technical/racy-git.html

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