test: Enforce application layering using import linter (#36581)

* test: add import linters to promote clean dependencies

* docs: update a few comments / todos

mypy.ini

@@ -9,6 +9,8 @@ files =
     cms/lib/xblock/upstream_sync.py,
     cms/djangoapps/contentstore/rest_api/v2/views/downstreams.py,
     openedx/core/djangoapps/content/learning_sequences,
+    # FIXME: need to solve type issues and add 'search' app here:
+    # openedx/core/djangoapps/content/search,
     openedx/core/djangoapps/content_staging,
     openedx/core/djangoapps/content_libraries,
     openedx/core/djangoapps/programs/rest_api,

openedx/core/djangoapps/content_staging/models.py

@@ -131,7 +131,6 @@ class UserClipboard(models.Model):
 
     def clean(self):
         """ Check that this model is being used correctly. """
-        # These could probably be replaced with constraints in Django 4.1+
         if self.user.id != self.content.user.id:
             raise ValidationError("User ID mismatch.")
         if self.content.purpose != CLIPBOARD_PURPOSE:

openedx/core/djangoapps/content_staging/views.py

@@ -81,6 +81,12 @@ class ClipboardEndpoint(APIView):
     def post(self, request):
         """
         Put some piece of content into the user's clipboard.
+
+        FIXME: This API needs to be deprecated and replaced by dedicated APIs
+        within each learning context (POST /course/foo/bar/copy, POST
+        /library/foo/bar/copy, etc.) We don't want to encode course- and
+        library-specific logic in content staging, and it shouldn't import
+        course or library modules.
         """
         # Check if the content exists and the user has permission to read it.
         # Parse the usage key:

setup.cfg

@@ -194,3 +194,36 @@ source_modules =
 forbidden_modules =
     openedx_learning.apps
 allow_indirect_imports = True
+
+[importlinter:contract:4]
+name = Low-level apps should not depend on high-level apps
+type = layers
+layers =
+    # Layers from high-level to low-level. Imports should only occur from higher to lower.
+    cms.lib.xblock.upstream_sync | openedx.core.djangoapps.content.search | openedx.core.djangoapps.olx_rest_api
+    openedx.core.djangoapps.content_libraries
+    openedx.core.djangoapps.content_staging
+    openedx.core.djangoapps.xblock
+    openedx.core.lib.xblock_serializer
+    openedx.core.djangoapps.content_tagging
+ignore_imports =
+    # Test code can break these layering rules
+    **.tests.** -> **
+
+    # FIXME: the exceptions below are from before we added this import linting rule. Should refactor to eliminate them.
+    # In particular, the contentstore.helpers module is too big and has too many imports - split it up?
+
+    # The CSV export hard-codes support for courses and libraries. Refactor to do something like learning_context.get_children()
+    openedx.core.djangoapps.content_tagging.helpers.objecttag_export_helpers -> openedx.core.djangoapps.content_libraries.api
+    # The permissions checking code for tagging requires libraries model data to get all the orgs that a user is using:
+    openedx.core.djangoapps.content_tagging.utils -> openedx.core.djangoapps.content_libraries.api
+    # Content staging POST to clipboard API uses libraries APIs. We're working on moving this code to content_libraries
+    openedx.core.djangoapps.content_staging.views -> openedx.core.djangoapps.content_libraries.api
+    # content_staging.serializers imports contentstore.helpers which imports contentstore.utils which imports the libraries API.
+    openedx.core.djangoapps.content_staging.serializers -> cms.djangoapps.contentstore.helpers
+    # content_libraries.rest_api.libraries imports cms.djangoapps.contentstore.views.course which imports
+    # contentstore.toggles which imports djangoapps.content.search.api
+    openedx.core.djangoapps.content_libraries.rest_api.libraries -> cms.djangoapps.contentstore.views.course
+    # Content libraries imports contentstore.helpers which imports upstream_sync
+    openedx.core.djangoapps.content_libraries.api.blocks -> cms.djangoapps.contentstore.helpers
+    openedx.core.djangoapps.content_libraries.api.libraries -> cms.djangoapps.contentstore.helpers