fix: fall back gracefully on malformed persisted selection

Address review feedback: a saved `selected` value that doesn't have
the expected (block_type, block_id) shape would raise during prefetch
and take the render down. Fall back to the full-pool behavior instead,
consistent with every other case here where the saved state can't be
used.

Author: rahulkanneri-2u

lms/djangoapps/courseware/model_data.py

@@ -28,6 +28,7 @@
 from collections import defaultdict, namedtuple
 
 from django.db import DatabaseError, IntegrityError, transaction
+from opaque_keys import InvalidKeyError
 from opaque_keys.edx.asides import AsideUsageKeyV1, AsideUsageKeyV2
 from opaque_keys.edx.block_types import BlockTypeKeyV1
 from opaque_keys.edx.keys import LearningContextKey
@@ -832,7 +833,14 @@ def _persisted_selection_usage_keys(self, block):
             return None
 
         course_key = block.location.course_key
-        return [course_key.make_usage_key(block_type, block_id) for block_type, block_id in selected]
+        try:
+            return [course_key.make_usage_key(block_type, block_id) for block_type, block_id in selected]
+        except (TypeError, ValueError, InvalidKeyError):
+            log.warning(
+                "Malformed 'selected' state for %s / %s while narrowing FieldDataCache prefetch",
+                self.user.id, block.location,
+            )
+            return None
 
     @classmethod
     def cache_for_block_descendents(cls, course_id, user, block, depth=None,

lms/djangoapps/courseware/tests/test_model_data.py

@@ -543,3 +543,24 @@ def test_ignores_empty_selection(self):
 
         parent.get_children.assert_called_once()
         parent.get_child.assert_not_called()
+
+    def test_falls_back_on_malformed_selection(self):
+        """
+        A saved `selected` value that isn't a list of (block_type, block_id)
+        pairs shouldn't blow up the render -- fall back to the full pool, same
+        as any other case where we can't make sense of what's saved.
+        """
+        StudentModule.objects.create(
+            student=self.user,
+            course_id=COURSE_KEY,
+            module_state_key=self.parent_location,
+            module_type='library_content',
+            state=json.dumps({'selected': ['not-a-pair', 'also-not-a-pair']}),
+        )
+        parent = self._make_parent_block()
+
+        cache = FieldDataCache([], COURSE_KEY, self.user)
+        cache.add_block_descendents(parent)
+
+        parent.get_children.assert_called_once()
+        parent.get_child.assert_not_called()