Once files are added (or removed or moved), need to send those changes to
all the other git clones, at both the git level and the key/value level.

## immediate action items

* At startup, and possibly periodically, look for files we have that
  location tracking indicates remotes do not, and enqueue Uploads for
  them. Also, enqueue Downloads for any files we're missing.
* After git sync, identify content that we don't have that is now available
  on remotes, and transfer. But first, need to ensure that when a remote
  receives content, and updates its location log, it syncs that update
  out.
* When MountWatcher detects a newly mounted drive, rescan git remotes
  in order to get ones on the drive, and do a git sync and file transfers
  to sync any repositories on it.

## longer-term TODO

* Test MountWatcher on Gnome (should work ok) and LXDE (dunno).
* git-annex needs a simple speed control knob, which can be plumbed
  through to, at least, rsync. A good job for an hour in an
  airport somewhere.
* Find a way to probe available outgoing bandwidth, to throttle so
  we don't bufferbloat the network to death.
* Investigate the XMPP approach like dvcs-autosync does, or other ways of
   signaling a change out of band.
* Add a hook, so when there's a change to sync, a program can be run
   and do its own signaling.
* --debug will show often unnecessary work being done. Optimise.
* This assumes the network is connected. It's often not, so the
  [[cloud]] needs to be used to bridge between LANs.
* Configurablity, including only enabling git syncing but not data transfer;
  only uploading new files but not downloading, and only downloading
  files in some directories and not others. See for use cases:
  [[forum/Wishlist:_options_for_syncing_meta-data_and_data]]
* speed up git syncing by using the cached ssh connection for it too
  (will need to use `GIT_SSH`, which needs to point to a command to run,
  not a shell command line)

## data syncing

There are two parts to data syncing. First, map the network and second,
decide what to sync when.

Mapping the network can reuse code in `git annex map`. Once the map is
built, we want to find paths through the network that reach all nodes
eventually, with the least cost. This is a minimum spanning tree problem,
except with a directed graph, so really a Arborescence problem.

With the map, we can determine which nodes to push new content to. Then we
need to control those data transfers, sending to the cheapest nodes first,
and with appropriate rate limiting and control facilities.

This probably will need lots of refinements to get working well.

### first pass: flood syncing

Before mapping the network, the best we can do is flood all files out to every
reachable remote. This is worth doing first, since it's the simplest way to
get the basic functionality of the assistant to work. And we'll need this
anyway.

## done

1. Can use `git annex sync`, which already handles bidirectional syncing.
   When a change is committed, launch the part of `git annex sync` that pushes
   out changes. **done**; changes are pushed out to all remotes in parallel
1. Watch `.git/refs/remotes/` for changes (which would be pushed in from
   another node via `git annex sync`), and run the part of `git annex sync`
   that merges in received changes, and follow it by the part that pushes out
   changes (sending them to any other remotes).
   [The watching can be done with the existing inotify code! This avoids needing
   any special mechanism to notify a remote that it's been synced to.]  
   **done**
1. Periodically retry pushes that failed.  **done** (every half an hour)
1. Also, detect if a push failed due to not being up-to-date, pull,
   and repush. **done**
2. Use a git merge driver that adds both conflicting files,
   so conflicts never break a sync. **done**

* on-disk transfers in progress information files (read/write/enumerate)
  **done**
* locking for the files, so redundant transfer races can be detected,
  and failed transfers noticed **done**
* transfer info for git-annex-shell **done**
* update files as transfers proceed. See [[progressbars]]
  (updating for downloads is easy; for uploads is hard)
* add Transfer queue TChan **done**
* add TransferInfo Map to DaemonStatus for tracking transfers in progress.
  **done**
* Poll transfer in progress info files for changes (use inotify again!
  wow! hammer, meet nail..), and update the TransferInfo Map **done**
* enqueue Transfers (Uploads) as new files are added to the annex by
  Watcher. **done**
* enqueue Tranferrs (Downloads) as new dangling symlinks are noticed by
  Watcher. **done**
  (Note: Needs git-annex branch to be merged before the tree is merged,
  so it knows where to download from. Checked and this is the case.)
* Write basic Transfer handling thread. Multiple such threads need to be
  able to be run at once. Each will need its own independant copy of the 
  Annex state monad. **done**
* Write transfer control thread, which decides when to launch transfers.
  **done**
* Transfer watching has a race on kqueue systems, which makes finished
  fast transfers not be noticed by the TransferWatcher. Which in turn
  prevents the transfer slot being freed and any further transfers
  from happening. So, this approach is too fragile to rely on for
  maintaining the TransferSlots. Instead, need [[todo/assistant_threaded_runtime]],
  which would allow running something for sure when a transfer thread
  finishes. **done**
* Test MountWatcher on KDE, and add whatever dbus events KDE emits when
  drives are mounted. **done**