howto/revert-branch-rebase.html

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 736 <div id="header">
 737 <h1>How to revert an existing commit</h1>
 738 <span id="revdate">2023-09-22</span>
 739 </div>
 740 <div id="content">
 741 <div id="preamble">
 742 <div class="sectionbody">
 743 <div class="paragraph"><p>One of the changes I pulled into the <em>master</em> branch turns out to
 744 break building Git with GCC 2.95.  While they were well-intentioned
 745 portability fixes, keeping things working with gcc-2.95 was also
 746 important.  Here is what I did to revert the change in the <em>master</em>
 747 branch and to adjust the <em>seen</em> branch, using core Git tools and
 748 barebone Porcelain.</p></div>
 749 <div class="paragraph"><p>First, prepare a throw-away branch in case I screw things up.</p></div>
 750 <div class="listingblock">
 751 <div class="content">
 752 <pre><code>$ git checkout -b revert-c99 master</code></pre>
 753 </div></div>
 754 <div class="paragraph"><p>Now I am on the <em>revert-c99</em> branch.  Let&#8217;s figure out which commit to
 755 revert.  I happen to know that the top of the <em>master</em> branch is a
 756 merge, and its second parent (i.e. foreign commit I merged from) has
 757 the change I would want to undo.  Further I happen to know that that
 758 merge introduced 5 commits or so:</p></div>
 759 <div class="listingblock">
 760 <div class="content">
 761 <pre><code>$ git show-branch --more=4 master master^2 | head
 762 * [master] Merge refs/heads/portable from http://www.cs.berkeley....
 763  ! [master^2] Replace C99 array initializers with code.
 764 --
 765 -  [master] Merge refs/heads/portable from http://www.cs.berkeley....
 766 *+ [master^2] Replace C99 array initializers with code.
 767 *+ [master^2~1] Replace unsetenv() and setenv() with older putenv().
 768 *+ [master^2~2] Include sys/time.h in daemon.c.
 769 *+ [master^2~3] Fix ?: statements.
 770 *+ [master^2~4] Replace zero-length array decls with [].
 771 *  [master~1] tutorial note about git branch</code></pre>
 772 </div></div>
 773 <div class="paragraph"><p>The <em>--more=4</em> above means "after we reach the merge base of refs,
 774 show until we display four more common commits".  That last commit
 775 would have been where the "portable" branch was forked from the main
 776 git.git repository, so this would show everything on both branches
 777 since then.  I just limited the output to the first handful using
 778 <em>head</em>.</p></div>
 779 <div class="paragraph"><p>Now I know <em>master^2~4</em> (pronounce it as "find the second parent of
 780 the <em>master</em>, and then go four generations back following the first
 781 parent") is the one I would want to revert.  Since I also want to say
 782 why I am reverting it, the <em>-n</em> flag is given to <em>git revert</em>.  This
 783 prevents it from actually making a commit, and instead <em>git revert</em>
 784 leaves the commit log message it wanted to use in <em>.msg</em> file:</p></div>
 785 <div class="listingblock">
 786 <div class="content">
 787 <pre><code>$ git revert -n master^2~4
 788 $ cat .msg
 789 Revert "Replace zero-length array decls with []."
 790
 791 This reverts 6c5f9baa3bc0d63e141e0afc23110205379905a4 commit.
 792 $ git diff HEAD ;# to make sure what we are reverting makes sense.
 793 $ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage.
 794 $ make clean test ;# make sure it did not cause other breakage.</code></pre>
 795 </div></div>
 796 <div class="paragraph"><p>The reverted change makes sense (from reading the <em>diff</em> output), does
 797 fix the problem (from <em>make CC=gcc-2.95</em> test), and does not cause new
 798 breakage (from the last <em>make test</em>).  I&#8217;m ready to commit:</p></div>
 799 <div class="listingblock">
 800 <div class="content">
 801 <pre><code>$ git commit -a -s ;# read .msg into the log,
 802                     # and explain why I am reverting.</code></pre>
 803 </div></div>
 804 <div class="paragraph"><p>I could have screwed up in any of the above steps, but in the worst
 805 case I could just have done <em>git checkout master</em> to start over.
