howto/recover-corrupted-blob-object.html

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 440 </head>
 441 <body class="article">
 442 <div id="header">
 443 <h1>How to recover a corrupted blob object</h1>
 444 <div class="details">
 445 <span id="revdate">2024-11-20</span>
 446 </div>
 447 </div>
 448 <div id="content">
 449 <div class="listingblock">
 450 <div class="content">
 451 <pre>On Fri, 9 Nov 2007, Yossi Leybovich wrote:
 452 &gt;
 453 &gt; Did not help still the repository look for this object?
 454 &gt; Any one know how can I track this object and understand which file is it</pre>
 455 </div>
 456 </div>
 457 <div class="paragraph">
 458 <p>So exactly <strong>because</strong> the SHA-1 hash is cryptographically secure, the hash
 459 itself doesn&#8217;t actually tell you anything, in order to fix a corrupt
 460 object you basically have to find the "original source" for it.</p>
 461 </div>
 462 <div class="paragraph">
 463 <p>The easiest way to do that is almost always to have backups, and find the
 464 same object somewhere else. Backups really are a good idea, and Git makes
 465 it pretty easy (if nothing else, just clone the repository somewhere else,
 466 and make sure that you do <strong>not</strong> use a hard-linked clone, and preferably
 467 not the same disk/machine).</p>
 468 </div>
 469 <div class="paragraph">
 470 <p>But since you don&#8217;t seem to have backups right now, the good news is that
 471 especially with a single blob being corrupt, these things <strong>are</strong> somewhat
 472 debuggable.</p>
 473 </div>
 474 <div class="paragraph">
 475 <p>First off, move the corrupt object away, and <strong>save</strong> it. The most common
 476 cause of corruption so far has been memory corruption, but even so, there
 477 are people who would be interested in seeing the corruption - but it&#8217;s
 478 basically impossible to judge the corruption until we can also see the
 479 original object, so right now the corrupt object is useless, but it&#8217;s very
 480 interesting for the future, in the hope that you can re-create a
 481 non-corrupt version.</p>
 482 </div>
 483 <div class="listingblock">
 484 <div class="content">
 485 <pre>So:
 486
 487 &gt; ib]$ mv .git/objects/4b/9458b3786228369c63936db65827de3cc06200 ../</pre>
 488 </div>
 489 </div>
 490 <div class="paragraph">
 491 <p>This is the right thing to do, although it&#8217;s usually best to save it under
 492 it&#8217;s full SHA-1 name (you just dropped the "4b" from the result ;).</p>
 493 </div>
 494 <div class="paragraph">
 495 <p>Let&#8217;s see what that tells us:</p>
 496 </div>
 497 <div class="listingblock">
 498 <div class="content">
 499 <pre>&gt; ib]$ git-fsck --full
 500 &gt; broken link from    tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
 501 &gt;              to    blob 4b9458b3786228369c63936db65827de3cc06200
 502 &gt; missing blob 4b9458b3786228369c63936db65827de3cc06200</pre>
 503 </div>
 504 </div>
 505 <div class="paragraph">
 506 <p>Ok, I removed the "dangling commit" messages, because they are just
 507 messages about the fact that you probably have rebased etc, so they&#8217;re not
 508 at all interesting. But what remains is still very useful. In particular,
 509 we now know which tree points to it!</p>
 510 </div>
 511 <div class="paragraph">
 512 <p>Now you can do</p>
 513 </div>
 514 <div class="literalblock">
 515 <div class="content">
 516 <pre>git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8</pre>
 517 </div>
 518 </div>
 519 <div class="paragraph">
 520 <p>which will show something like</p>
 521 </div>
 522 <div class="literalblock">
 523 <div class="content">
 524 <pre>100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8    .gitignore
 525 100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883    .mailmap
 526 100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c    COPYING
 527 100644 blob ee909f2cc49e54f0799a4739d24c4cb9151ae453    CREDITS
 528 040000 tree 0f5f709c17ad89e72bdbbef6ea221c69807009f6    Documentation
 529 100644 blob 1570d248ad9237e4fa6e4d079336b9da62d9ba32    Kbuild
 530 100644 blob 1c7c229a092665b11cd46a25dbd40feeb31661d9    MAINTAINERS
 531 ...</pre>
