technical/racy-git.html

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 735 <body class="article">
 736 <div id="header">
 737 <h1>Use of index and Racy Git problem</h1>
 738 <span id="revdate">2024-03-07</span>
 739 </div>
 740 <div id="content">
 741 <div class="sect1">
 742 <h2 id="_background">Background</h2>
 743 <div class="sectionbody">
 744 <div class="paragraph"><p>The index is one of the most important data structures in Git.
 745 It represents a virtual working tree state by recording list of
 746 paths and their object names and serves as a staging area to
 747 write out the next tree object to be committed.  The state is
 748 "virtual" in the sense that it does not necessarily have to, and
 749 often does not, match the files in the working tree.</p></div>
 750 <div class="paragraph"><p>There are cases where Git needs to examine the differences between the
 751 virtual working tree state in the index and the files in the
 752 working tree.  The most obvious case is when the user asks <code>git
 753 diff</code> (or its low level implementation, <code>git diff-files</code>) or
 754 <code>git-ls-files --modified</code>.  In addition, Git internally checks
 755 if the files in the working tree are different from what are
 756 recorded in the index to avoid stomping on local changes in them
 757 during patch application, switching branches, and merging.</p></div>
 758 <div class="paragraph"><p>In order to speed up this comparison between the files in the
 759 working tree and the index entries, the index entries record the
 760 information obtained from the filesystem via <code>lstat(2)</code> system
 761 call when they were last updated.  When checking if they differ,
 762 Git first runs <code>lstat(2)</code> on the files and compares the result
 763 with this information (this is what was originally done by the
 764 <code>ce_match_stat()</code> function, but the current code does it in
 765 <code>ce_match_stat_basic()</code> function).  If some of these "cached
 766 stat information" fields do not match, Git can tell that the
 767 files are modified without even looking at their contents.</p></div>
 768 <div class="paragraph"><p>Note: not all members in <code>struct stat</code> obtained via <code>lstat(2)</code>
 769 are used for this comparison.  For example, <code>st_atime</code> obviously
 770 is not useful.  Currently, Git compares the file type (regular
 771 files vs symbolic links) and executable bits (only for regular
 772 files) from <code>st_mode</code> member, <code>st_mtime</code> and <code>st_ctime</code>
 773 timestamps, <code>st_uid</code>, <code>st_gid</code>, <code>st_ino</code>, and <code>st_size</code> members.
 774 With a <code>USE_STDEV</code> compile-time option, <code>st_dev</code> is also
 775 compared, but this is not enabled by default because this member
 776 is not stable on network filesystems.  With <code>USE_NSEC</code>
 777 compile-time option, <code>st_mtim.tv_nsec</code> and <code>st_ctim.tv_nsec</code>
 778 members are also compared. On Linux, this is not enabled by default
 779 because in-core timestamps can have finer granularity than
 780 on-disk timestamps, resulting in meaningless changes when an
 781 inode is evicted from the inode cache.  See commit 8ce13b0
 782 of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
 783 ([PATCH] Sync in core time granularity with filesystems,
 784 2005-01-04). This patch is included in kernel 2.6.11 and newer, but
 785 only fixes the issue for file systems with exactly 1 ns or 1 s
 786 resolution. Other file systems are still broken in current Linux
 787 kernels (e.g. CEPH, CIFS, NTFS, UDF), see
 788 <a href="https://lore.kernel.org/lkml/5577240D.7020309@gmail.com/">https://lore.kernel.org/lkml/5577240D.7020309@gmail.com/</a></p></div>
 789 </div>
 790 </div>
 791 <div class="sect1">
 792 <h2 id="_racy_git">Racy Git</h2>
 793 <div class="sectionbody">
 794 <div class="paragraph"><p>There is one slight problem with the optimization based on the
 795 cached stat information.  Consider this sequence:</p></div>
 796 <div class="literalblock">
 797 <div class="content">
 798 <pre><code>: modify 'foo'
 799 $ git update-index 'foo'
 800 : modify 'foo' again, in-place, without changing its size</code></pre>
 801 </div></div>
 802 <div class="paragraph"><p>The first <code>update-index</code> computes the object name of the
 803 contents of file <code>foo</code> and updates the index entry for <code>foo</code>
 804 along with the <code>struct stat</code> information.  If the modification
 805 that follows it happens very fast so that the file&#8217;s <code>st_mtime</code>
 806 timestamp does not change, after this sequence, the cached stat
 807 information the index entry records still exactly match what you
 808 would see in the filesystem, even though the file <code>foo</code> is now
 809 different.
