Add a regression test for AMF metadata with no type

The suite had no AMF test at all, so the null attribute could come back
unnoticed. A typeless <metadata> inside an <object> now has to make
load_amf fail, with the same file carrying a typed one as the control.

tests/libslic3r/CMakeLists.txt

@@ -3,6 +3,7 @@ get_filename_component(_TEST_NAME ${CMAKE_CURRENT_LIST_DIR} NAME)
 add_executable(${_TEST_NAME}_tests
     ${_TEST_NAME}_tests.cpp
     test_3mf.cpp
+    test_amf.cpp
     # Round-trip seam metadata and active/dormant volume settings in both formats.
     test_precise_seam_3mf.cpp
     test_aabbindirect.cpp

tests/libslic3r/test_amf.cpp

@@ -0,0 +1,79 @@
+#include <catch2/catch_test_macros.hpp>
+
+#include "libslic3r/Config.hpp"
+#include "libslic3r/Format/AMF.hpp"
+#include "libslic3r/Model.hpp"
+#include "libslic3r/PrintConfig.hpp"
+
+#include "test_utils.hpp"
+
+#include <boost/nowide/fstream.hpp>
+
+#include <string>
+
+using namespace Slic3r;
+
+namespace {
+
+// The smallest AMF the loader accepts: one object holding one volume, a tetrahedron. The metadata
+// element of the object is dropped in verbatim, so a test can hand the parser a malformed one.
+std::string amf_with_object_metadata(const std::string &object_metadata)
+{
+    return "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"
+           "<amf unit=\"millimeter\">\n"
+           " <object id=\"0\">\n"
+           "  " + object_metadata + "\n"
+           "  <mesh>\n"
+           "   <vertices>\n"
+           "    <vertex><coordinates><x>0</x><y>0</y><z>0</z></coordinates></vertex>\n"
+           "    <vertex><coordinates><x>1</x><y>0</y><z>0</z></coordinates></vertex>\n"
+           "    <vertex><coordinates><x>0</x><y>1</y><z>0</z></coordinates></vertex>\n"
+           "    <vertex><coordinates><x>0</x><y>0</y><z>1</z></coordinates></vertex>\n"
+           "   </vertices>\n"
+           "   <volume>\n"
+           "    <triangle><v1>0</v1><v2>2</v2><v3>1</v3></triangle>\n"
+           "    <triangle><v1>0</v1><v2>1</v2><v3>3</v3></triangle>\n"
+           "    <triangle><v1>0</v1><v2>3</v2><v3>2</v3></triangle>\n"
+           "    <triangle><v1>1</v1><v2>2</v2><v3>3</v3></triangle>\n"
+           "   </volume>\n"
+           "  </mesh>\n"
+           " </object>\n"
+           "</amf>\n";
+}
+
+void write_file(const std::string &path, const std::string &content)
+{
+    boost::nowide::ofstream f(path, std::ios::binary);
+    f << content;
+}
+
+bool load(const std::string &path, Model &model)
+{
+    DynamicPrintConfig        config;
+    ConfigSubstitutionContext substitutions(ForwardCompatibilitySubstitutionRule::Disable);
+    return load_amf(path.c_str(), &config, &substitutions, &model, nullptr);
+}
+
+} // namespace
+
+TEST_CASE("An AMF object metadata element with no type attribute is rejected", "[AMF]")
+{
+    ScopedTemporaryFile tmp(".amf");
+
+    SECTION("with the attribute the file loads")
+    {
+        write_file(tmp.string(), amf_with_object_metadata("<metadata type=\"name\">tetra</metadata>"));
+
+        Model model;
+        REQUIRE(load(tmp.string(), model));
+        CHECK(model.objects.size() == 1);
+    }
+
+    SECTION("without it the load fails instead of reading a null attribute")
+    {
+        write_file(tmp.string(), amf_with_object_metadata("<metadata>tetra</metadata>"));
+
+        Model model;
+        CHECK_FALSE(load(tmp.string(), model));
+    }
+}