howto/keep-canonical-history-correct.html

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 736 <div id="header">
 737 <h1>Keep authoritative canonical history correct with git pull</h1>
 738 <span id="revdate">2023-10-29</span>
 739 </div>
 740 <div id="content">
 741 <div id="preamble">
 742 <div class="sectionbody">
 743 <div class="paragraph"><p>Sometimes a new project integrator will end up with project history
 744 that appears to be "backwards" from what other project developers
 745 expect. This howto presents a suggested integration workflow for
 746 maintaining a central repository.</p></div>
 747 <div class="paragraph"><p>Suppose that that central repository has this history:</p></div>
 748 <div class="listingblock">
 749 <div class="content">
 750 <pre><code>    ---o---o---A</code></pre>
 751 </div></div>
 752 <div class="paragraph"><p>which ends at commit <code>A</code> (time flows from left to right and each node
 753 in the graph is a commit, lines between them indicating parent-child
 754 relationship).</p></div>
 755 <div class="paragraph"><p>Then you clone it and work on your own commits, which leads you to
 756 have this history in <strong>your</strong> repository:</p></div>
 757 <div class="listingblock">
 758 <div class="content">
 759 <pre><code>    ---o---o---A---B---C</code></pre>
 760 </div></div>
 761 <div class="paragraph"><p>Imagine your coworker did the same and built on top of <code>A</code> in <strong>his</strong>
 762 repository in the meantime, and then pushed it to the
 763 central repository:</p></div>
 764 <div class="listingblock">
 765 <div class="content">
 766 <pre><code>    ---o---o---A---X---Y---Z</code></pre>
 767 </div></div>
 768 <div class="paragraph"><p>Now, if you <code>git push</code> at this point, because your history that leads
 769 to <code>C</code> lacks <code>X</code>, <code>Y</code> and <code>Z</code>, it will fail.  You need to somehow make
 770 the tip of your history a descendant of <code>Z</code>.</p></div>
 771 <div class="paragraph"><p>One suggested way to solve the problem is "fetch and then merge", aka
 772 <code>git pull</code>. When you fetch, your repository will have a history like
 773 this:</p></div>
 774 <div class="listingblock">
 775 <div class="content">
 776 <pre><code>    ---o---o---A---B---C
 777                 \
 778                  X---Y---Z</code></pre>
 779 </div></div>
 780 <div class="paragraph"><p>Once you run merge after that, while still on <strong>your</strong> branch, i.e. <code>C</code>,
 781 you will create a merge <code>M</code> and make the history look like this:</p></div>
 782 <div class="listingblock">
 783 <div class="content">
 784 <pre><code>    ---o---o---A---B---C---M
 785                 \         /
 786                  X---Y---Z</code></pre>
 787 </div></div>
 788 <div class="paragraph"><p><code>M</code> is a descendant of <code>Z</code>, so you can push to update the central
 789 repository.  Such a merge <code>M</code> does not lose any commit in both
 790 histories, so in that sense it may not be wrong, but when people want
 791 to talk about "the authoritative canonical history that is shared
 792 among the project participants", i.e. "the trunk", they often view
 793 it as "commits you see by following the first-parent chain", and use
 794 this command to view it:</p></div>
 795 <div class="listingblock">
 796 <div class="content">
 797 <pre><code>    $ git log --first-parent</code></pre>
 798 </div></div>
 799 <div class="paragraph"><p>For all other people who observed the central repository after your
 800 coworker pushed <code>Z</code> but before you pushed <code>M</code>, the commit on the trunk
 801 used to be <code>o-o-A-X-Y-Z</code>.  But because you made <code>M</code> while you were on
 802 <code>C</code>, <code>M</code>'s first parent is <code>C</code>, so by pushing <code>M</code> to advance the
 803 central repository, you made <code>X-Y-Z</code> a side branch, not on the trunk.</p></div>
 804 <div class="paragraph"><p>You would rather want to have a history of this shape:</p></div>
 805 <div class="listingblock">
 806 <div class="content">
 807 <pre><code>    ---o---o---A---X---Y---Z---M'
 808                 \             /
 809                  B-----------C</code></pre>
 810 </div></div>
 811 <div class="paragraph"><p>so that in the first-parent chain, it is clear that the project first
 812 did <code>X</code> and then <code>Y</code> and then <code>Z</code> and merged a change that consists of
 813 two commits <code>B</code> and <code>C</code> that achieves a single goal.  You may have
 814 worked on fixing the bug #12345 with these two patches, and the merge
 815 <code>M'</code> with swapped parents can say in its log message "Merge
 816 fix-bug-12345". Having a way to tell <code>git pull</code> to create a merge
 817 but record the parents in reverse order may be a way to do so.</p></div>
 818 <div class="paragraph"><p>Note that I said "achieves a single goal" above, because this is
 819 important.  "Swapping the merge order" only covers a special case
 820 where the project does not care too much about having unrelated
 821 things done on a single merge but cares a lot about first-parent
 822 chain.</p></div>
 823 <div class="paragraph"><p>There are multiple schools of thought about the "trunk" management.</p></div>
 824 <div class="olist arabic"><ol class="arabic">
 825 <li>
 826 <p>
 827 Some projects want to keep a completely linear history without any
 828     merges.  Obviously, swapping the merge order would not match their
 829     taste.  You would need to flatten your history on top of the
 830     updated upstream to result in a history of this shape instead:
 831 </p>
 832 <div class="listingblock">
 833 <div class="content">
 834 <pre><code>    ---o---o---A---X---Y---Z---B---C</code></pre>
 835 </div></div>
 836 <div class="paragraph"><p>with <code>git pull --rebase</code> or something.</p></div>
 837 </li>
 838 <li>
 839 <p>
 840 Some projects tolerate merges in their history, but do not worry
 841     too much about the first-parent order, and allow fast-forward
 842     merges.  To them, swapping the merge order does not hurt, but
 843     it is unnecessary.
