Re: My git repo is broken, how to fix it ?

On Wed, 28 Feb 2007, Alexander Litvinov wrote:

Generally, the best way to fix things is (I've written this up at somewhat more length before, but I'm too lazy to find it):

 - back up all your state so that anything you do is re-doable if you 
   corrupt things more!

 - explode any corrupt pack-files

   See "man git-unpack-objects", and in particular the "-r" flag. Also, 
   please realize that it only unpacks objects that aren't already 
   available, so you need to move the pack-file away from its normal 
   location first (otherwise git-unpack-objects will find all objects 
   that are in the pack-file in the pack-file itself, and not unpack 
   anything at all)

 - replace any broken and/or missing objects

   This is the challenging part. Sometimes (hopefully often!) you can find 
   the missing objects in other copies of the repositories. At other 
   times, you may need to try to find the data some other way (for 
   example, maybe your checked-out copy contains the file content that 
   when hashed will be the missing object?).

 - make sure everything is happy with "git-fsck --full"

 - repack everything to get back to an efficient state again.

And remember: git does _not_ make backups pointless. It hopefully makes backups *easy* (since cloning and pulling is easy), but the basic need for backups does not go away!

Shouldn't matter, at least as long as you are using the native git protocol: git will create objects fully under a temporary name, and then atomically rename things to their right names.

Using rsync and/or http may not be as safe.

HOWEVER! I do not know how well Windows and/or cygwin does file renames. If cygwin does a rename as a copy + delete, a lot of the safety assumptions just fly out the window.

Ok, that's a good thing, but see above: I don't think anything should have gotten unpacked, because it found all objects already existing in the very pack-file you tried to unpack.

So you might well need to do

	mv .git/objects/pack/pack-c4554978bbe079c9a43d6a13546a2fa314fe0884.pack oldpack
	git-unpack-objects -r < oldpack

(or rename the .idx file instead).

Alternatively (and in many ways this migth be better when you're trying to recover something) just create a totally *new* git repo, by doing

	mkdir new-repo
	cd new-repo
	git init
	git unpack-objects -r < ../other-repo/.git/pack/pack-.....pack

and re-create the objects somewhere else - you can do all of this without at all disturbing the old repository (but you'd need to copy all the refs and all the loose objects by hand, of course!)

You didn't do "--full", so it's not looking inside your pack, so the fsck wasn't very interesting in this case.

And no, you cannot reconstruct the file by sha1 sum, although you may be able to reconstruct the file some *other* way (by looking at the other blobs and remembering what the missing case is), and then you can obviously use the sha1-sum to *confirm* that you reconstructed the file exactly as it was!

So yes, reconstruction of missing objects is possible, but no, you can't do it based purely based on SHA1, you need to base reconstruction on some other information. That's kind of what "cryptographically secure hash" means ;^p

			Linus