technical/racy-git.html

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   8 <title>Use of index and Racy Git problem</title>
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 435 </head>
 436 <body class="article">
 437 <div id="header">
 438 <h1>Use of index and Racy Git problem</h1>
 439 <div class="details">
 440 <span id="revdate">2024-08-07</span>
 441 </div>
 442 </div>
 443 <div id="content">
 444 <div class="sect1">
 445 <h2 id="_background">Background</h2>
 446 <div class="sectionbody">
 447 <div class="paragraph">
 448 <p>The index is one of the most important data structures in Git.
 449 It represents a virtual working tree state by recording list of
 450 paths and their object names and serves as a staging area to
 451 write out the next tree object to be committed.  The state is
 452 "virtual" in the sense that it does not necessarily have to, and
 453 often does not, match the files in the working tree.</p>
 454 </div>
 455 <div class="paragraph">
 456 <p>There are cases where Git needs to examine the differences between the
 457 virtual working tree state in the index and the files in the
 458 working tree.  The most obvious case is when the user asks <code>git
 459 diff</code> (or its low level implementation, <code>git diff-files</code>) or
 460 <code>git-ls-files --modified</code>.  In addition, Git internally checks
 461 if the files in the working tree are different from what are
 462 recorded in the index to avoid stomping on local changes in them
 463 during patch application, switching branches, and merging.</p>
 464 </div>
 465 <div class="paragraph">
 466 <p>In order to speed up this comparison between the files in the
 467 working tree and the index entries, the index entries record the
 468 information obtained from the filesystem via <code>lstat(2)</code> system
 469 call when they were last updated.  When checking if they differ,
 470 Git first runs <code>lstat(2)</code> on the files and compares the result
 471 with this information (this is what was originally done by the
 472 <code>ce_match_stat()</code> function, but the current code does it in
 473 <code>ce_match_stat_basic()</code> function).  If some of these "cached
 474 stat information" fields do not match, Git can tell that the
 475 files are modified without even looking at their contents.</p>
 476 </div>
 477 <div class="paragraph">
 478 <p>Note: not all members in <code>struct stat</code> obtained via <code>lstat(2)</code>
 479 are used for this comparison.  For example, <code>st_atime</code> obviously
 480 is not useful.  Currently, Git compares the file type (regular
 481 files vs symbolic links) and executable bits (only for regular
 482 files) from <code>st_mode</code> member, <code>st_mtime</code> and <code>st_ctime</code>
 483 timestamps, <code>st_uid</code>, <code>st_gid</code>, <code>st_ino</code>, and <code>st_size</code> members.
 484 With a <code>USE_STDEV</code> compile-time option, <code>st_dev</code> is also
 485 compared, but this is not enabled by default because this member
 486 is not stable on network filesystems.  With <code>USE_NSEC</code>
 487 compile-time option, <code>st_mtim.tv_nsec</code> and <code>st_ctim.tv_nsec</code>
 488 members are also compared. On Linux, this is not enabled by default
 489 because in-core timestamps can have finer granularity than
 490 on-disk timestamps, resulting in meaningless changes when an
 491 inode is evicted from the inode cache.  See commit 8ce13b0
 492 of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
 493 ([PATCH] Sync in core time granularity with filesystems,
 494 2005-01-04). This patch is included in kernel 2.6.11 and newer, but
 495 only fixes the issue for file systems with exactly 1 ns or 1 s
 496 resolution. Other file systems are still broken in current Linux
 497 kernels (e.g. CEPH, CIFS, NTFS, UDF), see
 498 <a href="https://lore.kernel.org/lkml/5577240D.7020309@gmail.com/" class="bare">https://lore.kernel.org/lkml/5577240D.7020309@gmail.com/</a></p>
 499 </div>
 500 </div>
 501 </div>
 502 <div class="sect1">
 503 <h2 id="_racy_git">Racy Git</h2>
 504 <div class="sectionbody">
 505 <div class="paragraph">
 506 <p>There is one slight problem with the optimization based on the
 507 cached stat information.  Consider this sequence:</p>
 508 </div>
 509 <div class="literalblock">
 510 <div class="content">
 511 <pre>: modify 'foo'
 512 $ git update-index 'foo'
 513 : modify 'foo' again, in-place, without changing its size</pre>
 514 </div>
 515 </div>
 516 <div class="paragraph">
 517 <p>The first <code>update-index</code> computes the object name of the
 518 contents of file <code>foo</code> and updates the index entry for <code>foo</code>
 519 along with the <code>struct stat</code> information.  If the modification
 520 that follows it happens very fast so that the file&#8217;s <code>st_mtime</code>
 521 timestamp does not change, after this sequence, the cached stat
 522 information the index entry records still exactly match what you
 523 would see in the filesystem, even though the file <code>foo</code> is now
 524 different.