 844 </p>
 845 </li>
 846 <li>
 847 <p>
 848 Some projects want each commit on the "trunk" to do one single
 849     thing.  The output of <code>git log --first-parent</code> in such a project
 850     would show either a merge of a side branch that completes a single
 851     theme, or a single commit that completes a single theme by itself.
 852     If your two commits <code>B</code> and <code>C</code> (or they may even be two groups of
 853     commits) were solving two independent issues, then the merge <code>M'</code>
 854     we made in the earlier example by swapping the merge order is
 855     still not up to the project standard.  It merges two unrelated
 856     efforts <code>B</code> and <code>C</code> at the same time.
 857 </p>
 858 </li>
 859 </ol></div>
 860 <div class="paragraph"><p>For projects in the last category (Git itself is one of them),
 861 individual developers would want to prepare a history more like
 862 this:</p></div>
 863 <div class="listingblock">
 864 <div class="content">
 865 <pre><code>                 C0--C1--C2     topic-c
 866                 /
 867     ---o---o---A                master
 868                 \
 869                  B0--B1--B2     topic-b</code></pre>
 870 </div></div>
 871 <div class="paragraph"><p>That is, keeping separate topics on separate branches, perhaps like
 872 so:</p></div>
 873 <div class="listingblock">
 874 <div class="content">
 875 <pre><code>    $ git clone $URL work &amp;&amp; cd work
 876     $ git checkout -b topic-b master
 877     $ ... work to create B0, B1 and B2 to complete one theme
 878     $ git checkout -b topic-c master
 879     $ ... same for the theme of topic-c</code></pre>
 880 </div></div>
 881 <div class="paragraph"><p>And then</p></div>
 882 <div class="listingblock">
 883 <div class="content">
 884 <pre><code>    $ git checkout master
 885     $ git pull --ff-only</code></pre>
 886 </div></div>
 887 <div class="paragraph"><p>would grab <code>X</code>, <code>Y</code> and <code>Z</code> from the upstream and advance your master
 888 branch:</p></div>
 889 <div class="listingblock">
 890 <div class="content">
 891 <pre><code>                 C0--C1--C2     topic-c
 892                 /
 893     ---o---o---A---X---Y---Z    master
 894                 \
 895                  B0--B1--B2     topic-b</code></pre>
 896 </div></div>
 897 <div class="paragraph"><p>And then you would merge these two branches separately:</p></div>
 898 <div class="listingblock">
 899 <div class="content">
 900 <pre><code>    $ git merge topic-b
 901     $ git merge topic-c</code></pre>
 902 </div></div>
 903 <div class="paragraph"><p>to result in</p></div>
 904 <div class="listingblock">
 905 <div class="content">
 906 <pre><code>                 C0--C1---------C2
 907                 /                 \
 908     ---o---o---A---X---Y---Z---M---N
 909                 \             /
 910                  B0--B1-----B2</code></pre>
 911 </div></div>
 912 <div class="paragraph"><p>and push it back to the central repository.</p></div>
 913 <div class="paragraph"><p>It is very much possible that while you are merging topic-b and
 914 topic-c, somebody again advanced the history in the central repository
 915 to put <code>W</code> on top of <code>Z</code>, and make your <code>git push</code> fail.</p></div>
 916 <div class="paragraph"><p>In such a case, you would rewind to discard <code>M</code> and <code>N</code>, update the
 917 tip of your <em>master</em> again and redo the two merges:</p></div>
 918 <div class="listingblock">
 919 <div class="content">
 920 <pre><code>    $ git reset --hard origin/master
 921     $ git pull --ff-only
 922     $ git merge topic-b
 923     $ git merge topic-c</code></pre>
 924 </div></div>
 925 <div class="paragraph"><p>The procedure will result in a history that looks like this:</p></div>
 926 <div class="listingblock">
 927 <div class="content">
 928 <pre><code>                 C0--C1--------------C2
 929                 /                     \
 930     ---o---o---A---X---Y---Z---W---M'--N'
 931                 \                 /
 932                  B0--B1---------B2</code></pre>
 933 </div></div>
 934 <div class="paragraph"><p>See also <a href="http://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html">http://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html</a></p></div>
 935 </div>
 936 </div>
 937 </div>
 938 <div id="footnotes"><hr /></div>
 939 <div id="footer">
 940 <div id="footer-text">
 941 Last updated
 942  2023-10-30 08:42:32 JST
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