XWikiDocument toXML lists attachments in an inconsistent order

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Prerequisites

• The student will need to have a Jira account, if it's not the case you can signup here
• The student will also need a https://www.github.com account. You might also want to take a look at Git and GitHub interface documentations.

• Assign yourself to the Jira issue

Issue

Since toXML lists attachment in the order in the internal List, and that the internal list reads them in the order the database gives them, which gives them (database specific) in the order of creation in the DB, the XML output of XWiki can be inconsistent accross wikis especially when importing XMLs.

This leads to differences when comparing the XML although they are the same (problem verified with the SVN commit app).

We need to make sure that the attachments are always listed in the same order (ideally the creation date).

Since getting the creation order is complex we should probably just use ASCII order (any order is good as long as it's stable).

The best is probably the refactor XWikiDocument#attachmentList into a TreeMap (as a bonus it will improve the speed of methods like XWikiDocument#getAttachment()).

Expected Outcome

• The student must commit the changes and create a pull request on https://github.com/xwiki/xwiki-platform
  • The summary of the Pull Request must have a first line matching the JIRA issue following by a colon and then the title of this issue
  • Add a comment in the jira issue mentioning your pull request and provide a link to it

Links

• https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/doc/XWikiDocument.java

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