IA.BAK and another project both need a way to let untrusted clients push
git-annex branch changes to a central server. It's desired to only
let a client make non-malicious pushes; a malicious client could screw
up a lot of info in the branch.

I propose adding a git-annex command that can be used in a git pre-receive
hook to do this. --[[Joey]]

There are two levels of checking it seems such a command could do:

1. Only allow certain files to be changed. For example, maybe clients are only
   expected to change location tracking files, and the activity.log
   file, but not others like trust.log.

2. Only allow modifications of data about a specific UUID. The UUID
   would be provided to the command (and could be determined based on a
   per-client ssh key or etc).

  The changes to the branch would be checked, so this needs centralized
  knowledge about the format of each file on the branch. I think this
  mostly exists already in Logs.hs.

Of these the second seems more likely to be useful, but the first would
be by far the easier to add. So, do both?

This might be too limiting for some situations:

* If someone has 2 clients, that are talking with one-another,
  then a push would include changes involving the UUIDs of both clients.
  The command could be given multiple UUIDs to allow, to allow
  for these kinds of setups.

* A client might add a special remote somewhere, but this would need
  changes to remote.log, which the first level of checking would not allow.
  And, it would add another UUID, which the second level of checking would
  need to be configured to allow.

Python implementation
---------------------

I started doing an implementation of this in Python here. The code is
available in [pre-receive-gitannex-check.py](https://gitlab.com/anarcat/puppet-git-annex/raw/a7333fd26af19eb5ee662c261a498c868b8b67e3/files/pre-receive-gitannex-check.py) (permalink, see also
the [latest version](https://gitlab.com/anarcat/puppet-git-annex/blob/master/files/pre-receive-gitannex-check.py)).

I went through what seems to be a rather convoluted design with libgit
because I wanted to have some proper unit tests and generating git
commands by hand in a shell script is rather painful. Also, it
currently adopts a "blocking" approach, ie. it blocks known problems,
but maybe it should be based on an "allow" approach, that is: only
allow certain things to go through. 

So far it only forbids removals and changes to trust.log.  A bunch of
stuff is still missing like parameters (to allow changing the list of
protected files) and checking the log tracking info. Feedback
welcome. --[[anarcat]]

Similar: [[todo/append-only_mode]]