fix: store tags in TaggedBlockMixin.tags_v1 field

Previously, we relied on the presence of the xml_attributes field, which
is added by the XMLMixin. However, XMLMixin is not part of the default
XBLOCK_MIXINS, so custom XBlocks weren't able to copy/paste tags.

cms/djangoapps/contentstore/helpers.py

@@ -348,6 +348,7 @@ def _import_xml_node_to_parent(
 
     from cms.lib.xblock.tagging.tagged_block_mixin import TaggedBlockMixin
     if isinstance(new_xblock, TaggedBlockMixin):
+        new_xblock.read_tags_from_node(node)
         new_xblock.add_tags_from_xml()
 
     return new_xblock

cms/lib/xblock/tagging/tagged_block_mixin.py

@@ -3,11 +3,29 @@
 """
 from urllib.parse import quote, unquote
 
+from xblock.fields import Scope, String
+
+
+def _(text):
+    """
+    A noop underscore function that marks strings for extraction.
+    """
+    return text
+
 
 class TaggedBlockMixin:
     """
     Mixin containing XML serializing and parsing functionality for tagged blocks
     """
+
+    tags_v1 = String(
+        display_name=_("Tags v1"),
+        name="tags-v1",
+        help=_("Serialized content tags"),
+        default="",
+        scope=Scope.settings
+    )
+
     def studio_post_duplicate(self, store, source_item):
         """
         Duplicates content tags from the source_item.
@@ -17,7 +35,7 @@ def studio_post_duplicate(self, store, source_item):
 
         if hasattr(source_item, 'serialize_tag_data'):
             tags = source_item.serialize_tag_data()
-            self.xml_attributes['tags-v1'] = tags
+            self.tags_v1 = tags
         self.add_tags_from_xml()
 
     def serialize_tag_data(self):
@@ -67,17 +85,17 @@ def read_tags_from_node(self, node):
         Deserialize and read tag data from node
         """
         if 'tags-v1' in node.attrib:
-            self.xml_attributes['tags-v1'] = str(node.attrib['tags-v1'])
+            self.tags_v1 = str(node.attrib['tags-v1'])
 
     def add_tags_from_xml(self):
         """
-        Parse and add tag data from xml_attributes
+        Parse and add tag data from tags_v1 field
         """
         # This import is done here since we import and use TaggedBlockMixin in the cms settings, but the
         # content_tagging app wouldn't have loaded yet, so importing it outside causes an error
         from openedx.core.djangoapps.content_tagging.api import set_object_tags
 
-        tag_data = self.xml_attributes.get('tags-v1', None) if hasattr(self, 'xml_attributes') else None
+        tag_data = self.tags_v1
         if not tag_data:
             return
 

openedx/core/lib/xblock_serializer/block_serializer.py

@@ -89,6 +89,11 @@ def _serialize_normal_block(self, block) -> etree.Element:
                     with filesystem.open(file_path, 'rb') as fh:
                         data = fh.read()
                     self.static_files.append(StaticFile(name=unit_file.name, data=data, url=None))
+
+        # Serialize and add tag data if any
+        if isinstance(block, TaggedBlockMixin):
+            block.add_tags_to_node(olx_node)
+
         if block.has_children:
             self._serialize_children(block, olx_node)
         return olx_node