Thanks Caelyn.  This patch also needs a doc patch and a whatsnew entry in order to be complete.  It's not obvious to me where the relevant documentation is, though, so perhaps we instead have missing documentation that should be addressed in a separate issue.  The whatsnew entry would still be needed, though.

However, I think there is something important missing here in the patch itself.  It is specified (section 4.2.7 Inclusion Loops) that it is a fatal error for an already included document to be specified in a recursively processed xinclude directive, and indeed failing to detect such a loop has DOS implications.  So, the patch needs to address that before it can be committed.