howto/revert-a-faulty-merge.html

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 735 <body class="article">
 736 <div id="header">
 737 <h1>How to revert a faulty merge</h1>
 738 <span id="revdate">2023-08-15</span>
 739 </div>
 740 <div id="content">
 741 <div id="preamble">
 742 <div class="sectionbody">
 743 <div class="paragraph"><p>Alan &lt;<a href="mailto:alan@clueserver.org">alan@clueserver.org</a>&gt; said:</p></div>
 744 <div class="literalblock">
 745 <div class="content">
 746 <pre><code>I have a master branch.  We have a branch off of that that some
 747 developers are doing work on.  They claim it is ready. We merge it
 748 into the master branch.  It breaks something so we revert the merge.
 749 They make changes to the code.  they get it to a point where they say
 750 it is ok and we merge again.</code></pre>
 751 </div></div>
 752 <div class="literalblock">
 753 <div class="content">
 754 <pre><code>When examined, we find that code changes made before the revert are
 755 not in the master branch, but code changes after are in the master
 756 branch.</code></pre>
 757 </div></div>
 758 <div class="paragraph"><p>and asked for help recovering from this situation.</p></div>
 759 <div class="paragraph"><p>The history immediately after the "revert of the merge" would look like
 760 this:</p></div>
 761 <div class="literalblock">
 762 <div class="content">
 763 <pre><code>---o---o---o---M---x---x---W
 764               /
 765       ---A---B</code></pre>
 766 </div></div>
 767 <div class="paragraph"><p>where A and B are on the side development that was not so good, M is the
 768 merge that brings these premature changes into the mainline, x are changes
 769 unrelated to what the side branch did and already made on the mainline,
 770 and W is the "revert of the merge M" (doesn&#8217;t W look M upside down?).
 771 IOW, <code>"diff W^..W"</code> is similar to <code>"diff -R M^..M"</code>.</p></div>
 772 <div class="paragraph"><p>Such a "revert" of a merge can be made with:</p></div>
 773 <div class="literalblock">
 774 <div class="content">
 775 <pre><code>$ git revert -m 1 M</code></pre>
 776 </div></div>
 777 <div class="paragraph"><p>After the developers of the side branch fix their mistakes, the history
 778 may look like this:</p></div>
 779 <div class="literalblock">
 780 <div class="content">
 781 <pre><code>---o---o---o---M---x---x---W---x
 782               /
 783       ---A---B-------------------C---D</code></pre>
 784 </div></div>
 785 <div class="paragraph"><p>where C and D are to fix what was broken in A and B, and you may already
 786 have some other changes on the mainline after W.</p></div>
 787 <div class="paragraph"><p>If you merge the updated side branch (with D at its tip), none of the
 788 changes made in A or B will be in the result, because they were reverted
 789 by W.  That is what Alan saw.</p></div>
 790 <div class="paragraph"><p>Linus explains the situation:</p></div>
 791 <div class="literalblock">
 792 <div class="content">
 793 <pre><code>Reverting a regular commit just effectively undoes what that commit
 794 did, and is fairly straightforward. But reverting a merge commit also
 795 undoes the _data_ that the commit changed, but it does absolutely
 796 nothing to the effects on _history_ that the merge had.</code></pre>
 797 </div></div>
 798 <div class="literalblock">
 799 <div class="content">
 800 <pre><code>So the merge will still exist, and it will still be seen as joining
 801 the two branches together, and future merges will see that merge as
 802 the last shared state - and the revert that reverted the merge brought
 803 in will not affect that at all.</code></pre>
 804 </div></div>
 805 <div class="literalblock">
 806 <div class="content">
 807 <pre><code>So a "revert" undoes the data changes, but it's very much _not_ an
 808 "undo" in the sense that it doesn't undo the effects of a commit on
 809 the repository history.</code></pre>
 810 </div></div>
 811 <div class="literalblock">
 812 <div class="content">
 813 <pre><code>So if you think of "revert" as "undo", then you're going to always
 814 miss this part of reverts. Yes, it undoes the data, but no, it doesn't
 815 undo history.</code></pre>
 816 </div></div>
 817 <div class="paragraph"><p>In such a situation, you would want to first revert the previous revert,
 818 which would make the history look like this:</p></div>
 819 <div class="literalblock">
 820 <div class="content">
 821 <pre><code>---o---o---o---M---x---x---W---x---Y
 822               /
 823       ---A---B-------------------C---D</code></pre>
 824 </div></div>
 825 <div class="paragraph"><p>where Y is the revert of W.  Such a "revert of the revert" can be done
 826 with:</p></div>
 827 <div class="literalblock">
 828 <div class="content">
 829 <pre><code>$ git revert W</code></pre>
 830 </div></div>
 831 <div class="paragraph"><p>This history would (ignoring possible conflicts between what W and W..Y
 832 changed) be equivalent to not having W or Y at all in the history:</p></div>
 833 <div class="literalblock">
 834 <div class="content">
 835 <pre><code>---o---o---o---M---x---x-------x----
 836               /
 837       ---A---B-------------------C---D</code></pre>
