tweak-fetch is a new git hook I have developed (not yet accepted into
git, but looking bright). Amoung other things, the hook can be used to
observe what is being fetched, notice remote git-annex branches that might
be updated, and merge them into the git-annex branch.
This will solve problems where users do a git pull, immediately followed
by a push, and it refuses to push because their git-annex branch is
diverged, and they neither ran git annex merge by hand, nor ran other
git-annex commands that auto-merge.
The tweak-fetch is written by git annex init. Of course, existing
repositories won't have it, which is ok, because git-annex still
automatically does a merge if changed branches have appeared. Indeed,
it will always need to do that check, as long as it needs to support
support git-annex branches that might be updated by other means.
Eventually though, I will want to ensure all repositories have the
tweak-fetch hook. Perhaps a minor verison upgrade to ensure it is added?
A subtlety of the hook is that when it's run, the remote tracking refs
have not yet been updated. So Annex.Branch.updateTo has to be careful to
only use the sha1 that was fetched, not the branch name. The branch
name is only used in the commit message.
The other tricky thing is that git tweak-fetch hook should *only*
output lines in a specific format, and git will be unhappy if it also
outputs status messages, etc. So those messages are sent to stderr.
I spent 10 hours today writing code to feed and parse the output of this
hook in C. I had 10 minutes so thought I'd do the same in Haskell,
as a way to get all those pointers, explicit memory management, impure
code, and lack of abstractions out of my head.
Later, if the hook is actually accepted into git, git-annex will be able
to use it to run the git-annex merge, as well as perhaps enable a mode
where pulling from remotes known to git-annex automatically merges
their master branch into the local master.
