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How do you restore a corrupted object in a git repository (for newbies)?

Answer 1

+1 A:

Okay, so. We can see from the second error message that the corrupt object which you moved was a commit. (HEAD was pointing to it!) Unfortunately, this means that it's hard to manually repair it. (By "hard" I mean likely impossible unless you can remember exactly what the commit message was and what time you made the commit.) Fortunately, this does mean that it's easy to resurrect a new commit with the same file contents - you'll just have to write a new message for it.

Before you start, have a look at the contents of .git/HEAD - if it's a branch name, remember that for later.

First, we need to figure out what the parent of this commit should've been. You can use git reflog to look at the reflog of HEAD, and find the SHA1 of where HEAD was just before you made commit 016660b. It should look something like this:

016660b HEAD@{n}: commit: <subject of commit>
1234abc HEAD@{n-1}: ...

You can copy the SHA1 of the previous position of HEAD, and check out that commit:

git checkout 1234abc

Then you can read in the tree that your corrupted commit had:

git read-tree 2c1033501b82e301d47dbf53ba0a199003af25a8

And then commit!

git commit

Now, there's some question here about what should've happened to your branches. If HEAD was pointing to a branch (say master) which in turn pointed to the corrupted commit, we definitely want to fix that up:

git branch -d master       # remove the original master branch
git checkout -b master     # recreate it here

If there are other branches which contained the corrupted commit, you'll have to do some restoration on them too - let me know if you need help with that.

Jefromi 2010-09-01 14:37:04

With corrupt file displaced: $ git reflog fatal: bad object HEAD With corrupt file in place: $ git reflog fatal: object 016660b7605cfc2da85f631bbe809f7cb7962608 is corrupted I read the manual and tried: $ git reflog show HEAD@{1} Which gave me activity from 1 to 12, and then restarted at 1and continued until the initial pull I had done on my repositories. Is this restart typical of the command?

Humble Penguin 2010-09-02 03:36:51

Continuing on...$ git checkout bcbd203 #which was HEAD@{1}fatal: object 016660b7605cfc2da85f631bbe809f7cb7962608 is corruptedShould I simply move the file out and attempt the checkout again?Sorry about the absence of user-friendly formatting in these comments. I don't currently know a better way to do this.

Humble Penguin 2010-09-02 03:40:28

@Humble: Oops, sorry about that. I haven't actually had this happen to me, so I was writing untested things. I didn't know reflog actually tried to read the objects! Anyway, as long as you've figured out what the previous commit was, that part's all good. (You could also have looked directly at `.git/logs/HEAD`)

Jefromi 2010-09-02 03:52:37

@Humble: For the checkout... yeah, pretty much whatever you have to do to get it to succeed. Move the corrupt object out, and if that doesn't work... try `checkout -f`, then try `reset --hard <SHA1>`... once it works you should be able to continue on.

Jefromi 2010-09-02 03:53:56

I did `checkout -f`, which gave me a polite `detached HEAD state` warning. It didn't hit me until after committing to the detached head that the `reset --hard <SHA1>` you mentioned might be to "undetach" (re-attach?) the HEAD. Was this the case or was it another level of advice in case `checkout -f` had failed? Two more small questions: if I delete the branch and then check it out again, do I lose the commit history? And do you have any ideas (besides backing up the repository) to prevent this from happening in the future?

Humble Penguin 2010-09-02 17:03:01

The reset was in case the checkout failed, yes. You need to delete and recreate the branch there now. Lose the commit history? I believe all you've done is replace the broken commit with the new one you just made, so you will "lose" the broken one and gain the new one. As for prevention, in terms of software, backup is pretty much the way. (You could also look into RAID, but backups are still a good idea.)

Jefromi 2010-09-02 17:58:53

SO CLOSE! It won't let me delete the branch because if the corrupt file is not there, it `Couldn't look up commit object for 'refs/heads/RPG'`, and if it is there, the `object 016660b7605cfc2da85f631bbe809f7cb7962608 is corrupted`. I started to try forcing it (`git branch -d -f RPG`, `git branch -d RPG -f` and for kicks `git branch -D RPG`) but it threw the same errors or displayed the usage rules for `git branch`. Would it be best now to just force creation of the new branch on top of this old branch since it won't let me delete it? I would think not...

Humble Penguin 2010-09-02 19:50:31

Aha! Git is just doing its best not to let you throw away any information - unfortunately we know we have to. You're going to have to get rid of the old branch one way or another. If `git branch -f master; git checkout master` doesn't work, just go in the .git directory and yank it out yourself - remove `refs/heads/master` and `logs/refs/heads/master`, then create it normally.

Jefromi 2010-09-02 21:41:11

`git branch -f master; git checkout master` worked! I now have my repository up and working again. Thanks so much for your time and patience.

Humble Penguin 2010-09-03 03:37:28

@Humble: Phew! Glad I managed to get you there eventually.

Jefromi 2010-09-03 15:45:32

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How do you restore a corrupted object in a git repository (for newbies)?

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