How to recover Git objects damaged by hard disk failure?

Question

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?

Answer 1

Banengusk was putting me on the right track. For further reference, I want to post the steps I took to fix my repository corruption. I was lucky enough to find all needed objects either in older packs or in repository backups.

# Unpack last non-corrupted pack $ mv .git/objects/pack .git/objects/pack.old $ git unpack-objects -r < .git/objects/pack.old/pack-012066c998b2d171913aeb5bf0719fd4655fa7d0.pack $ git log fatal: bad object HEAD  $ cat .git/HEAD  ref: refs/heads/master  $ ls .git/refs/heads/  $ cat .git/packed-refs  # pack-refs with: peeled  aa268a069add6d71e162c4e2455c1b690079c8c1 refs/heads/master  $ git fsck --full  error: HEAD: invalid sha1 pointer aa268a069add6d71e162c4e2455c1b690079c8c1 error: refs/heads/master does not point to a valid object! missing blob 75405ef0e6f66e48c1ff836786ff110efa33a919 missing blob 27c4611ffbc3c32712a395910a96052a3de67c9b dangling tree 30473f109d87f4bcde612a2b9a204c3e322cb0dc  # Copy HEAD object from backup of repository $ cp repobackup/.git/objects/aa/268a069add6d71e162c4e2455c1b690079c8c1 .git/objects/aa # Now copy all missing objects from backup of repository and run "git fsck --full" afterwards # Repeat until git fsck --full only reports dangling objects  # Now garbage collect repo $ git gc warning: reflog of 'HEAD' references pruned commits warning: reflog of 'refs/heads/master' references pruned commits Counting objects: 3992, done. Delta compression using 2 threads. fatal: object bf1c4953c0ea4a045bf0975a916b53d247e7ca94 inconsistent object length (6093 vs 415232) error: failed to run repack  # Check reflogs... $ git reflog  # ...then clean $ git reflog expire --expire=0 --all  # Now garbage collect again $ git gc        Counting objects: 3992, done. Delta compression using 2 threads. Compressing objects: 100% (3970/3970), done. Writing objects: 100% (3992/3992), done. Total 3992 (delta 2060), reused 0 (delta 0) Removing duplicate objects: 100% (256/256), done. # Done! 

Answer 2

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.