From 57e9e7f7305ab3f73a2175542695b91e4badcbfe Mon Sep 17 00:00:00 2001
From: Thomas Madlener <thomas.madlener@desy.de>
Date: Wed, 5 Jun 2024 13:17:25 +0200
Subject: [PATCH] Make MetaDataHandle::get throw in case the value is not
 available and add more overloads (#195)

* Cleanup unnecessary includes

* Return to always returning a value or throwing an exception

* Add get_optional overload for retrieving the optional value

* Let exception propagate in tests
---
 k4FWCore/include/k4FWCore/MetaDataHandle.h    | 64 ++++++++++++++++---
 .../components/k4FWCoreTest_cellID_reader.cpp |  4 +-
 2 files changed, 56 insertions(+), 12 deletions(-)

diff --git a/k4FWCore/include/k4FWCore/MetaDataHandle.h b/k4FWCore/include/k4FWCore/MetaDataHandle.h
index 20b917c6..c3d55b09 100644
--- a/k4FWCore/include/k4FWCore/MetaDataHandle.h
+++ b/k4FWCore/include/k4FWCore/MetaDataHandle.h
@@ -19,15 +19,9 @@
 #ifndef K4FWCORE_METADATAHANDLE_H
 #define K4FWCORE_METADATAHANDLE_H
 
-// GAUDI
-#include "Gaudi/Algorithm.h"
-
 #include "k4FWCore/PodioDataSvc.h"
-#include "podio/GenericParameters.h"
-
-#include "GaudiKernel/MsgStream.h"
 
-#include <type_traits>
+#include "podio/podioVersion.h"
 
 template <typename T> class MetaDataHandle {
 public:
@@ -36,12 +30,35 @@ template <typename T> class MetaDataHandle {
   MetaDataHandle(const Gaudi::DataHandle& handle, const std::string& descriptor, Gaudi::DataHandle::Mode a);
   ~MetaDataHandle();
 
+  /// Get the value that is stored in this MetaDataHandle
+  ///
+  /// @returns The value for this MetaDataHandle
+  ///
+  /// @throws GaudiException in case the value is not (yet) available
   const T get() const;
-  void    put(T);
+
+  /// Get the (optional) value that is stored in this MetaDataHandle
+  ///
+  /// @returns An optional that contains the value if it was available from the
+  ///          data store and is not engaged otherwise
+  std::optional<T> get_optional() const;
+
+  /// Get the value that is stored in the MetaDataHandle or the provided default
+  /// value in case that is not available
+  ///
+  /// @returns The value stored in the Handle or the default value
+  const T get(const T& defaultValue) const;
+
+  /// Set the value for this MetaDataHandle
+  ///
+  /// @note This can only be called during initalize and/or finalize but not
+  /// during execute for algorithms that use it
+  void put(T);
 
 private:
   std::string fullDescriptor() const;
-  void        checkPodioDataSvc();
+
+  void checkPodioDataSvc();
 
 private:
   ServiceHandle<IDataProviderSvc> m_eds;
@@ -73,9 +90,36 @@ MetaDataHandle<T>::MetaDataHandle(const Gaudi::DataHandle& handle, const std::st
 }
 
 //---------------------------------------------------------------------------
-template <typename T> const T MetaDataHandle<T>::get() const {
+template <typename T> std::optional<T> MetaDataHandle<T>::get_optional() const {
   const auto& frame = m_podio_data_service->getMetaDataFrame();
+#if PODIO_BUILD_VERSION > PODIO_VERSION(0, 99, 0)
   return frame.getParameter<T>(fullDescriptor());
+#else
+  // explicitly make a copy here and move it into the optional below
+  auto val = frame.getParameter<T>(fullDescriptor());
+  // This is what happens if we have a non-present key, so we can use it as a
+  // way to determine if the parameter was (likely) unset. For more details see
+  // https://github.com/AIDASoft/podio/issues/576
+  if (val == T{}) {
+    return std::nullopt;
+  }
+  return std::optional<T>(std::move(val));
+#endif
+}
+
+//---------------------------------------------------------------------------
+template <typename T> const T MetaDataHandle<T>::get() const {
+  const auto maybeVal = get_optional();
+  if (!maybeVal.has_value()) {
+    throw GaudiException("MetaDataHandle empty handle access",
+                         "MetaDataHandle " + fullDescriptor() + " not (yet?) available", StatusCode::FAILURE);
+  }
+  return maybeVal.value();
+}
+
+//---------------------------------------------------------------------------
+template <typename T> const T MetaDataHandle<T>::get(const T& defaultValue) const {
+  return get_optional().value_or(defaultValue);
 }
 
 //---------------------------------------------------------------------------
diff --git a/test/k4FWCoreTest/src/components/k4FWCoreTest_cellID_reader.cpp b/test/k4FWCoreTest/src/components/k4FWCoreTest_cellID_reader.cpp
index 4e6ac18e..464f738c 100644
--- a/test/k4FWCoreTest/src/components/k4FWCoreTest_cellID_reader.cpp
+++ b/test/k4FWCoreTest/src/components/k4FWCoreTest_cellID_reader.cpp
@@ -41,10 +41,10 @@ StatusCode k4FWCoreTest_cellID_reader::initialize() {
 StatusCode k4FWCoreTest_cellID_reader::execute(const EventContext&) const {
   const auto simtrackerhits_coll = m_simTrackerHitReaderHandle.get();
 
-  auto       collID    = simtrackerhits_coll->getID();
   const auto cellIDstr = m_cellIDHandle.get();
+
   if (cellIDstr != cellIDtest) {
-    error() << "ERROR cellID is: " << cellIDstr << endmsg;
+    error() << "ERROR cellID is: " << cellIDstr << "expected (" << cellIDtest << ")" << endmsg;
     return StatusCode::FAILURE;
   }