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| <body class="article"> | |
| <div id="header"> | |
| <h1>How to recover a corrupted blob object</h1> | |
| <span id="revdate">2024-04-16</span> | |
| </div> | |
| <div id="content"> | |
| <div id="preamble"> | |
| <div class="sectionbody"> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code>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</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>So exactly <strong>because</strong> the SHA-1 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.</p></div> | |
| <div class="paragraph"><p>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 <strong>not</strong> use a hard-linked clone, and preferably | |
| not the same disk/machine).</p></div> | |
| <div class="paragraph"><p>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 <strong>are</strong> somewhat | |
| debuggable.</p></div> | |
| <div class="paragraph"><p>First off, move the corrupt object away, and <strong>save</strong> 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.</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code>So: | |
| > ib]$ mv .git/objects/4b/9458b3786228369c63936db65827de3cc06200 ../</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>This is the right thing to do, although it’s usually best to save it under | |
| it’s full SHA-1 name (you just dropped the "4b" from the result ;).</p></div> | |
| <div class="paragraph"><p>Let’s see what that tells us:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code>> ib]$ git-fsck --full | |
| > broken link from tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8 | |
| > to blob 4b9458b3786228369c63936db65827de3cc06200 | |
| > missing blob 4b9458b3786228369c63936db65827de3cc06200</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>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!</p></div> | |
| <div class="paragraph"><p>Now you can do</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>which will show something like</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>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 | |
| ...</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>and you should now have a line that looks like</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>10064 blob 4b9458b3786228369c63936db65827de3cc06200 my-magic-file</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>in the output. This already tells you a <strong>lot</strong> it tells you what file the | |
| corrupt blob came from!</p></div> | |
| <div class="paragraph"><p>Now, it doesn’t tell you quite enough, though: it doesn’t tell what | |
| <strong>version</strong> 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</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>git hash-object -w my-magic-file</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>again, and if it outputs the missing SHA-1 (4b945..) you’re now all done!</p></div> | |
| <div class="paragraph"><p>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?</p></div> | |
| <div class="paragraph"><p>The easiest way to do it is to do</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>git log --raw --all --full-history -- subdirectory/my-magic-file</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>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</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>commit abc | |
| Author: | |
| Date: | |
| .. | |
| :100644 100644 4b9458b... newsha... M somedirectory/my-magic-file</code></pre> | |
| </div></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>commit xyz | |
| Author: | |
| Date:</code></pre> | |
| </div></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code> .. | |
| :100644 100644 oldsha... 4b9458b... M somedirectory/my-magic-file</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>and this actually tells you what the <strong>previous</strong> and <strong>subsequent</strong> 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!</p></div> | |
| <div class="paragraph"><p>If you can do that, you can now recreate the missing object with</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>git hash-object -w <recreated-file></code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>and your repository is good again!</p></div> | |
| <div class="paragraph"><p>(Btw, you could have ignored the fsck, and started with doing a</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>git log --raw --all</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>and just looked for the sha of the missing object (4b9458b..) in that | |
| whole thing. It’s up to you - Git does <strong>have</strong> a lot of information, it is | |
| just missing one particular blob version.</p></div> | |
| <div class="paragraph"><p>Trying to recreate trees and especially commits is <strong>much</strong> harder. So you | |
| were lucky that it’s a blob. It’s quite possible that you can recreate the | |
| thing.</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>Linus</code></pre> | |
| </div></div> | |
| </div> | |
| </div> | |
| </div> | |
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| Last updated | |
| 2024-04-16 14:54:57 PDT | |
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