HDF5: Close operations also upon failure (#1866)

Replicate a Golang-inspired defer pattern in order to ensure resource cleanup also upon early return.

---------

Co-authored-by: AI Agent <ai@anthropic.com>

Author: franzpoeschel

CMakeLists.txt

@@ -826,6 +826,7 @@ if(openPMD_BUILD_TESTING)
         elseif(${test_name} STREQUAL "Core")
             list(APPEND ${out_list}
                 test/Files_Core/automatic_variable_encoding.cpp
+                test/Files_Core/read_nonexistent_attribute.cpp
             )
         endif()
     endmacro()

include/openPMD/auxiliary/Defer.hpp

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+namespace openPMD::auxiliary
+{
+template <typename F>
+struct defer_type
+{
+    F functor;
+    bool do_run_this = true;
+    ~defer_type()
+    {
+        if (!do_run_this)
+        {
+            return;
+        }
+        do_run_this = false;
+        std::move(functor)();
+    }
+
+    auto to_opaque() && -> defer_type<std::function<void()>>
+    {
+        do_run_this = false;
+        if (!do_run_this)
+        {
+            return defer_type<std::function<void()>>{{}, false};
+        }
+        else
+        {
+            return defer_type<std::function<void()>>{
+                std::function<void()>{std::move(functor)}};
+        }
+    }
+};
+
+using opaque_defer_type = defer_type<std::function<void()>>;
+
+template <typename F>
+auto defer(F &&functor) -> defer_type<std::remove_reference_t<F>>
+{
+    return defer_type<std::remove_reference_t<F>>{std::forward<F>(functor)};
+}
+} // namespace openPMD::auxiliary