 525 This way, Git can incorrectly think files in the working tree
 526 are unmodified even though they actually are.  This is called
 527 the "racy Git" problem (discovered by Pasky), and the entries
 528 that appear clean when they may not be because of this problem
 529 are called "racily clean".</p>
 530 </div>
 531 <div class="paragraph">
 532 <p>To avoid this problem, Git does two things:</p>
 533 </div>
 534 <div class="olist arabic">
 535 <ol class="arabic">
 536 <li>
 537 <p>When the cached stat information says the file has not been
 538 modified, and the <code>st_mtime</code> is the same as (or newer than)
 539 the timestamp of the index file itself (which is the time <code>git
 540 update-index foo</code> finished running in the above example), it
 541 also compares the contents with the object registered in the
 542 index entry to make sure they match.</p>
 543 </li>
 544 <li>
 545 <p>When the index file is updated that contains racily clean
 546 entries, cached <code>st_size</code> information is truncated to zero
 547 before writing a new version of the index file.</p>
 548 </li>
 549 </ol>
 550 </div>
 551 <div class="paragraph">
 552 <p>Because the index file itself is written after collecting all
 553 the stat information from updated paths, <code>st_mtime</code> timestamp of
 554 it is usually the same as or newer than any of the paths the
 555 index contains.  And no matter how quick the modification that
 556 follows <code>git update-index foo</code> finishes, the resulting
 557 <code>st_mtime</code> timestamp on <code>foo</code> cannot get a value earlier
 558 than the index file.  Therefore, index entries that can be
 559 racily clean are limited to the ones that have the same
 560 timestamp as the index file itself.</p>
 561 </div>
 562 <div class="paragraph">
 563 <p>The callers that want to check if an index entry matches the
 564 corresponding file in the working tree continue to call
 565 <code>ce_match_stat()</code>, but with this change, <code>ce_match_stat()</code> uses
 566 <code>ce_modified_check_fs()</code> to see if racily clean ones are
 567 actually clean after comparing the cached stat information using
 568 <code>ce_match_stat_basic()</code>.</p>
 569 </div>
 570 <div class="paragraph">
 571 <p>The problem the latter solves is this sequence:</p>
 572 </div>
 573 <div class="literalblock">
 574 <div class="content">
 575 <pre>$ git update-index 'foo'
 576 : modify 'foo' in-place without changing its size
 577 : wait for enough time
 578 $ git update-index 'bar'</pre>
 579 </div>
 580 </div>
 581 <div class="paragraph">
 582 <p>Without the latter, the timestamp of the index file gets a newer
 583 value, and falsely clean entry <code>foo</code> would not be caught by the
 584 timestamp comparison check done with the former logic anymore.
