Some backlinks are missing upon import of a bunch of terminal pages linking to each other

How to reproduce:

• export some pages from an older XWiki (before nested pages were introduced in 7.2, for example)
• make sure that there are a lot of wikilinks of these pages to each other
• import these pages on a new version of XWiki

Expected result :

• All backlinks corresponding to the wikilinks mentioned at bullet 2 are properly stored in the backlinks wiki table and UI

Actual result:

• Some of the backlinks are missing from the backlinks table and UI

This is a consequence of XWIKI-17933 , basically depending on the order in which the pages are imported (if the target of the wikilink is imported first, the backlink will be stored properly upon import of the source of the link, if the source is imported first the backlink will not be stored properly).

The full usecase from which the above steps are extracted includes an execution of the Nested pages migrator after the import of the content from the older XWiki version, to bring the content up to date.
Without properly stored backlinks, the nested pages migrator will not re-wire properly wikilinks, and thus will endup with either:

• lots of broken links (if redirects were not preserved during NS Migrator execution)
• or simply messy links if the redirects were preserved during execution of NS Migrator.