I have had a hard disk failure which resulted in some files of a Git repository getting damaged. When running git fsck --full I get the following output:
error: .git/objects/pack/pack-6863e0a0e4b4ded6090fac5d12eba6ca7346b19c.pack SHA1 checksum mismatch
error: index CRC mismatch for object 6c8cae4994b5ec7891ccb1527d30634997a978ee from .git/objects/pack/pack-6863e0a0e4b4ded6090fac5d12eba6ca7346b19c.pack at offset 97824129
error: inflate: data stream error (invalid code lengths set)
error: cannot unpack 6c8cae4994b5ec7891ccb1527d30634997a978ee from .git/objects/pack/pack-6863e0a0e4b4ded6090fac5d12eba6ca7346b19c.pack at offset 97824129
error: inflate: data stream error (invalid stored block lengths)
error: failed to read object 0dcf6723cc69cc7f91d4a7432d0f1a1f05e77eaa at offset 276988017 from .git/objects/pack/pack-6863e0a0e4b4ded6090fac5d12eba6ca7346b19c.pack
fatal: object 0dcf6723cc69cc7f91d4a7432d0f1a1f05e77eaa is corrupted
I have backups of the repository, but the only backup that includes the pack file has it already damaged. So I think that I have to find out a way to retrieve the single objects from different backups and somehow instruct Git to produce a new pack with only correct objects.
Can you please give me hints how to fix my repository?
In some previous backups, your bad objects may have been packed in different files or may be loose objects yet. So your objects may be recovered.
It seems there are a few bad objects in your database. So you could do it the manual way.
Because of git hash-object, git mktree and git commit-tree do not write the objects because they are found in the pack, then start doing this:
mv .git/objects/pack/* <somewhere>
for i in <somewhere>/*.pack; do
git unpack-objects -r < $i
done
rm <somewhere>/*
(Your packs are moved out from the repository, and unpacked again in it; only the good objects are now in the database)
You can do:
git cat-file -t 6c8cae4994b5ec7891ccb1527d30634997a978ee
and check the type of the object.
If the type is blob: retrieve the contents of the file from previous backups (with git show or git cat-file or git unpack-file; then you may git hash-object -w to rewrite the object in your current repository.
If the type is tree: you could use git ls-tree to recover the tree from previous backups; then git mktree to write it again in your current repository.
If the type is commit: the same with git show, git cat-file and git commit-tree.
Of course, I would backup your original working copy before starting this process.
Also, take a look at How to Recover Corrupted Blob Object.