howto/recover-corrupted-blob-object.html

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 735 <body class="article">
 736 <div id="header">
 737 <h1>How to recover a corrupted blob object</h1>
 738 <span id="revdate">2023-07-25</span>
 739 </div>
 740 <div id="content">
 741 <div id="preamble">
 742 <div class="sectionbody">
 743 <div class="listingblock">
 744 <div class="content">
 745 <pre><code>On Fri, 9 Nov 2007, Yossi Leybovich wrote:
 746 &gt;
 747 &gt; Did not help still the repository look for this object?
 748 &gt; Any one know how can I track this object and understand which file is it</code></pre>
 749 </div></div>
 750 <div class="paragraph"><p>So exactly <strong>because</strong> the SHA-1 hash is cryptographically secure, the hash
 751 itself doesn&#8217;t actually tell you anything, in order to fix a corrupt
 752 object you basically have to find the "original source" for it.</p></div>
 753 <div class="paragraph"><p>The easiest way to do that is almost always to have backups, and find the
 754 same object somewhere else. Backups really are a good idea, and Git makes
 755 it pretty easy (if nothing else, just clone the repository somewhere else,
 756 and make sure that you do <strong>not</strong> use a hard-linked clone, and preferably
 757 not the same disk/machine).</p></div>
 758 <div class="paragraph"><p>But since you don&#8217;t seem to have backups right now, the good news is that
 759 especially with a single blob being corrupt, these things <strong>are</strong> somewhat
 760 debuggable.</p></div>
 761 <div class="paragraph"><p>First off, move the corrupt object away, and <strong>save</strong> it. The most common
 762 cause of corruption so far has been memory corruption, but even so, there
 763 are people who would be interested in seeing the corruption - but it&#8217;s
 764 basically impossible to judge the corruption until we can also see the
 765 original object, so right now the corrupt object is useless, but it&#8217;s very
 766 interesting for the future, in the hope that you can re-create a
 767 non-corrupt version.</p></div>
 768 <div class="listingblock">
 769 <div class="content">
 770 <pre><code>So:
 771
 772 &gt; ib]$ mv .git/objects/4b/9458b3786228369c63936db65827de3cc06200 ../</code></pre>
 773 </div></div>
 774 <div class="paragraph"><p>This is the right thing to do, although it&#8217;s usually best to save it under
 775 it&#8217;s full SHA-1 name (you just dropped the "4b" from the result ;).</p></div>
 776 <div class="paragraph"><p>Let&#8217;s see what that tells us:</p></div>
 777 <div class="listingblock">
 778 <div class="content">
 779 <pre><code>&gt; ib]$ git-fsck --full
 780 &gt; broken link from    tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
 781 &gt;              to    blob 4b9458b3786228369c63936db65827de3cc06200
 782 &gt; missing blob 4b9458b3786228369c63936db65827de3cc06200</code></pre>
 783 </div></div>
 784 <div class="paragraph"><p>Ok, I removed the "dangling commit" messages, because they are just
 785 messages about the fact that you probably have rebased etc, so they&#8217;re not
 786 at all interesting. But what remains is still very useful. In particular,
 787 we now know which tree points to it!</p></div>
 788 <div class="paragraph"><p>Now you can do</p></div>
 789 <div class="literalblock">
 790 <div class="content">
 791 <pre><code>git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8</code></pre>
 792 </div></div>
 793 <div class="paragraph"><p>which will show something like</p></div>
 794 <div class="literalblock">
 795 <div class="content">
 796 <pre><code>100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8    .gitignore
 797 100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883    .mailmap
 798 100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c    COPYING
 799 100644 blob ee909f2cc49e54f0799a4739d24c4cb9151ae453    CREDITS
 800 040000 tree 0f5f709c17ad89e72bdbbef6ea221c69807009f6    Documentation
 801 100644 blob 1570d248ad9237e4fa6e4d079336b9da62d9ba32    Kbuild
 802 100644 blob 1c7c229a092665b11cd46a25dbd40feeb31661d9    MAINTAINERS
 803 ...</code></pre>
 804 </div></div>