 806 Fortunately I did not have to; what I have in the current branch
 807 <em>revert-c99</em> is what I want.  So merge that back into <em>master</em>:</p></div>
 808 <div class="listingblock">
 809 <div class="content">
 810 <pre><code>$ git checkout master
 811 $ git merge revert-c99 ;# this should be a fast-forward
 812 Updating from 10d781b9caa4f71495c7b34963bef137216f86a8 to e3a693c...
 813  cache.h        |    8 ++++----
 814  commit.c       |    2 +-
 815  ls-files.c     |    2 +-
 816  receive-pack.c |    2 +-
 817  server-info.c  |    2 +-
 818  5 files changed, 8 insertions(+), 8 deletions(-)</code></pre>
 819 </div></div>
 820 <div class="paragraph"><p>There is no need to redo the test at this point.  We fast-forwarded
 821 and we know <em>master</em> matches <em>revert-c99</em> exactly.  In fact:</p></div>
 822 <div class="listingblock">
 823 <div class="content">
 824 <pre><code>$ git diff master..revert-c99</code></pre>
 825 </div></div>
 826 <div class="paragraph"><p>says nothing.</p></div>
 827 <div class="paragraph"><p>Then we rebase the <em>seen</em> branch as usual.</p></div>
 828 <div class="listingblock">
 829 <div class="content">
 830 <pre><code>$ git checkout seen
 831 $ git tag seen-anchor seen
 832 $ git rebase master
 833 * Applying: Redo "revert" using three-way merge machinery.
 834 First trying simple merge strategy to cherry-pick.
 835 * Applying: Remove git-apply-patch-script.
 836 First trying simple merge strategy to cherry-pick.
 837 Simple cherry-pick fails; trying Automatic cherry-pick.
 838 Removing Documentation/git-apply-patch-script.txt
 839 Removing git-apply-patch-script
 840 * Applying: Document "git cherry-pick" and "git revert"
 841 First trying simple merge strategy to cherry-pick.
 842 * Applying: mailinfo and applymbox updates
 843 First trying simple merge strategy to cherry-pick.
 844 * Applying: Show commits in topo order and name all commits.
 845 First trying simple merge strategy to cherry-pick.
 846 * Applying: More documentation updates.
 847 First trying simple merge strategy to cherry-pick.</code></pre>
 848 </div></div>
 849 <div class="paragraph"><p>The temporary tag <em>seen-anchor</em> is me just being careful, in case <em>git
 850 rebase</em> screws up.  After this, I can do these for sanity check:</p></div>
 851 <div class="listingblock">
 852 <div class="content">
 853 <pre><code>$ git diff seen-anchor..seen ;# make sure we got the master fix.
 854 $ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage.
 855 $ make clean test ;# make sure it did not cause other breakage.</code></pre>
 856 </div></div>
 857 <div class="paragraph"><p>Everything is in the good order.  I do not need the temporary branch
 858 or tag anymore, so remove them:</p></div>
 859 <div class="listingblock">
 860 <div class="content">
 861 <pre><code>$ rm -f .git/refs/tags/seen-anchor
 862 $ git branch -d revert-c99</code></pre>
 863 </div></div>
 864 <div class="paragraph"><p>It was an emergency fix, so we might as well merge it into the
 865 <em>release candidate</em> branch, although I expect the next release would
 866 be some days off:</p></div>
 867 <div class="listingblock">
 868 <div class="content">
 869 <pre><code>$ git checkout rc
 870 $ git pull . master
 871 Packing 0 objects
 872 Unpacking 0 objects
 873
 874 * commit-ish: e3a693c...        refs/heads/master from .
 875 Trying to merge e3a693c... into 8c1f5f0... using 10d781b...
 876 Committed merge 7fb9b7262a1d1e0a47bbfdcbbcf50ce0635d3f8f
 877  cache.h        |    8 ++++----
 878  commit.c       |    2 +-
 879  ls-files.c     |    2 +-
 880  receive-pack.c |    2 +-
 881  server-info.c  |    2 +-
 882  5 files changed, 8 insertions(+), 8 deletions(-)</code></pre>
 883 </div></div>
 884 <div class="paragraph"><p>And the final repository status looks like this:</p></div>
 885 <div class="listingblock">
 886 <div class="content">
 887 <pre><code>$ git show-branch --more=1 master seen rc
 888 ! [master] Revert "Replace zero-length array decls with []."
 889  ! [seen] git-repack: Add option to repack all objects.
 890   * [rc] Merge refs/heads/master from .
 891 ---
 892  +  [seen] git-repack: Add option to repack all objects.
 893  +  [seen~1] More documentation updates.
 894  +  [seen~2] Show commits in topo order and name all commits.
 895  +  [seen~3] mailinfo and applymbox updates
 896  +  [seen~4] Document "git cherry-pick" and "git revert"
 897  +  [seen~5] Remove git-apply-patch-script.
 898  +  [seen~6] Redo "revert" using three-way merge machinery.
 899   - [rc] Merge refs/heads/master from .
 900 ++* [master] Revert "Replace zero-length array decls with []."
 901   - [rc~1] Merge refs/heads/master from .
 902 ... [master~1] Merge refs/heads/portable from http://www.cs.berkeley....</code></pre>
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