 532 </div>
 533 </div>
 534 <div class="paragraph">
 535 <p>and you should now have a line that looks like</p>
 536 </div>
 537 <div class="literalblock">
 538 <div class="content">
 539 <pre>10064 blob 4b9458b3786228369c63936db65827de3cc06200        my-magic-file</pre>
 540 </div>
 541 </div>
 542 <div class="paragraph">
 543 <p>in the output. This already tells you a <strong>lot</strong> it tells you what file the
 544 corrupt blob came from!</p>
 545 </div>
 546 <div class="paragraph">
 547 <p>Now, it doesn&#8217;t tell you quite enough, though: it doesn&#8217;t tell what
 548 <strong>version</strong> of the file didn&#8217;t get correctly written! You might be really
 549 lucky, and it may be the version that you already have checked out in your
 550 working tree, in which case fixing this problem is really simple, just do</p>
 551 </div>
 552 <div class="literalblock">
 553 <div class="content">
 554 <pre>git hash-object -w my-magic-file</pre>
 555 </div>
 556 </div>
 557 <div class="paragraph">
 558 <p>again, and if it outputs the missing SHA-1 (4b945..) you&#8217;re now all done!</p>
 559 </div>
 560 <div class="paragraph">
 561 <p>But that&#8217;s the really lucky case, so let&#8217;s assume that it was some older
 562 version that was broken. How do you tell which version it was?</p>
 563 </div>
 564 <div class="paragraph">
 565 <p>The easiest way to do it is to do</p>
 566 </div>
 567 <div class="literalblock">
 568 <div class="content">
 569 <pre>git log --raw --all --full-history -- subdirectory/my-magic-file</pre>
 570 </div>
 571 </div>
 572 <div class="paragraph">
 573 <p>and that will show you the whole log for that file (please realize that
 574 the tree you had may not be the top-level tree, so you need to figure out
 575 which subdirectory it was in on your own), and because you&#8217;re asking for
 576 raw output, you&#8217;ll now get something like</p>
 577 </div>
 578 <div class="literalblock">
 579 <div class="content">
 580 <pre>commit abc
 581 Author:
 582 Date:
 583   ..
 584 :100644 100644 4b9458b... newsha... M  somedirectory/my-magic-file</pre>
 585 </div>
 586 </div>
 587 <div class="literalblock">
 588 <div class="content">
 589 <pre>commit xyz
 590 Author:
 591 Date:</pre>
 592 </div>
 593 </div>
 594 <div class="literalblock">
 595 <div class="content">
 596 <pre>  ..
 597 :100644 100644 oldsha... 4b9458b... M   somedirectory/my-magic-file</pre>
 598 </div>
 599 </div>
 600 <div class="paragraph">
 601 <p>and this actually tells you what the <strong>previous</strong> and <strong>subsequent</strong> versions
 602 of that file were! So now you can look at those ("oldsha" and "newsha"
 603 respectively), and hopefully you have done commits often, and can
 604 re-create the missing my-magic-file version by looking at those older and
 605 newer versions!</p>
 606 </div>
 607 <div class="paragraph">
 608 <p>If you can do that, you can now recreate the missing object with</p>
 609 </div>
 610 <div class="literalblock">
 611 <div class="content">
 612 <pre>git hash-object -w &lt;recreated-file&gt;</pre>
 613 </div>
 614 </div>
 615 <div class="paragraph">
 616 <p>and your repository is good again!</p>
 617 </div>
 618 <div class="paragraph">
 619 <p>(Btw, you could have ignored the fsck, and started with doing a</p>
 620 </div>
 621 <div class="literalblock">
 622 <div class="content">
 623 <pre>git log --raw --all</pre>
 624 </div>
 625 </div>
 626 <div class="paragraph">
 627 <p>and just looked for the sha of the missing object (4b9458b..) in that
 628 whole thing. It&#8217;s up to you - Git does <strong>have</strong> a lot of information, it is
 629 just missing one particular blob version.</p>
 630 </div>
 631 <div class="paragraph">
 632 <p>Trying to recreate trees and especially commits is <strong>much</strong> harder. So you
 633 were lucky that it&#8217;s a blob. It&#8217;s quite possible that you can recreate the
 634 thing.</p>
 635 </div>
 636 <div class="literalblock">
 637 <div class="content">
 638 <pre>Linus</pre>
 639 </div>
 640 </div>
 641 </div>
 642 <div id="footer">
 643 <div id="footer-text">
 644 Last updated 2024-11-20 15:41:29 +0900
 645 </div>
 646 </div>
 647 </body>
 648 </html>