fix: ComponentLinks were not deleted when parent container was deleted

[bradenmacdonald]

Description

Fixes #38820

Bug description:

From my investigation, it appears that when the unit is deleted, the corresponding entry is removed from the ContainerLink table as expected, but the associated entries in the ComponentLink table are not removed. As a result, these component links remain in the system even though their downstream usage (the unit) no longer exists in the modulestore.

Explanation: The reason this was happening is that the handler that listens for the item_deleted signal tries to load the deleted item and then delete the ComponentLink entries for each of its children. However, since the item is already deleted, it fails to load the item's children and then fails silently.

openedx-platform/cms/djangoapps/contentstore/signals/handlers.py

Lines 211 to 216 in 17d8ef7

	try:
	deleted_block = modulestore().get_item(usage_key)
	except ItemNotFoundError:
	return
	id_list = {deleted_block.location}
	for block in yield_dynamic_block_descendants(deleted_block, kwargs.get('user_id')):

The simple fix is to use the pre_item_delete signal instead of the item_deleted signal.

Testing instructions

Find a unit in a library that contains at least one component.
Link that unit into a course.
Modify the unit's components in the library, and publish the changes.
Delete the unit from the course.
Without this fix, the ComponentLink will remain and the "Library Updates" tab will show an incorrect number of updates.

Deadline

ASAP, priority for Verawood.

Private ref MNG-4987

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