The rsync special remote supports exporttree; it would be useful to also support importtree.

The adb special remote shows this is possible using just some fairly standard commands, although find and stat are not well enough standardized to work on all unixes, it would be easy to at least support linux.

So far, the rsync special remote really only needs rsync on the server. And can be used with servers that lock down their shell to only allow rsync to be run. It would be good to also only need rsync for importtree, but there are several roadblocks:

• listing contents: Using rsync would involve a command like rsync --checksum -avz -i --dry-run --out-format='%C %l %M %f\n' $rsync-url empty-directory; that makes the remote rsync checksum all the files, which could be very expensive. But without checksum, only mtime and size is available, which is not really enough to be sure all edits to a file are imported (eg a rename swap of two files that have the same mtime would not be noticed).

  It would be a lot nicer to use find | stat

• retrieving file with a specific content identifier: If rsync --checksum is used, git-annex can just do the same checksum on the downloaded file and make sure rsync retrieved the same content identifier that was requested.

• store/remove/checkpresent with a content identifier: If the only way available to check a content identifier is to run rsync to get the current remote checksum of a file, very wide race windows will be open when the file is large. So a file that is unmodified at the start may get modified and that modification be overwritten.

  This is not acceptable

So, it seems that, importtree would need to be able to run commands other than rsync on the server. --Joey