[[!meta title="streaming for transitive transfers"]]

This todo was originally about `git-annex copy --from --to`, which got
implemented, but without the ability to stream content between the two
remotes.

So this todo remains open, but is now only concerned with
streaming an object that is being received from one remote out to another
repository without first needing to buffer the whole object on disk.

git-annex's remote interface does not currently support that.
`retrieveKeyFile` stores the object into a file. And `storeKey`
sends an object file to a remote.

The interface would need to be extended to support this, and the external
special remote interface as well. As well as git remotes and special
remotes needing to be updated to support the new interface.

There's also a question of buffering. If a object is being received from a
remote faster than it is being sent to the other remote, it has to be
buffered somewhere, eg not in memory. Or the receive interface needs to
include a way to get the sender to pause.

----

Recieving to a file, and sending from the same file as it grows is one
possibility, since that would handle buffering, and it might avoid needing
to change interfaces as much. It would still need a new interface since the
current one does not guarantee the file is written in-order.

A fifo is a possibility, but would certianly not work with remotes
that don't write to the file in-order. Also resuming a download would not
work with a fifo, the sending remote wouldn't know where to resume from.