144 lines
5.4 KiB
Text
144 lines
5.4 KiB
Text
Date: Fri, 9 Nov 2007 08:28:38 -0800 (PST)
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From: Linus Torvalds <torvalds@linux-foundation.org>
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Subject: corrupt object on git-gc
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Abstract: Some tricks to reconstruct blob objects in order to fix
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a corrupted repository.
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Content-type: text/asciidoc
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How to recover a corrupted blob object
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======================================
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-----------------------------------------------------------
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On Fri, 9 Nov 2007, Yossi Leybovich wrote:
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>
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> Did not help still the repository look for this object?
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> Any one know how can I track this object and understand which file is it
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-----------------------------------------------------------
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So exactly *because* the SHA-1 hash is cryptographically secure, the hash
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itself doesn't actually tell you anything, in order to fix a corrupt
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object you basically have to find the "original source" for it.
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The easiest way to do that is almost always to have backups, and find the
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same object somewhere else. Backups really are a good idea, and Git makes
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it pretty easy (if nothing else, just clone the repository somewhere else,
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and make sure that you do *not* use a hard-linked clone, and preferably
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not the same disk/machine).
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But since you don't seem to have backups right now, the good news is that
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especially with a single blob being corrupt, these things *are* somewhat
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debuggable.
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First off, move the corrupt object away, and *save* it. The most common
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cause of corruption so far has been memory corruption, but even so, there
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are people who would be interested in seeing the corruption - but it's
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basically impossible to judge the corruption until we can also see the
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original object, so right now the corrupt object is useless, but it's very
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interesting for the future, in the hope that you can re-create a
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non-corrupt version.
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-----------------------------------------------------------
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So:
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> ib]$ mv .git/objects/4b/9458b3786228369c63936db65827de3cc06200 ../
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-----------------------------------------------------------
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This is the right thing to do, although it's usually best to save it under
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it's full SHA-1 name (you just dropped the "4b" from the result ;).
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Let's see what that tells us:
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-----------------------------------------------------------
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> ib]$ git-fsck --full
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> broken link from tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
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> to blob 4b9458b3786228369c63936db65827de3cc06200
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> missing blob 4b9458b3786228369c63936db65827de3cc06200
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-----------------------------------------------------------
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Ok, I removed the "dangling commit" messages, because they are just
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messages about the fact that you probably have rebased etc, so they're not
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at all interesting. But what remains is still very useful. In particular,
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we now know which tree points to it!
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Now you can do
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git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
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which will show something like
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100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8 .gitignore
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100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883 .mailmap
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100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c COPYING
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100644 blob ee909f2cc49e54f0799a4739d24c4cb9151ae453 CREDITS
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040000 tree 0f5f709c17ad89e72bdbbef6ea221c69807009f6 Documentation
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100644 blob 1570d248ad9237e4fa6e4d079336b9da62d9ba32 Kbuild
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100644 blob 1c7c229a092665b11cd46a25dbd40feeb31661d9 MAINTAINERS
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...
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and you should now have a line that looks like
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10064 blob 4b9458b3786228369c63936db65827de3cc06200 my-magic-file
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in the output. This already tells you a *lot* it tells you what file the
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corrupt blob came from!
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Now, it doesn't tell you quite enough, though: it doesn't tell what
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*version* of the file didn't get correctly written! You might be really
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lucky, and it may be the version that you already have checked out in your
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working tree, in which case fixing this problem is really simple, just do
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git hash-object -w my-magic-file
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again, and if it outputs the missing SHA-1 (4b945..) you're now all done!
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But that's the really lucky case, so let's assume that it was some older
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version that was broken. How do you tell which version it was?
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The easiest way to do it is to do
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git log --raw --all --full-history -- subdirectory/my-magic-file
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and that will show you the whole log for that file (please realize that
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the tree you had may not be the top-level tree, so you need to figure out
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which subdirectory it was in on your own), and because you're asking for
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raw output, you'll now get something like
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commit abc
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Author:
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Date:
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..
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:100644 100644 4b9458b... newsha... M somedirectory/my-magic-file
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commit xyz
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Author:
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Date:
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..
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:100644 100644 oldsha... 4b9458b... M somedirectory/my-magic-file
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and this actually tells you what the *previous* and *subsequent* versions
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of that file were! So now you can look at those ("oldsha" and "newsha"
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respectively), and hopefully you have done commits often, and can
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re-create the missing my-magic-file version by looking at those older and
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newer versions!
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If you can do that, you can now recreate the missing object with
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git hash-object -w <recreated-file>
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and your repository is good again!
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(Btw, you could have ignored the fsck, and started with doing a
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git log --raw --all
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and just looked for the sha of the missing object (4b9458b..) in that
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whole thing. It's up to you - Git does *have* a lot of information, it is
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just missing one particular blob version.
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Trying to recreate trees and especially commits is *much* harder. So you
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were lucky that it's a blob. It's quite possible that you can recreate the
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thing.
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Linus
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