exporttree=yes special remotes.
* `git-annex post-receive` of a proxied exporttree=yes special remote's
- annex-tracking-branch needs to exporttree.
-* When there is an incomplete export and a key is received, the proxy
- should check if it's the *last* key that is needed to complete the
- export, and when so, do a final exporttree.
+ annex-tracking-branch should check if the special remote contains all
+ keys in the tree. If so, it can exporttree. If not, record
+ the keys that are needed. (It could always exporttree,
+ but better to avoid leaving it incomplete.)
+* After a key is received, the proxy should check if it's the *last* key
+ that is needed to complete the export, and exporttree when so.
* Handle cases where a single key is used by multiple files in the exported
tree. Need to download from the special remote in order to export
multiple copies to it.