80 lines
4.1 KiB
Markdown
80 lines
4.1 KiB
Markdown
Yesterday I spent making a release, and shopping for a new laptop, since
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this one is dying. (Soon I'll be able to compile git-annex fast-ish! Yay!)
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And thinking about [[todo/wishlist:_dropping_git-annex_history]].
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Today, I added the `git annex forget` command. It's currently been lightly
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tested, seems to work, and is living in the `forget` branch until I gain
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confidence with it. It should be perfectly safe to use, even if it's buggy,
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because you can use `git reflog git-annex` to pull out and revert to an old
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version of your git-annex branch. So if you're been wanting this feature,
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please beta test!
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----
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I actually implemented something more generic than just forgetting git
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history. There's now a whole mechanism for git-annex doing distributed
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transitions of whatever sort is needed.
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There were several subtleties involved in distributed transitions:
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First is how to tell when a given transition has already been done on a
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branch. At first I was thinking that the transition log should include the
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sha of the first commit on the old branch that got rewritten. However, that
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would mean that after a single transition had been done, every git-annex
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branch merge would need to look up the first commit of the current branch,
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to see if it's done the transition yet. That's slow! Instead, transitions
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are logged with a timestamp, and as long as a branch contains a transition
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with the same timestamp, it's been done.
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A really tricky problem is what to do if the local repository has
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transitioned, but a remote has not, and changes keep being made to the
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remote. What it does so far is incorporate the changes from the remote into
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the index, and re-run the transition code over the whole thing to yeild a
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single new commit. This might not be very efficient (once I write the more
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full-featured transition code), but it lets the local repo keep up with
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what's going on in the remote, without directly merging with it (which
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would revert the transition). And once the remote repository has its
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git-annex upgraded to one that knows about transitions, it will finish up
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the transition on its side automatically, and the two branches will once
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again merge.
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Related to the previous problem, we don't want to keep trying to merge
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from a remote branch when it's not yet transitioned. So a blacklist is
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used, of untransitioned commits that have already been integrated.
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One really subtle thing is that when the user does a transition more
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complicated than `git annex forget`, like the `git annex forget --dead`
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that I need to implement to forget dead remotes, they're not just telling
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git-annex to forget whatever dead remotes it knows right now. They're
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actually telling git-annex to perform the transition one time on every
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existing clone of the repository, at some point in the future. Repositories
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with unfinished transitions could hang around for years, and at some future
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point when git-annex runs in the repository again, it would merge in the
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current state of the world, and re-do the transition. So you might tell it
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to forget dead remotes today, and then the very repository you ran that in
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later becomes dead, and a long-slumbering repo wakes up and forgets about
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the repo that started the whole process! I hope users don't find this
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massively confusing, but that's how the implementation works right now.
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----
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I think I have at least two more days of work to do to finish up this
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feature.
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* I still need to add some extra features like forgetting about dead remotes,
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and forgetting about keys that are no longer present on any remote.
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* After `git annex forget`, `git annex sync`
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will fail to push the synced/annex branch to remotes, since the branch
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is no longer a fast-forward of the old one. I will probably fix this by
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making `git annex sync` do a fallback push of a unique branch in this case,
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like the assistant already does. Although I may need to adjust that code
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to handle this case, too..
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* For some reason the automatic transitioning code triggers
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a "(recovery from race)" commit. This is certianly a bug somewhere,
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because you can't have a race with only 1 participant.
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----
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Today's work was sponsored by Richard Hartmann.
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