 838 </div></div>
 839 <div class="paragraph"><p>and merging the side branch again will not have conflict arising from an
 840 earlier revert and revert of the revert.</p></div>
 841 <div class="literalblock">
 842 <div class="content">
 843 <pre><code>---o---o---o---M---x---x-------x-------*
 844               /                       /
 845       ---A---B-------------------C---D</code></pre>
 846 </div></div>
 847 <div class="paragraph"><p>Of course the changes made in C and D still can conflict with what was
 848 done by any of the x, but that is just a normal merge conflict.</p></div>
 849 <div class="paragraph"><p>On the other hand, if the developers of the side branch discarded their
 850 faulty A and B, and redone the changes on top of the updated mainline
 851 after the revert, the history would have looked like this:</p></div>
 852 <div class="literalblock">
 853 <div class="content">
 854 <pre><code>---o---o---o---M---x---x---W---x---x
 855               /                 \
 856       ---A---B                   A'--B'--C'</code></pre>
 857 </div></div>
 858 <div class="paragraph"><p>If you reverted the revert in such a case as in the previous example:</p></div>
 859 <div class="literalblock">
 860 <div class="content">
 861 <pre><code>---o---o---o---M---x---x---W---x---x---Y---*
 862               /                 \         /
 863       ---A---B                   A'--B'--C'</code></pre>
 864 </div></div>
 865 <div class="paragraph"><p>where Y is the revert of W, A' and B' are rerolled A and B, and there may
 866 also be a further fix-up C' on the side branch.  <code>"diff Y^..Y"</code> is similar
 867 to <code>"diff -R W^..W"</code> (which in turn means it is similar to <code>"diff M^..M"</code>),
 868 and <code>"diff A'^..C'"</code> by definition would be similar but different from that,
 869 because it is a rerolled series of the earlier change.  There will be a
 870 lot of overlapping changes that result in conflicts.  So do not do "revert
 871 of revert" blindly without thinking..</p></div>
 872 <div class="literalblock">
 873 <div class="content">
 874 <pre><code>---o---o---o---M---x---x---W---x---x
 875               /                 \
 876       ---A---B                   A'--B'--C'</code></pre>
 877 </div></div>
 878 <div class="paragraph"><p>In the history with rebased side branch, W (and M) are behind the merge
 879 base of the updated branch and the tip of the mainline, and they should
 880 merge without the past faulty merge and its revert getting in the way.</p></div>
 881 <div class="paragraph"><p>To recap, these are two very different scenarios, and they want two very
 882 different resolution strategies:</p></div>
 883 <div class="ulist"><ul>
 884 <li>
 885 <p>
 886 If the faulty side branch was fixed by adding corrections on top, then
 887    doing a revert of the previous revert would be the right thing to do.
 888 </p>
 889 </li>
 890 <li>
 891 <p>
 892 If the faulty side branch whose effects were discarded by an earlier
 893    revert of a merge was rebuilt from scratch (i.e. rebasing and fixing,
 894    as you seem to have interpreted), then re-merging the result without
 895    doing anything else fancy would be the right thing to do.
 896    (See the ADDENDUM below for how to rebuild a branch from scratch
 897    without changing its original branching-off point.)
 898 </p>
 899 </li>
 900 </ul></div>
 901 <div class="paragraph"><p>However, there are things to keep in mind when reverting a merge (and
 902 reverting such a revert).</p></div>
 903 <div class="paragraph"><p>For example, think about what reverting a merge (and then reverting the
 904 revert) does to bisectability. Ignore the fact that the revert of a revert
 905 is undoing it - just think of it as a "single commit that does a lot".
 906 Because that is what it does.</p></div>
 907 <div class="paragraph"><p>When you have a problem you are chasing down, and you hit a "revert this
 908 merge", what you&#8217;re hitting is essentially a single commit that contains
 909 all the changes (but obviously in reverse) of all the commits that got
 910 merged. So it&#8217;s debugging hell, because now you don&#8217;t have lots of small
 911 changes that you can try to pinpoint which <em>part</em> of it changes.</p></div>
 912 <div class="paragraph"><p>But does it all work? Sure it does. You can revert a merge, and from a
 913 purely technical angle, Git did it very naturally and had no real
 914 troubles. It just considered it a change from "state before merge" to
 915 "state after merge", and that was it. Nothing complicated, nothing odd,
 916 nothing really dangerous. Git will do it without even thinking about it.</p></div>
 917 <div class="paragraph"><p>So from a technical angle, there&#8217;s nothing wrong with reverting a merge,
 918 but from a workflow angle it&#8217;s something that you generally should try to
 919 avoid.</p></div>
 920 <div class="paragraph"><p>If at all possible, for example, if you find a problem that got merged
 921 into the main tree, rather than revert the merge, try <em>really</em> hard to
 922 bisect the problem down into the branch you merged, and just fix it, or
 923 try to revert the individual commit that caused it.</p></div>
