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detailed design for git repository repair

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Joey Hess 2013-10-18 14:00:27 -04:00
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doc/design/assistant/disaster_recovery.mdwn
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@ -79,3 +79,41 @@ 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.
### detailed design
Run `git fsck` and parse output to find bad objects, and determine
from its output if they are a commit, a tree, or a blob.
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 it, and inject them
back into .git/objects to recover.
How to best re-get bad objects from a remote? May need to re-clone from
the remote, and rsync .git/objects from the clone.
Otherwise, find commits in each local branch that are broken by
all found bad objects. Some of this can be parsed from git fsck
output, but for eg bad blobs, the commits need to be walked to
walk the trees, to find trees that refer to the blobs.
For each branch that is affected, look in the reflog and/or `git log
$branch` to find the last good change 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.)
Reset the branch to the last good change. 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.
(Special handling for git-annex branch: Commit .git/annex/index over
top of the reset git-annex branch, and then run a `git annex fsck --fast`
to fix up any object location info.)
Also should check all remote tracking branches. If such a branch refers
to a bad object, it is sufficient to remove the tracking
branch and then `git fetch` from the remote, which will re-download missing
objects from it and reinstate the tracking branch.