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I have a corrupt blob object which can be seen here

> git fsck --full
error: unable to unpack 5426a4097ea6a3597a1674b0b7fa67f395006f2a header
error: inflateEnd: stream consistency error (no message)
fatal: loose object 5426a4097ea6a3597a1674b0b7fa67f395006f2a (stored in .git/obj
ects/54/26a4097ea6a3597a1674b0b7fa67f395006f2a) is corrupt

git cat-file -t 542... also errors with the same unable to unpack header

I've checked other machines, and all have the corrupt version.

I worked through this process - http://www.kernel.org/pub/software/scm/git/docs/v1.7.10.1/howto/recover-corrupted-blob-object.txt and narrowed it down to a particular file (Site.css) and a date range (Commits before and after) but I'm not going to be able to reconstruct the changes to recover the file, as it was a part of a fairly major set of changes made a long time ago: So long ago that I really don't care about that bit of history.

As I now have the hashes of the commits before and after, can I do something to just forget about that bit of the history - kind of a rebase (although I imagine a rebase would fail - not tried it yet!)

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2 Answers 2

Reset to default
4

Take a look at the git-replace manpage. The replacement mechanism allows you to drop in a new blob with a different hash as a substitute for an old one. This may work for you.

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2

Here's what I'd do. There may be more elegant solutions, but for something that'll get you where you need to be, this'll do the trick.

What you'll need:

  • The SHA1 of the commit immediately before the one that introduced the duff blob, <LastGoodCommit>.
  • The SHA1 of the commit that contains the duff blob, <BadCommit>.

What you need to do:

  1. Get a list of all the branches and tags that contain the duff commit, and save these off:

    git branch --contains <BadCommit> >branches.txt
git tag --contains <BadCommit> >tags.txt

  2. Check out the good commit:

    git checkout <LastGoodCommit>

  3. Create a new commit that will replace the commit containing the bad blob.

    You may be able to use git cherry-pick -n <BadCommit> to get the changes, or possibly not. I can't test it, so you'll need to try yourself and see what happens. If it doesn't work, you should be able to checkout individual files and directories using git checkout <BadCommit>:<path>.

    You'll need to work out what you want that site.css file to look like in this commit. I can see two options:

    • Just leave it as it was in <LastGoodCommit>

    • Find the next time you have a good blob for site.css in one of your branches (ideal situation: it's the same in all branches) and use that.

    Whichever you do, note the new commit hash <NewGoodCommit>.

  4. Rebase all your branches, listed in branches.txt from above, that contained the bad commit onto this new commit. Depending on which option you picked at step 3, you may have merge conflicts; just resolve these using the new version of the file.

    If all the branches have the same following version of site.css, you should be able to be confident there will be no clashes, and use the following one-liner to do all the rebases:

    while read branchname; do git rebase --onto <NewGoodCommit> <BadCommit> "$branchname" || echo "Failed rebasing $branchname" && break; done

  5. Go through your tags (in tags.txt) and create replacements for each one on the newly rebased branches. Sadly I don't know a way to automate this.

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