feat: Upstream Sync with Content Library Blocks

This introdues the idea of "upstream" and "downstream" content,
where downstreams (like course components) can pull content updates from
upstreams (like learning core-backed content library blocks). This
supports the upcoming Content Libraries Relaunch Beta for Sumac.
New features include:

* A new XBlockMixin: UpstreamSyncMixin.
* A new CMS Python API: cms.lib.xblock.upstream_sync
* A new CMS JSON API: /api/contentstore/v2/downstreams
* A temporary, very basic UI for syncing from Content Library blocks

Implements:
https://github.com/kdmccormick/edx-platform/blob/kdmccormick/upstream-proto/docs/decisions/0020-upstream-block.rst

Co-Authored-By: Braden MacDonald <braden@opencraft.com>

cms/djangoapps/contentstore/helpers.py

@@ -9,6 +9,7 @@
 
 from attrs import frozen, Factory
 from django.conf import settings
+from django.contrib.auth import get_user_model
 from django.utils.translation import gettext as _
 from opaque_keys.edx.keys import AssetKey, CourseKey, UsageKey
 from opaque_keys.edx.locator import DefinitionLocator, LocalId
@@ -22,6 +23,7 @@
 from xmodule.xml_block import XmlMixin
 
 from cms.djangoapps.models.settings.course_grading import CourseGradingModel
+from cms.lib.xblock.upstream_sync import UpstreamLink, BadUpstream, BadDownstream, fetch_customizable_fields
 from openedx.core.djangoapps.site_configuration import helpers as configuration_helpers
 import openedx.core.djangoapps.content_staging.api as content_staging_api
 import openedx.core.djangoapps.content_tagging.api as content_tagging_api
@@ -30,6 +32,10 @@
 
 log = logging.getLogger(__name__)
 
+
+User = get_user_model()
+
+
 # Note: Grader types are used throughout the platform but most usages are simply in-line
 # strings.  In addition, new grader types can be defined on the fly anytime one is needed
 # (because they're just strings). This dict is an attempt to constrain the sprawl in Studio.
@@ -282,9 +288,10 @@ def import_staged_content_from_user_clipboard(parent_key: UsageKey, request) ->
             node,
             parent_xblock,
             store,
-            user_id=request.user.id,
+            user=request.user,
             slug_hint=user_clipboard.source_usage_key.block_id,
             copied_from_block=str(user_clipboard.source_usage_key),
+            copied_from_version_num=user_clipboard.content.version_num,
             tags=user_clipboard.content.tags,
         )
     # Now handle static files that need to go into Files & Uploads:
@@ -293,7 +300,6 @@ def import_staged_content_from_user_clipboard(parent_key: UsageKey, request) ->
         staged_content_id=user_clipboard.content.id,
         static_files=static_files,
     )
-
     return new_xblock, notices
 
 
@@ -302,12 +308,15 @@ def _import_xml_node_to_parent(
     parent_xblock: XBlock,
     # The modulestore we're using
     store,
-    # The ID of the user who is performing this operation
-    user_id: int,
+    # The user who is performing this operation
+    user: User,
     # Hint to use as usage ID (block_id) for the new XBlock
     slug_hint: str | None = None,
     # UsageKey of the XBlock that this one is a copy of
     copied_from_block: str | None = None,
+    # Positive int version of source block, if applicable (e.g., library block).
+    # Zero if not applicable (e.g., course block).
+    copied_from_version_num: int = 0,
     # Content tags applied to the source XBlock(s)
     tags: dict[str, str] | None = None,
 ) -> XBlock:
@@ -373,12 +382,32 @@ def _import_xml_node_to_parent(
         raise NotImplementedError("We don't yet support pasting XBlocks with children")
     temp_xblock.parent = parent_key
     if copied_from_block:
-        # Store a reference to where this block was copied from, in the 'copied_from_block' field (AuthoringMixin)
-        temp_xblock.copied_from_block = copied_from_block
+        # Try to link the pasted block (downstream) to the copied block (upstream).
+        temp_xblock.upstream = copied_from_block
+        try:
+            UpstreamLink.get_for_block(temp_xblock)
+        except (BadDownstream, BadUpstream):
+            # Usually this will fail. For example, if the copied block is a modulestore course block, it can't be an
+            # upstream. That's fine! Instead, we store a reference to where this block was copied from, in the
+            # 'copied_from_block' field (from AuthoringMixin).
+            temp_xblock.upstream = None
+            temp_xblock.copied_from_block = copied_from_block
+        else:
+            # But if it doesn't fail, then populate the `upstream_version` field based on what was copied. Note that
+            # this could be the latest published version, or it could be an an even newer draft version.
+            temp_xblock.upstream_version = copied_from_version_num
+            # Also, fetch upstream values (`upstream_display_name`, etc.).
+            # Recall that the copied block could be a draft. So, rather than fetching from the published upstream (which
+            # could be older), fetch from the copied block itself. That way, if an author customizes a field, but then
+            # later wants to restore it, it will restore to the value that the field had when the block was pasted. Of
+            # course, if the author later syncs updates from a *future* published upstream version, then that will fetch
+            # new values from the published upstream content.
+            fetch_customizable_fields(upstream=temp_xblock, downstream=temp_xblock, user=user)
+
     # Save the XBlock into modulestore. We need to save the block and its parent for this to work:
-    new_xblock = store.update_item(temp_xblock, user_id, allow_not_found=True)
+    new_xblock = store.update_item(temp_xblock, user.id, allow_not_found=True)
     parent_xblock.children.append(new_xblock.location)
-    store.update_item(parent_xblock, user_id)
+    store.update_item(parent_xblock, user.id)
 