 924 <div class="paragraph"><p>Yes, it&#8217;s more complex, and no, it&#8217;s not always going to work (sometimes
 925 the answer is: "oops, I really shouldn&#8217;t have merged it, because it wasn&#8217;t
 926 ready yet, and I really need to undo <em>all</em> of the merge"). So then you
 927 really should revert the merge, but when you want to re-do the merge, you
 928 now need to do it by reverting the revert.</p></div>
 929 <div class="paragraph"><p>ADDENDUM</p></div>
 930 <div class="paragraph"><p>Sometimes you have to rewrite one of a topic branch&#8217;s commits <strong>and</strong> you can&#8217;t
 931 change the topic&#8217;s branching-off point.  Consider the following situation:</p></div>
 932 <div class="literalblock">
 933 <div class="content">
 934 <pre><code>P---o---o---M---x---x---W---x
 935  \         /
 936   A---B---C</code></pre>
 937 </div></div>
 938 <div class="paragraph"><p>where commit W reverted commit M because it turned out that commit B was wrong
 939 and needs to be rewritten, but you need the rewritten topic to still branch
 940 from commit P (perhaps P is a branching-off point for yet another branch, and
 941 you want be able to merge the topic into both branches).</p></div>
 942 <div class="paragraph"><p>The natural thing to do in this case is to checkout the A-B-C branch and use
 943 "rebase -i P" to change commit B.  However this does not rewrite commit A,
 944 because "rebase -i" by default fast-forwards over any initial commits selected
 945 with the "pick" command.  So you end up with this:</p></div>
 946 <div class="literalblock">
 947 <div class="content">
 948 <pre><code>P---o---o---M---x---x---W---x
 949  \         /
 950   A---B---C   &lt;-- old branch
 951    \
 952     B'---C'   &lt;-- naively rewritten branch</code></pre>
 953 </div></div>
 954 <div class="paragraph"><p>To merge A-B'-C' into the mainline branch you would still have to first revert
 955 commit W in order to pick up the changes in A, but then it&#8217;s likely that the
 956 changes in B' will conflict with the original B changes re-introduced by the
 957 reversion of W.</p></div>
 958 <div class="paragraph"><p>However, you can avoid these problems if you recreate the entire branch,
 959 including commit A:</p></div>
 960 <div class="literalblock">
 961 <div class="content">
 962 <pre><code>  A'---B'---C'  &lt;-- completely rewritten branch
 963  /
 964 P---o---o---M---x---x---W---x
 965  \         /
 966   A---B---C</code></pre>
 967 </div></div>
 968 <div class="paragraph"><p>You can merge A'-B'-C' into the mainline branch without worrying about first
 969 reverting W.  Mainline&#8217;s history would look like this:</p></div>
 970 <div class="literalblock">
 971 <div class="content">
 972 <pre><code>  A'---B'---C'------------------
 973  /                              \
 974 P---o---o---M---x---x---W---x---M2
 975  \         /
 976   A---B---C</code></pre>
 977 </div></div>
 978 <div class="paragraph"><p>But if you don&#8217;t actually need to change commit A, then you need some way to
 979 recreate it as a new commit with the same changes in it.  The rebase command&#8217;s
 980 --no-ff option provides a way to do this:</p></div>
 981 <div class="literalblock">
 982 <div class="content">
 983 <pre><code>$ git rebase [-i] --no-ff P</code></pre>
 984 </div></div>
 985 <div class="paragraph"><p>The --no-ff option creates a new branch A'-B'-C' with all-new commits (all the
 986 SHA IDs will be different) even if in the interactive case you only actually
 987 modify commit B.  You can then merge this new branch directly into the mainline
 988 branch and be sure you&#8217;ll get all of the branch&#8217;s changes.</p></div>
 989 <div class="paragraph"><p>You can also use --no-ff in cases where you just add extra commits to the topic
 990 to fix it up.  Let&#8217;s revisit the situation discussed at the start of this howto:</p></div>
 991 <div class="literalblock">
 992 <div class="content">
 993 <pre><code>P---o---o---M---x---x---W---x
 994  \         /
 995   A---B---C----------------D---E   &lt;-- fixed-up topic branch</code></pre>
 996 </div></div>
 997 <div class="paragraph"><p>At this point, you can use --no-ff to recreate the topic branch:</p></div>
 998 <div class="literalblock">
 999 <div class="content">
1000 <pre><code>$ git checkout E
1001 $ git rebase --no-ff P</code></pre>
1002 </div></div>
1003 <div class="paragraph"><p>yielding</p></div>
1004 <div class="literalblock">
1005 <div class="content">
1006 <pre><code>  A'---B'---C'------------D'---E'  &lt;-- recreated topic branch
1007  /
1008 P---o---o---M---x---x---W---x
1009  \         /
1010   A---B---C----------------D---E</code></pre>
1011 </div></div>
1012 <div class="paragraph"><p>You can merge the recreated branch into the mainline without reverting commit W,
1013 and mainline&#8217;s history will look like this:</p></div>
1014 <div class="literalblock">
1015 <div class="content">
1016 <pre><code>  A'---B'---C'------------D'---E'
1017  /                              \
1018 P---o---o---M---x---x---W---x---M2
1019  \         /
1020   A---B---C</code></pre>
1021 </div></div>
1022 </div>
1023 </div>
1024 </div>
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