git-annex/ design/ assistant/ disaster recovery

The assistant should help the user recover their repository when things go wrong.

## dangling lock files

There are a few ways a git repository can get broken that are easily fixed. One is left over index.lck files. When a commit to a repository fails, check that nothing else is using it, fix the problem, and redo the commit.

- done for .git/annex/index.lock, can be handled safely and automatically.
- done for .git/index.lock, only when the assistant is starting up.
- What about local remotes, eg removable drives? git-annex does attempt to commit to the git-annex branch of those. It will use the automatic fix if any are dangling. It does not commit to the master branch; indeed a removable drive typically has a bare repository. However, it does a scan for broken locks anyway if there's a problem syncing. done
- What about git-annex-shell? If the ssh remote has the assistant running, it can take care of it, and if not, it's a server, and perhaps the user should be required to fix up if it crashes during a commit. This should not affect the assistant anyway.
- done Seems that refs can also have stale lock files, for example '/storage/emulated/legacy/DCIM/.git/refs/remotes/flick_phonecamera/synced/git-annex.lock' All git lock files are now handled (except gc lock files).

## incremental fsck

Add webapp UI to enable incremental fsck done

Of course, incremental fsck will run as an niced (and ioniced) background job. There will need to be a button in the webapp to stop it, in case it's annoying. done

When fsck finds a damanged file, queue a download of the file from a remote. done

Detect when a removable drive is connected in the Cronner, and check and try to run its remote fsck jobs. done (Same mechanism will work for network remotes becoming connected.)

TODO: If no accessible remote has a file that fsck reported missing, prompt the user to eg, connect a drive containing it. Or perhaps this is a special case of a general problem, and the webapp should prompt the user when any desired file is available on a remote that's not mounted?

## git-annex-shell remote fsck

TODO: git-annex-shell fsck support, which would allow cheap fast fscks of ssh remotes.

Would be nice; otherwise remote fsck is too expensive (downloads everything) to have the assistant do.

Note that Remote.Git already tries to use this, but the assistant does not call it for non-local remotes.

## git fsck and repair

Add git fsck to scheduled self fsck done

TODO: git fsck on ssh remotes? Probably not worth the complexity..

TODO: If committing to the repository fails, after resolving any dangling lock files (see above), it should git fsck. This is difficult, because git commit will also fail if the commit turns out to be empty, or due to other transient problems.. So commit failures are currently ignored by the assistant.

If git fsck finds problems, launch git repository repair. done

git annex fsck --fast at end of repository repair to ensure git-annex branch is accurate. done

If syncing with a local repository fails, try to repair it. done

TODO: "Repair" gcrypt remotes, by removing all refs and objects, and re-pushing. (Since the objects are encrypted data, there is no way to pull missing ones from anywhere..) Need to preserve gcrypt-id while doing this!

TODO: along with displaying alert when there is a problem detected by consistency check, send an email alert. (Using system MTA?)

## nudge user to schedule fscks

Make the webapp encourage users to schedule fscks of their local repositories. The goal here was that it should not be obnoxious about repeatedly pestering the user to set that up, but should still encourage anyone who cares to set it up.

Maybe: Display a message only once per week, and only after the repository has existed for at least one full day. But, this will require storing quite a lot of state.

Or: Display a message whenever a removable drive is detected to have been connected. I like this, but what about nudging the main repo? Could do it every webapp startup, perhaps? done

There should be a "No thanks" button that prevents it nudging again for a repo. done

## git repository repair

There are several ways git repositories can get damanged.

The most common is empty files in .git/annex/objects and commits that refer to those objects. When the objects have not yet been pushed anywhere. I've several times recovered from this manually by removing the bad files and resetting to before the commits that referred to them. Then re-staging any divergence in the working tree. This could perhaps be automated.

As long as the git repository has at least one remote, another method is to clone the remote, sync from all other remotes, move over .git/config and .git/annex/objects, and tar up the old broken git repo and `git annex add` it. This should be automatable and get the user back on their feet. User could just click a button and have this be done.

This is useful outside git-annex as well, so make it a git-recover-repository command.

### detailed design

Run `git fsck` and parse output to find bad objects. done Note that fsck may fall over and fail to print out all bad objects, when files are corrupt. So if the fsck exits nonzero, need to collect all bad objects it did find, and:

1. If the local repository contains packs, the packs may be corrupt. So, start by using `git unpack-objects` to unpack all packs it can handle (which may include parts of corrupt packs) back to loose objects. And delete all packs. done
2. Delete all loose corrupt objects. done

Repeat until fsck finds no new problems. done

Check if there's a remote. If so, and if the bad objects are all present on it, can simply get all bad objects from the remote, and inject them back into .git/objects to recover:

1. Make a new (bare) clone from the remote. (Note: git does not seem to provide a way to fetch specific missing objects from the remote. Also, cannot use `--reference` against a repository with missing refs. So this seems unavoidably network-expensive.) done
2. Rsync objects over. (Turned out to work better than git-cat-file, because we don't have to walk the graph to add missing objects.) done
3. If each bad object was able to be repaired this way, we're done! (If not, can reuse the clone for getting objects from the next remote.) done

If some missing objects cannot be recovered from remotes, find commits in each local branch that are broken by all remaining missing objects. Some of this can be parsed from git fsck output, but for eg blobs, the commits need to be walked to walk the trees, to find trees that refer to the blobs. done

For each branch that is affected, look in the reflog and/or `git log $branch` to find the last good commit that predates all broken commits. (If the head commit of a branch is broken, git log is not going to show anything useful, but the reflog can be used to find past refs for the branch -- have to first delete the .git/HEAD file if it points to the broken ref.) done

The basic idea then is to reset the branch to the last good commit that was found for it.

- For the HEAD branch, can just reset it. (If no last good commit was found for the HEAD branch, reset it to a dummy empty commit.) This will leave git showing any changes made since then as staged in the index and uncommitted. Or if the index is missing/corrupt, any files in the tree will show as modified and uncommitted. User (or git-annex assistant) can then commit as appropriate. Print appropriate warning message. done
- Special handling for git-annex branch and index. done
- Remote tracking branches can just be removed, and then `git fetch` from the remote, which will re-download missing objects from it and reinstate the tracking branch. done
- For other branches, reset them to last good commit, or delete if none was found. done
- (Decided not to touch tags.)

The index file can still refer to objects that were missing. Rewrite to remove them. done

comment 1

The restriction on fetching over the Git protocol is, partly, for security reasons – e.g. if one accidentally pushes a commit with private data, and then `push --force`'s a cleaned-up version, Git needs to prevent anyone from downloading the old commit by just giving its SHA1 (e.g. obtained from an IRC/email push notification). So it restricts fetching to the tips of any ref. (I've been told that it could check if the given object is merely reachable from any ref, but it doesn't do so for performance reasons.)

git 1.8 has a minor way to relax this requirement – it allows giving a SHA1 to `git fetch` (although I think the protocol already worked this way), and it allows refs to be hidden server-side but still remain fetchable, so in theory there could be a (hidden) ref for every object, for easy fetching...

Comment by http://nullroute.eu.org/~grawity/ — Wed Oct 29 14:05:27 2025
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