fix: error pasting a unit containing a completely blank problem

cms/djangoapps/contentstore/helpers.py

@@ -302,6 +302,16 @@ def _import_xml_node_to_parent(
     # and VAL will thus make the transcript available.
 
     child_nodes = []
+
+    if issubclass(xblock_class, XmlMixin):
+        # Hack: XBlocks that use "XmlMixin" have their own XML parsing behavior, and in particular if they encounter
+        # an XML node that has no children and has only a "url_name" attribute, they'll try to load the XML data
+        # from an XML file in runtime.resources_fs. But that file doesn't exist here. So we set at least one
+        # additional attribute here to make sure that url_name is not the only attribute; otherwise in some cases,
+        # XmlMixin.parse_xml will try to load an XML file that doesn't exist, giving an error. The name and value
+        # of this attribute don't matter and should be ignored.
+        node.attrib["x-is-pointer-node"] = "no"
+
     if not xblock_class.has_children:
         # No children to worry about. The XML may contain child nodes, but they're not XBlocks.
         temp_xblock = xblock_class.parse_xml(node, runtime, keys, id_generator)
@@ -314,14 +324,6 @@ def _import_xml_node_to_parent(
         # serialization of a child block, in order. For blocks that don't support children, their XML content/nodes
         # could be anything (e.g. HTML, capa)
         node_without_children = etree.Element(node.tag, **node.attrib)
-        if issubclass(xblock_class, XmlMixin):
-            # Hack: XBlocks that use "XmlMixin" have their own XML parsing behavior, and in particular if they encounter
-            # an XML node that has no children and has only a "url_name" attribute, they'll try to load the XML data
-            # from an XML file in runtime.resources_fs. But that file doesn't exist here. So we set at least one
-            # additional attribute here to make sure that url_name is not the only attribute; otherwise in some cases,
-            # XmlMixin.parse_xml will try to load an XML file that doesn't exist, giving an error. The name and value
-            # of this attribute don't matter and should be ignored.
-            node_without_children.attrib["x-is-pointer-node"] = "no"
         temp_xblock = xblock_class.parse_xml(node_without_children, runtime, keys, id_generator)
         child_nodes = list(node)
     if xblock_class.has_children and temp_xblock.children:

cms/djangoapps/contentstore/views/tests/test_clipboard_paste.py

@@ -3,6 +3,7 @@
 allow users to paste XBlocks that were copied using the staged_content/clipboard
 APIs.
 """
+import ddt
 from opaque_keys.edx.keys import UsageKey
 from rest_framework.test import APIClient
 from xmodule.modulestore.django import contentstore
@@ -13,6 +14,7 @@
 XBLOCK_ENDPOINT = "/xblock/"
 
 
+@ddt.ddt
 class ClipboardPasteTestCase(ModuleStoreTestCase):
     """
     Test Clipboard Paste functionality
@@ -99,6 +101,41 @@ def test_copy_and_paste_unit(self):
         # The new block should store a reference to where it was copied from
         assert dest_unit.copied_from_block == str(unit_key)
 
+    @ddt.data(
+        # A problem with absolutely no fields set. A previous version of copy-paste had an error when pasting this.
+        {"category": "problem", "display_name": None, "data": ""},
+    )
+    def test_copy_and_paste_component(self, block_args):
+        """
+        Test copying a component (XBlock) from one course into another
+        """
+        source_course = CourseFactory.create(display_name='Destination Course')
+        source_block = BlockFactory.create(parent_location=source_course.location, **block_args)
+
+        dest_course = CourseFactory.create(display_name='Destination Course')
+        with self.store.bulk_operations(dest_course.id):
+            dest_chapter = BlockFactory.create(parent=dest_course, category='chapter', display_name='Section')
+            dest_sequential = BlockFactory.create(parent=dest_chapter, category='sequential', display_name='Subsection')
+
+        # Copy the block
+        client = APIClient()
+        client.login(username=self.user.username, password=self.user_password)
+        copy_response = client.post(CLIPBOARD_ENDPOINT, {"usage_key": str(source_block.location)}, format="json")
+        assert copy_response.status_code == 200
+
+        # Paste the unit
+        paste_response = client.post(XBLOCK_ENDPOINT, {
+            "parent_locator": str(dest_sequential.location),
+            "staged_content": "clipboard",
+        }, format="json")
+        assert paste_response.status_code == 200
+        dest_block_key = UsageKey.from_string(paste_response.json()["locator"])
+
+        dest_block = self.store.get_item(dest_block_key)
+        assert dest_block.display_name == source_block.display_name
+        # The new block should store a reference to where it was copied from
+        assert dest_block.copied_from_block == str(source_block.location)
+
     def test_paste_with_assets(self):
         """
         When pasting into a different course, any required static assets should