     children_handled = False
     if hasattr(new_xblock, 'studio_post_paste'):
@@ -394,7 +423,7 @@ def _import_xml_node_to_parent(
                 child_node,
                 new_xblock,
                 store,
-                user_id=user_id,
+                user=user,
                 copied_from_block=str(child_copied_from),
                 tags=tags,
             )

cms/djangoapps/contentstore/rest_api/v1/serializers/vertical_block.py

@@ -103,6 +103,18 @@ def get_assets_url(self, obj):
         return None
 
 
+class UpstreamLinkSerializer(serializers.Serializer):
+    """
+    Serializer holding info for syncing a block with its upstream (eg, a library block).
+    """
+    upstream_ref = serializers.CharField()
+    version_synced = serializers.IntegerField()
+    version_available = serializers.IntegerField(allow_null=True)
+    version_declined = serializers.IntegerField(allow_null=True)
+    error_message = serializers.CharField(allow_null=True)
+    ready_to_sync = serializers.BooleanField()
+
+
 class ChildVerticalContainerSerializer(serializers.Serializer):
     """
     Serializer for representing a xblock child of vertical container.
@@ -113,6 +125,7 @@ class ChildVerticalContainerSerializer(serializers.Serializer):
     block_type = serializers.CharField()
     user_partition_info = serializers.DictField()
     user_partitions = serializers.ListField()
+    upstream_link = UpstreamLinkSerializer(allow_null=True)
     actions = serializers.SerializerMethodField()
     validation_messages = MessageValidation(many=True)
     render_error = serializers.CharField()

cms/djangoapps/contentstore/rest_api/v1/views/tests/test_vertical_block.py

@@ -70,6 +70,8 @@ def setup_xblock(self):
             parent=self.vertical.location,
             category="html",
             display_name="Html Content 2",
+            upstream="lb:FakeOrg:FakeLib:html:FakeBlock",
+            upstream_version=5,
         )
 
     def create_block(self, parent, category, display_name, **kwargs):
@@ -193,6 +195,7 @@ def test_children_content(self):
                 "name": self.html_unit_first.display_name_with_default,
                 "block_id": str(self.html_unit_first.location),
                 "block_type": self.html_unit_first.location.block_type,
+                "upstream_link": None,
                 "user_partition_info": expected_user_partition_info,
                 "user_partitions": expected_user_partitions,
                 "actions": {
@@ -218,12 +221,21 @@ def test_children_content(self):
                     "can_delete": True,
                     "can_manage_tags": True,
                 },
+                "upstream_link": {
+                    "upstream_ref": "lb:FakeOrg:FakeLib:html:FakeBlock",
+                    "version_synced": 5,
+                    "version_available": None,
+                    "version_declined": None,
+                    "error_message": "Linked library item was not found in the system",
+                    "ready_to_sync": False,
+                },
                 "user_partition_info": expected_user_partition_info,
                 "user_partitions": expected_user_partitions,
                 "validation_messages": [],
                 "render_error": "",
             },
         ]
+        self.maxDiff = None
         self.assertEqual(response.data["children"], expected_response)
 
     def test_not_valid_usage_key_string(self):

