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diff --git a/Documentation/howto/recover-corrupted-blob-object.txt b/Documentation/howto/recover-corrupted-blob-object.txt new file mode 100644 index 0000000000..323b513ed0 --- /dev/null +++ b/Documentation/howto/recover-corrupted-blob-object.txt @@ -0,0 +1,134 @@ +Date: Fri, 9 Nov 2007 08:28:38 -0800 (PST) +From: Linus Torvalds <torvalds@linux-foundation.org> +Subject: corrupt object on git-gc +Abstract: Some tricks to reconstruct blob objects in order to fix + a corrupted repository. + +On Fri, 9 Nov 2007, Yossi Leybovich wrote: +> +> Did not help still the repository look for this object? +> Any one know how can I track this object and understand which file is it + +So exactly *because* the SHA1 hash is cryptographically secure, the hash +itself doesn't actually tell you anything, in order to fix a corrupt +object you basically have to find the "original source" for it. + +The easiest way to do that is almost always to have backups, and find the +same object somewhere else. Backups really are a good idea, and git makes +it pretty easy (if nothing else, just clone the repository somewhere else, +and make sure that you do *not* use a hard-linked clone, and preferably +not the same disk/machine). + +But since you don't seem to have backups right now, the good news is that +especially with a single blob being corrupt, these things *are* somewhat +debuggable. + +First off, move the corrupt object away, and *save* it. The most common +cause of corruption so far has been memory corruption, but even so, there +are people who would be interested in seeing the corruption - but it's +basically impossible to judge the corruption until we can also see the +original object, so right now the corrupt object is useless, but it's very +interesting for the future, in the hope that you can re-create a +non-corrupt version. + +So: + +> ib]$ mv .git/objects/4b/9458b3786228369c63936db65827de3cc06200 ../ + +This is the right thing to do, although it's usually best to save it under +it's full SHA1 name (you just dropped the "4b" from the result ;). + +Let's see what that tells us: + +> ib]$ git-fsck --full +> broken link from tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8 +> to blob 4b9458b3786228369c63936db65827de3cc06200 +> missing blob 4b9458b3786228369c63936db65827de3cc06200 + +Ok, I removed the "dangling commit" messages, because they are just +messages about the fact that you probably have rebased etc, so they're not +at all interesting. But what remains is still very useful. In particular, +we now know which tree points to it! + +Now you can do + + git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8 + +which will show something like + + 100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8 .gitignore + 100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883 .mailmap + 100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c COPYING + 100644 blob ee909f2cc49e54f0799a4739d24c4cb9151ae453 CREDITS + 040000 tree 0f5f709c17ad89e72bdbbef6ea221c69807009f6 Documentation + 100644 blob 1570d248ad9237e4fa6e4d079336b9da62d9ba32 Kbuild + 100644 blob 1c7c229a092665b11cd46a25dbd40feeb31661d9 MAINTAINERS + ... + +and you should now have a line that looks like + + 10064 blob 4b9458b3786228369c63936db65827de3cc06200 my-magic-file + +in the output. This already tells you a *lot* it tells you what file the +corrupt blob came from! + +Now, it doesn't tell you quite enough, though: it doesn't tell what +*version* of the file didn't get correctly written! You might be really +lucky, and it may be the version that you already have checked out in your +working tree, in which case fixing this problem is really simple, just do + + git hash-object -w my-magic-file + +again, and if it outputs the missing SHA1 (4b945..) you're now all done! + +But that's the really lucky case, so let's assume that it was some older +version that was broken. How do you tell which version it was? + +The easiest way to do it is to do + + git log --raw --all --full-history -- subdirectory/my-magic-file + +and that will show you the whole log for that file (please realize that +the tree you had may not be the top-level tree, so you need to figure out +which subdirectory it was in on your own), and because you're asking for +raw output, you'll now get something like + + commit abc + Author: + Date: + .. + :100644 100644 4b9458b... newsha... M somedirectory/my-magic-file + + + commit xyz + Author: + Date: + + .. + :100644 100644 oldsha... 4b9458b... M somedirectory/my-magic-file + +and this actually tells you what the *previous* and *subsequent* versions +of that file were! So now you can look at those ("oldsha" and "newsha" +respectively), and hopefully you have done commits often, and can +re-create the missing my-magic-file version by looking at those older and +newer versions! + +If you can do that, you can now recreate the missing object with + + git hash-object -w <recreated-file> + +and your repository is good again! + +(Btw, you could have ignored the fsck, and started with doing a + + git log --raw --all + +and just looked for the sha of the missing object (4b9458b..) in that +whole thing. It's up to you - git does *have* a lot of information, it is +just missing one particular blob version. + +Trying to recreate trees and especially commits is *much* harder. So you +were lucky that it's a blob. It's quite possible that you can recreate the +thing. + + Linus |