 805 <div class="paragraph"><p>and you should now have a line that looks like</p></div>
 806 <div class="literalblock">
 807 <div class="content">
 808 <pre><code>10064 blob 4b9458b3786228369c63936db65827de3cc06200     my-magic-file</code></pre>
 809 </div></div>
 810 <div class="paragraph"><p>in the output. This already tells you a <strong>lot</strong> it tells you what file the
 811 corrupt blob came from!</p></div>
 812 <div class="paragraph"><p>Now, it doesn&#8217;t tell you quite enough, though: it doesn&#8217;t tell what
 813 <strong>version</strong> of the file didn&#8217;t get correctly written! You might be really
 814 lucky, and it may be the version that you already have checked out in your
 815 working tree, in which case fixing this problem is really simple, just do</p></div>
 816 <div class="literalblock">
 817 <div class="content">
 818 <pre><code>git hash-object -w my-magic-file</code></pre>
 819 </div></div>
 820 <div class="paragraph"><p>again, and if it outputs the missing SHA-1 (4b945..) you&#8217;re now all done!</p></div>
 821 <div class="paragraph"><p>But that&#8217;s the really lucky case, so let&#8217;s assume that it was some older
 822 version that was broken. How do you tell which version it was?</p></div>
 823 <div class="paragraph"><p>The easiest way to do it is to do</p></div>
 824 <div class="literalblock">
 825 <div class="content">
 826 <pre><code>git log --raw --all --full-history -- subdirectory/my-magic-file</code></pre>
 827 </div></div>
 828 <div class="paragraph"><p>and that will show you the whole log for that file (please realize that
 829 the tree you had may not be the top-level tree, so you need to figure out
 830 which subdirectory it was in on your own), and because you&#8217;re asking for
 831 raw output, you&#8217;ll now get something like</p></div>
 832 <div class="literalblock">
 833 <div class="content">
 834 <pre><code>commit abc
 835 Author:
 836 Date:
 837   ..
 838 :100644 100644 4b9458b... newsha... M  somedirectory/my-magic-file</code></pre>
 839 </div></div>
 840 <div class="literalblock">
 841 <div class="content">
 842 <pre><code>commit xyz
 843 Author:
 844 Date:</code></pre>
 845 </div></div>
 846 <div class="literalblock">
 847 <div class="content">
 848 <pre><code>  ..
 849 :100644 100644 oldsha... 4b9458b... M   somedirectory/my-magic-file</code></pre>
 850 </div></div>
 851 <div class="paragraph"><p>and this actually tells you what the <strong>previous</strong> and <strong>subsequent</strong> versions
 852 of that file were! So now you can look at those ("oldsha" and "newsha"
 853 respectively), and hopefully you have done commits often, and can
 854 re-create the missing my-magic-file version by looking at those older and
 855 newer versions!</p></div>
 856 <div class="paragraph"><p>If you can do that, you can now recreate the missing object with</p></div>
 857 <div class="literalblock">
 858 <div class="content">
 859 <pre><code>git hash-object -w &lt;recreated-file&gt;</code></pre>
 860 </div></div>
 861 <div class="paragraph"><p>and your repository is good again!</p></div>
 862 <div class="paragraph"><p>(Btw, you could have ignored the fsck, and started with doing a</p></div>
 863 <div class="literalblock">
 864 <div class="content">
 865 <pre><code>git log --raw --all</code></pre>
 866 </div></div>
 867 <div class="paragraph"><p>and just looked for the sha of the missing object (4b9458b..) in that
 868 whole thing. It&#8217;s up to you - Git does <strong>have</strong> a lot of information, it is
 869 just missing one particular blob version.</p></div>
 870 <div class="paragraph"><p>Trying to recreate trees and especially commits is <strong>much</strong> harder. So you
 871 were lucky that it&#8217;s a blob. It&#8217;s quite possible that you can recreate the
 872 thing.</p></div>
 873 <div class="literalblock">
 874 <div class="content">
 875 <pre><code>Linus</code></pre>
 876 </div></div>
 877 </div>
 878 </div>
 879 </div>
 880 <div id="footnotes"><hr /></div>
 881 <div id="footer">
 882 <div id="footer-text">
 883 Last updated
 884  2023-07-25 13:20:33 PDT
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