cms/djangoapps/contentstore/rest_api/v1/views/vertical_block.py

@@ -20,6 +20,7 @@
     ContainerHandlerSerializer,
     VerticalContainerSerializer,
 )
+from cms.lib.xblock.upstream_sync import UpstreamLink
 from openedx.core.lib.api.view_utils import view_auth_classes
 from xmodule.modulestore.django import modulestore  # lint-amnesty, pylint: disable=wrong-import-order
 from xmodule.modulestore.exceptions import ItemNotFoundError  # lint-amnesty, pylint: disable=wrong-import-order
@@ -198,6 +199,7 @@ def get(self, request: Request, usage_key_string: str):
                     "block_type": "drag-and-drop-v2",
                     "user_partition_info": {},
                     "user_partitions": {}
+                    "upstream_link": null,
                     "actions": {
                         "can_copy": true,
                         "can_duplicate": true,
@@ -215,6 +217,13 @@ def get(self, request: Request, usage_key_string: str):
                     "block_type": "video",
                     "user_partition_info": {},
                     "user_partitions": {}
+                    "upstream_link": {
+                        "upstream_ref": "lb:org:mylib:video:404",
+                        "version_synced": 16
+                        "version_available": null,
+                        "error_message": "Linked library item not found: lb:org:mylib:video:404",
+                        "ready_to_sync": false,
+                    },
                     "actions": {
                         "can_copy": true,
                         "can_duplicate": true,
@@ -232,6 +241,13 @@ def get(self, request: Request, usage_key_string: str):
                     "block_type": "html",
                     "user_partition_info": {},
                     "user_partitions": {},
+                    "upstream_link": {
+                        "upstream_ref": "lb:org:mylib:html:abcd",
+                        "version_synced": 43,
+                        "version_available": 49,
+                        "error_message": null,
+                        "ready_to_sync": true,
+                    },
                     "actions": {
                         "can_copy": true,
                         "can_duplicate": true,
@@ -267,6 +283,7 @@ def get(self, request: Request, usage_key_string: str):
                     child_info = modulestore().get_item(child)
                     user_partition_info = get_visibility_partition_info(child_info, course=course)
                     user_partitions = get_user_partition_info(child_info, course=course)
+                    upstream_link = UpstreamLink.try_get_for_block(child_info)
                     validation_messages = get_xblock_validation_messages(child_info)
                     render_error = get_xblock_render_error(request, child_info)
 
@@ -277,6 +294,12 @@ def get(self, request: Request, usage_key_string: str):
                         "block_type": child_info.location.block_type,
                         "user_partition_info": user_partition_info,
                         "user_partitions": user_partitions,
+                        "upstream_link": (
+                            # If the block isn't linked to an upstream (which is by far the most common case) then just
+                            # make this field null, which communicates the same info, but with less noise.
+                            upstream_link.to_json() if upstream_link.upstream_ref
+                            else None
+                        ),
                         "validation_messages": validation_messages,
                         "render_error": render_error,
                     })

cms/djangoapps/contentstore/rest_api/v2/urls.py

@@ -1,15 +1,31 @@
 """Contenstore API v2 URLs."""
 
-from django.urls import path
-
-from cms.djangoapps.contentstore.rest_api.v2.views import HomePageCoursesViewV2
+from django.conf import settings
+from django.urls import path, re_path
 
+from cms.djangoapps.contentstore.rest_api.v2.views import home, downstreams
 app_name = "v2"
 
 urlpatterns = [
     path(
         "home/courses",
-        HomePageCoursesViewV2.as_view(),
+        home.HomePageCoursesViewV2.as_view(),
         name="courses",
     ),
+    # TODO: Potential future path.
+    # re_path(
+    #     fr'^downstreams/$',
+    #     downstreams.DownstreamsListView.as_view(),
+    #     name="downstreams_list",
+    # ),
+    re_path(
+        fr'^downstreams/{settings.USAGE_KEY_PATTERN}$',
+        downstreams.DownstreamView.as_view(),
+        name="downstream"
+    ),
+    re_path(
+        fr'^downstreams/{settings.USAGE_KEY_PATTERN}/sync$',
+        downstreams.SyncFromUpstreamView.as_view(),
+        name="sync_from_upstream"
+    ),
 ]

cms/djangoapps/contentstore/rest_api/v2/views/__init__.py

@@ -1,3 +0,0 @@
-"""Module for v2 views."""
-
-from cms.djangoapps.contentstore.rest_api.v2.views.home import HomePageCoursesViewV2