Use xapian for dovecot fts

[arodier]

Starting with Debian buster, dovecot supports xapian indexing engine for full text search. I think it is more up to date and more secure than using solr inside a java engine.

• https://packages.debian.org/bullseye/dovecot-fts-xapian
• https://github.com/grosjo/fts-xapian

[arodier]

For the attachments, we have to wait for this bug to be fixed: https://github.com/grosjo/fts-xapian/issues/68 Otherwise, the backend seems to work fine.

[grosjo]

@arodier Supposedly, latest version fixed the case

[arodier]

I can tell the version in Bullseye does not, from the logs:

doveadm(camille): Info: FTS Xapian: New part (Header=(null),Type=text/csv,Disposition=attachment; filename="file.csv") doveadm(camille): Info: FTS Xapian: Skipping part of type 'text/csv' and disposition 'attachment; filename="file.csv"' doveadm(camille): Info: FTS Xapian: fts_backend_xapian_update_set_mailbox doveadm(camille): Info: FTS Xapian: Unset box 'INBOX' (c0d4e304584e5460dae30000075d7e67)

[arodier]

Bug report filled on Debian Bullseye: https://bugs.debian.org/985654

[arodier]

Fixed in the bullseye branch.

[progmaticltd]

Implemented