More work on git annex export. Made initremote exporttree=yes be required to enable exporting to a special remote. Added a sqlite database to keep track of what files have been exported. That let me fix the known problems with exporting multiple files that have the same content.

The same database lets git annex get (etc) download content from exports. Since an export is not a key/value store, git-annex has to do more verification of content downloaded from an export. Some types of keys, that are not based on checksums (eg WORM and URL), cannot be downloaded from an export. And, git-annex will never trust an export to retain the content of a key, since some other tree could be exported over it at any time.

With git annex get working from exports, it might be nice to also support git annex copy --to export for exporting specific files to them. However, that needs information that is not currently stored in the sqlite database until the export has already completed. One way it could work is for git annex export --fast treeish --to export to put all the filenames in the database but not export anything, and then git annex copy --to export (or even git annex sync --content to send the contents). I don't know if this complication is worth it.

Otherwise, the export feature is fairly close to being complete now. Still need to make renames be handled efficiently, and add support for exporting to more special remotes.

Today's work was supported by the NSF-funded DataLad project.