fix(library_tools): duplicate legacy library content synchronously to avoid race

[kingoftech-v01]

Description

Duplicating a unit containing a LegacyLibraryContentBlock (randomized content / V1 library content) produces a duplicate block with no children: "There are no problems in the specified library of type any. Select another problem type." The bug only appears in production — CI's CELERY_ALWAYS_EAGER=True hides it.

User impact (Course Author): Duplicating a unit with randomized content now produces a functional duplicate with the same children as the source.

Root Cause

Race condition. duplicate_block() (cms/djangoapps/contentstore/utils.py:1149-1219) wraps the whole operation in store.bulk_operations(course_key). Inside, it calls store.create_item(...) and dest_block.studio_post_duplicate(store, source_item), which for LegacyLibraryContentBlock calls self.get_tools().trigger_duplication(...), which fires library_tasks.duplicate_children.delay(...).

bulk_operations on split modulestore buffers writes thread-locally and only flushes on exit. The Celery worker runs in a separate process — its store.get_item(BlockUsageLocator.from_string(dest_block_id)) executes before the parent request ends its bulk op, so the worker sees either ItemNotFoundError or an empty dest block. _sync_children then operates on a block with no children, and the duplicated library_content block ends up with len(children) == 0.

The identical race was fixed for sync_from_library in commit 2b47b8a379 (PR #34030, TNL-11339, 2024-01-09) by switching to .apply(). trigger_duplication was overlooked.

Typo (same file, line 124): The cross-learning-context guard compares source_block to itself — it's always False and silently disabled. Has been broken since the original 2023-11-20 commit e800ae7622.

Fix

Two surgical changes in xmodule/library_tools.py:

1. Fix the self-comparison typo: source_block.scope_ids.usage_id.context_key != source_block.scope_ids.usage_id.context_key → source_block.scope_ids.usage_id.context_key != dest_block.scope_ids.usage_id.context_key.
2. Switch library_tasks.duplicate_children.delay(...) → library_tasks.duplicate_children.apply(kwargs=...) to run in-process. This mirrors the sync_from_library.apply(...) call just above and is the same fix applied to that sibling task in feat: make course -> lib import synchronous #34030.

A TODO comment points at #36544 and #34029 for future readers.

Supporting Information

• Issue: Duplicating units or subsections with units that contain randomized content blocks does not duplicate the contents of the randomized content blocks successfully #36544
• Prior fix for the analogous race: PR feat: make course -> lib import synchronous #34030, commit 2b47b8a379 ("feat: make course -> lib import synchronous")
• Typo introduced: commit e800ae7622 (PR feat: provisionally support V2 libraries in LibraryContentBlock (randomized only) #33263, 2023-11-20)
• Why CI never caught this: CELERY_ALWAYS_EAGER=True in openedx/envs/test.py runs tasks in-thread, hiding the bulk-op race.

Testing Instructions

1. Create a content library with at least one problem.
2. In a course, add a Randomized Content Block pointing at that library.
3. Sync the library and verify the block has children.
4. Duplicate the parent unit.
5. Before the fix: the duplicated randomized block displays "There are no problems in the specified library of type any."
6. After the fix: the duplicated randomized block has the same children as the source.

Three new tests in xmodule/tests/test_library_tools.py:

• test_unit_trigger_duplication_does_not_enqueue_async_task — Unit: asserts .apply() called, not .delay()
• test_integration_trigger_duplication_inside_bulk_operations — Integration: asserts duplicate has children inside an outer bulk_operations
• test_bug_36544_regression_cross_context_guard — Regression: asserts the typo fix rejects cross-course duplication

Deadline

None

Other Information

• No database migrations
• No new dependencies
• No deprecations
• V2 libraries unaffected (studio_post_duplicate gates on is_migrated_to_v2)
• .apply() blocks the CMS request thread for the duration of the sync, matching the already-shipped sync_from_library.apply() path. Duplicate is a user action; blocking until completion is the correct semantics.

Closes #36544

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