 585 The latter makes sure that the cached stat information for <code>foo</code>
 586 would never match with the file in the working tree, so later
 587 checks by <code>ce_match_stat_basic()</code> would report that the index entry
 588 does not match the file and Git does not have to fall back on more
 589 expensive <code>ce_modified_check_fs()</code>.</p>
 590 </div>
 591 </div>
 592 </div>
 593 <div class="sect1">
 594 <h2 id="_runtime_penalty">Runtime penalty</h2>
 595 <div class="sectionbody">
 596 <div class="paragraph">
 597 <p>The runtime penalty of falling back to <code>ce_modified_check_fs()</code>
 598 from <code>ce_match_stat()</code> can be very expensive when there are many
 599 racily clean entries.  An obvious way to artificially create
 600 this situation is to give the same timestamp to all the files in
 601 the working tree in a large project, run <code>git update-index</code> on
 602 them, and give the same timestamp to the index file:</p>
 603 </div>
 604 <div class="literalblock">
 605 <div class="content">
 606 <pre>$ date &gt;.datestamp
 607 $ git ls-files | xargs touch -r .datestamp
 608 $ git ls-files | git update-index --stdin
 609 $ touch -r .datestamp .git/index</pre>
 610 </div>
 611 </div>
 612 <div class="paragraph">
 613 <p>This will make all index entries racily clean.  The linux project, for
 614 example, there are over 20,000 files in the working tree.  On my
 615 Athlon 64 X2 3800+, after the above:</p>
 616 </div>
 617 <div class="literalblock">
 618 <div class="content">
 619 <pre>$ /usr/bin/time git diff-files
 620 1.68user 0.54system 0:02.22elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
 621 0inputs+0outputs (0major+67111minor)pagefaults 0swaps
 622 $ git update-index MAINTAINERS
 623 $ /usr/bin/time git diff-files
 624 0.02user 0.12system 0:00.14elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
 625 0inputs+0outputs (0major+935minor)pagefaults 0swaps</pre>
 626 </div>
 627 </div>
 628 <div class="paragraph">
 629 <p>Running <code>git update-index</code> in the middle checked the racily
 630 clean entries, and left the cached <code>st_mtime</code> for all the paths
 631 intact because they were actually clean (so this step took about
 632 the same amount of time as the first <code>git diff-files</code>).  After
 633 that, they are not racily clean anymore but are truly clean, so
 634 the second invocation of <code>git diff-files</code> fully took advantage
 635 of the cached stat information.</p>
 636 </div>
 637 </div>
 638 </div>
 639 <div class="sect1">
 640 <h2 id="_avoiding_runtime_penalty">Avoiding runtime penalty</h2>
 641 <div class="sectionbody">
 642 <div class="paragraph">
 643 <p>In order to avoid the above runtime penalty, post 1.4.2 Git used
 644 to have a code that made sure the index file
 645 got a timestamp newer than the youngest files in the index when
 646 there were many young files with the same timestamp as the
 647 resulting index file otherwise would have by waiting
 648 before finishing writing the index file out.</p>
 649 </div>
 650 <div class="paragraph">
 651 <p>I suspected that in practice the situation where many paths in the
 652 index are all racily clean was quite rare.  The only code paths
 653 that can record recent timestamp for large number of paths are:</p>
 654 </div>
 655 <div class="olist arabic">
 656 <ol class="arabic">
 657 <li>
 658 <p>Initial <code>git add .</code> of a large project.</p>
 659 </li>
 660 <li>
 661 <p><code>git checkout</code> of a large project from an empty index into an
 662 unpopulated working tree.</p>
 663 </li>
 664 </ol>
 665 </div>
 666 <div class="paragraph">
 667 <p>Note: switching branches with <code>git checkout</code> keeps the cached
 668 stat information of existing working tree files that are the
 669 same between the current branch and the new branch, which are
 670 all older than the resulting index file, and they will not
 671 become racily clean.  Only the files that are actually checked
 672 out can become racily clean.</p>
 673 </div>
 674 <div class="paragraph">
 675 <p>In a large project where raciness avoidance cost really matters,
 676 however, the initial computation of all object names in the
 677 index takes more than one second, and the index file is written
 678 out after all that happens.  Therefore the timestamp of the
 679 index file will be more than one second later than the
 680 youngest file in the working tree.  This means that in these
 681 cases there actually will not be any racily clean entry in
 682 the resulting index.</p>
 683 </div>
 684 <div class="paragraph">
 685 <p>Based on this discussion, the current code does not use the
 686 "workaround" to avoid the runtime penalty that does not exist in
 687 practice anymore.  This was done with commit 0fc82cff on Aug 15,
 688 2006.</p>
 689 </div>
 690 </div>
 691 </div>
 692 </div>
 693 <div id="footer">
 694 <div id="footer-text">
 695 Last updated 2023-10-23 14:43:46 -0700
 696 </div>
 697 </div>
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