 810 This way, Git can incorrectly think files in the working tree
 811 are unmodified even though they actually are.  This is called
 812 the "racy Git" problem (discovered by Pasky), and the entries
 813 that appear clean when they may not be because of this problem
 814 are called "racily clean".</p></div>
 815 <div class="paragraph"><p>To avoid this problem, Git does two things:</p></div>
 816 <div class="olist arabic"><ol class="arabic">
 817 <li>
 818 <p>
 819 When the cached stat information says the file has not been
 820   modified, and the <code>st_mtime</code> is the same as (or newer than)
 821   the timestamp of the index file itself (which is the time <code>git
 822   update-index foo</code> finished running in the above example), it
 823   also compares the contents with the object registered in the
 824   index entry to make sure they match.
 825 </p>
 826 </li>
 827 <li>
 828 <p>
 829 When the index file is updated that contains racily clean
 830   entries, cached <code>st_size</code> information is truncated to zero
 831   before writing a new version of the index file.
 832 </p>
 833 </li>
 834 </ol></div>
 835 <div class="paragraph"><p>Because the index file itself is written after collecting all
 836 the stat information from updated paths, <code>st_mtime</code> timestamp of
 837 it is usually the same as or newer than any of the paths the
 838 index contains.  And no matter how quick the modification that
 839 follows <code>git update-index foo</code> finishes, the resulting
 840 <code>st_mtime</code> timestamp on <code>foo</code> cannot get a value earlier
 841 than the index file.  Therefore, index entries that can be
 842 racily clean are limited to the ones that have the same
 843 timestamp as the index file itself.</p></div>
 844 <div class="paragraph"><p>The callers that want to check if an index entry matches the
 845 corresponding file in the working tree continue to call
 846 <code>ce_match_stat()</code>, but with this change, <code>ce_match_stat()</code> uses
 847 <code>ce_modified_check_fs()</code> to see if racily clean ones are
 848 actually clean after comparing the cached stat information using
 849 <code>ce_match_stat_basic()</code>.</p></div>
 850 <div class="paragraph"><p>The problem the latter solves is this sequence:</p></div>
 851 <div class="literalblock">
 852 <div class="content">
 853 <pre><code>$ git update-index 'foo'
 854 : modify 'foo' in-place without changing its size
 855 : wait for enough time
 856 $ git update-index 'bar'</code></pre>
 857 </div></div>
 858 <div class="paragraph"><p>Without the latter, the timestamp of the index file gets a newer
 859 value, and falsely clean entry <code>foo</code> would not be caught by the
 860 timestamp comparison check done with the former logic anymore.
