user-manual: recovering from corruption
Some instructions on dealing with corruption of the object database. Most of this text is from an example by Linus, identified by Nicolas Pitre <nico@cam.org> with a little further editing by me. Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
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J. Bruce Fields
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@ -1560,6 +1560,11 @@ This may be time-consuming. Unlike most other git operations (including
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git-gc when run without any options), it is not safe to prune while
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other git operations are in progress in the same repository.
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If gitlink:git-fsck[1] complains about sha1 mismatches or missing
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objects, you may have a much more serious problem; your best option is
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probably restoring from backups. See
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<<recovering-from-repository-corruption>> for a detailed discussion.
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[[recovering-lost-changes]]
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Recovering lost changes
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~~~~~~~~~~~~~~~~~~~~~~~
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@ -3220,6 +3225,127 @@ confusing and scary messages, but it won't actually do anything bad. In
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contrast, running "git prune" while somebody is actively changing the
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repository is a *BAD* idea).
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[[recovering-from-repository-corruption]]
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Recovering from repository corruption
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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By design, git treats data trusted to it with caution. However, even in
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the absence of bugs in git itself, it is still possible that hardware or
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operating system errors could corrupt data.
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The first defense against such problems is backups. You can back up a
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git directory using clone, or just using cp, tar, or any other backup
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mechanism.
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As a last resort, you can search for the corrupted objects and attempt
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to replace them by hand. Back up your repository before attempting this
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in case you corrupt things even more in the process.
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We'll assume that the problem is a single missing or corrupted blob,
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which is sometimes a solveable problem. (Recovering missing trees and
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especially commits is *much* harder).
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Before starting, verify that there is corruption, and figure out where
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it is with gitlink:git-fsck[1]; this may be time-consuming.
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Assume the output looks like this:
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------------------------------------------------
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$ 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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(Typically there will be some "dangling object" messages too, but they
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aren't interesting.)
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Now you know that blob 4b9458b3 is missing, and that the tree 2d9263c6
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points to it. If you could find just one copy of that missing blob
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object, possibly in some other repository, you could move it into
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.git/objects/4b/9458b3... and be done. Suppose you can't. You can
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still examine the tree that pointed to it with gitlink:git-ls-tree[1],
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which might output something like:
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------------------------------------------------
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$ git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
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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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...
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100644 blob 4b9458b3786228369c63936db65827de3cc06200 myfile
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...
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------------------------------------------------
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So now you know that the missing blob was the data for a file named
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"myfile". And chances are you can also identify the directory--let's
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say it's in "somedirectory". If you're lucky the missing copy might be
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the same as the copy you have checked out in your working tree at
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"somedirectory/myfile"; you can test whether that's right with
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gitlink:git-hash-object[1]:
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------------------------------------------------
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$ git hash-object -w somedirectory/myfile
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------------------------------------------------
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which will create and store a blob object with the contents of
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somedirectory/myfile, and output the sha1 of that object. if you're
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extremely lucky it might be 4b9458b3786228369c63936db65827de3cc06200, in
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which case you've guessed right, and the corruption is fixed!
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Otherwise, you need more information. How do you tell which version of
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the file has been lost?
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The easiest way to do this is with:
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------------------------------------------------
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$ git log --raw --all --full-history -- somedirectory/myfile
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------------------------------------------------
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Because you're asking for raw output, you'll now get something like
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------------------------------------------------
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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/myfile
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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/myfile
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------------------------------------------------
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This tells you that the immediately preceding version of the file was
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"newsha", and that the immediately following version was "oldsha".
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You also know the commit messages that went with the change from oldsha
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to 4b9458b and with the change from 4b9458b to newsha.
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If you've been committing small enough changes, you may now have a good
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shot at reconstructing the contents of the in-between state 4b9458b.
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If you can do that, you can now recreate the missing object with
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------------------------------------------------
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$ git hash-object -w <recreated-file>
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------------------------------------------------
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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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------------------------------------------------
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$ git log --raw --all
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------------------------------------------------
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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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[[the-index]]
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The index
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-----------
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@ -4429,4 +4555,7 @@ Write a chapter on using plumbing and writing scripts.
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Alternates, clone -reference, etc.
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git unpack-objects -r for recovery
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More on recovery from repository corruption. See:
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http://marc.theaimsgroup.com/?l=git&m=117263864820799&w=2
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http://marc.theaimsgroup.com/?l=git&m=117147855503798&w=2
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http://marc.theaimsgroup.com/?l=git&m=117147855503798&w=2
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