations for '" << varName << "':";
+        for(auto const & op: operators)
         {
-            operators = defaultOperators;
+            std::cout << " '" << op.op.Type() << "'";
         }
+        std::cout << std::endl;
+#endif
+
         parameters.warnUnusedParameters(
-            options,
+            config,
             "adios2",
             "Warning: parts of the backend configuration for ADIOS2 dataset '" +
                 varName + "' remain unused:\n");
@@ -1936,7 +1985,7 @@ namespace detail
         }
 
         // Operators in reading needed e.g. for setting decompression threads
-        for (auto const &operation : impl->defaultOperators)
+        for (auto const &operation : impl->readOperators)
         {
             if (operation.op)
             {