 861 The latter makes sure that the cached stat information for <code>foo</code>
 862 would never match with the file in the working tree, so later
 863 checks by <code>ce_match_stat_basic()</code> would report that the index entry
 864 does not match the file and Git does not have to fall back on more
 865 expensive <code>ce_modified_check_fs()</code>.</p></div>
 866 </div>
 867 </div>
 868 <div class="sect1">
 869 <h2 id="_runtime_penalty">Runtime penalty</h2>
 870 <div class="sectionbody">
 871 <div class="paragraph"><p>The runtime penalty of falling back to <code>ce_modified_check_fs()</code>
 872 from <code>ce_match_stat()</code> can be very expensive when there are many
 873 racily clean entries.  An obvious way to artificially create
 874 this situation is to give the same timestamp to all the files in
 875 the working tree in a large project, run <code>git update-index</code> on
 876 them, and give the same timestamp to the index file:</p></div>
 877 <div class="literalblock">
 878 <div class="content">
 879 <pre><code>$ date &gt;.datestamp
 880 $ git ls-files | xargs touch -r .datestamp
 881 $ git ls-files | git update-index --stdin
 882 $ touch -r .datestamp .git/index</code></pre>
 883 </div></div>
 884 <div class="paragraph"><p>This will make all index entries racily clean.  The linux project, for
 885 example, there are over 20,000 files in the working tree.  On my
 886 Athlon 64 X2 3800+, after the above:</p></div>
 887 <div class="literalblock">
 888 <div class="content">
 889 <pre><code>$ /usr/bin/time git diff-files
 890 1.68user 0.54system 0:02.22elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
 891 0inputs+0outputs (0major+67111minor)pagefaults 0swaps
 892 $ git update-index MAINTAINERS
 893 $ /usr/bin/time git diff-files
 894 0.02user 0.12system 0:00.14elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
 895 0inputs+0outputs (0major+935minor)pagefaults 0swaps</code></pre>
 896 </div></div>
 897 <div class="paragraph"><p>Running <code>git update-index</code> in the middle checked the racily
 898 clean entries, and left the cached <code>st_mtime</code> for all the paths
 899 intact because they were actually clean (so this step took about
 900 the same amount of time as the first <code>git diff-files</code>).  After
 901 that, they are not racily clean anymore but are truly clean, so
 902 the second invocation of <code>git diff-files</code> fully took advantage
 903 of the cached stat information.</p></div>
 904 </div>
 905 </div>
 906 <div class="sect1">
 907 <h2 id="_avoiding_runtime_penalty">Avoiding runtime penalty</h2>
 908 <div class="sectionbody">
 909 <div class="paragraph"><p>In order to avoid the above runtime penalty, post 1.4.2 Git used
 910 to have a code that made sure the index file
 911 got a timestamp newer than the youngest files in the index when
 912 there were many young files with the same timestamp as the
 913 resulting index file otherwise would have by waiting
 914 before finishing writing the index file out.</p></div>
 915 <div class="paragraph"><p>I suspected that in practice the situation where many paths in the
 916 index are all racily clean was quite rare.  The only code paths
 917 that can record recent timestamp for large number of paths are:</p></div>
 918 <div class="olist arabic"><ol class="arabic">
 919 <li>
 920 <p>
 921 Initial <code>git add .</code> of a large project.
 922 </p>
 923 </li>
 924 <li>
 925 <p>
 926 <code>git checkout</code> of a large project from an empty index into an
 927   unpopulated working tree.
 928 </p>
 929 </li>
 930 </ol></div>
 931 <div class="paragraph"><p>Note: switching branches with <code>git checkout</code> keeps the cached
 932 stat information of existing working tree files that are the
 933 same between the current branch and the new branch, which are
 934 all older than the resulting index file, and they will not
 935 become racily clean.  Only the files that are actually checked
 936 out can become racily clean.</p></div>
 937 <div class="paragraph"><p>In a large project where raciness avoidance cost really matters,
 938 however, the initial computation of all object names in the
 939 index takes more than one second, and the index file is written
 940 out after all that happens.  Therefore the timestamp of the
 941 index file will be more than one second later than the
 942 youngest file in the working tree.  This means that in these
 943 cases there actually will not be any racily clean entry in
 944 the resulting index.</p></div>
 945 <div class="paragraph"><p>Based on this discussion, the current code does not use the
 946 "workaround" to avoid the runtime penalty that does not exist in
 947 practice anymore.  This was done with commit 0fc82cff on Aug 15,
 948 2006.</p></div>
 949 </div>
 950 </div>
 951 </div>
 952 <div id="footnotes"><hr /></div>
 953 <div id="footer">
 954 <div id="footer-text">
 955 Last updated
 956  2023-10-23 14:43:46 PDT
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