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554 <h1>Reverting an existing commit
</h1>
558 <div class=
"sectionbody">
559 <div class=
"paragraph"><p>One of the changes I pulled into the
<em>master
</em> branch turns out to
560 break building GIT with GCC
2.95. While they were well intentioned
561 portability fixes, keeping things working with gcc-
2.95 was also
562 important. Here is what I did to revert the change in the
<em>master
</em>
563 branch and to adjust the
<em>pu
</em> branch, using core GIT tools and
564 barebone Porcelain.
</p></div>
565 <div class=
"paragraph"><p>First, prepare a throw-away branch in case I screw things up.
</p></div>
566 <div class=
"listingblock">
567 <div class=
"content">
568 <pre><tt>$ git checkout -b revert-c99 master
</tt></pre>
570 <div class=
"paragraph"><p>Now I am on the
<em>revert-c99
</em> branch. Let
’s figure out which commit to
571 revert. I happen to know that the top of the
<em>master
</em> branch is a
572 merge, and its second parent (i.e. foreign commit I merged from) has
573 the change I would want to undo. Further I happen to know that that
574 merge introduced
5 commits or so:
</p></div>
575 <div class=
"listingblock">
576 <div class=
"content">
577 <pre><tt>$ git show-branch --more=
4 master master^
2 | head
578 * [master] Merge refs/heads/portable from http://www.cs.berkeley....
579 ! [master^
2] Replace C99 array initializers with code.
581 - [master] Merge refs/heads/portable from http://www.cs.berkeley....
582 *+ [master^
2] Replace C99 array initializers with code.
583 *+ [master^
2~
1] Replace unsetenv() and setenv() with older putenv().
584 *+ [master^
2~
2] Include sys/time.h in daemon.c.
585 *+ [master^
2~
3] Fix ?: statements.
586 *+ [master^
2~
4] Replace zero-length array decls with [].
587 * [master~
1] tutorial note about git branch
</tt></pre>
589 <div class=
"paragraph"><p>The
<em>--more=
4</em> above means
"after we reach the merge base of refs,
590 show until we display four more common commits". That last commit
591 would have been where the
"portable" branch was forked from the main
592 git.git repository, so this would show everything on both branches
593 since then. I just limited the output to the first handful using
594 <em>head
</em>.
</p></div>
595 <div class=
"paragraph"><p>Now I know
<em>master^
2~
4</em> (pronounce it as
"find the second parent of
596 the <em>master</em>, and then go four generations back following the first
597 parent") is the one I would want to revert. Since I also want to say
598 why I am reverting it, the
<em>-n
</em> flag is given to
<em>git revert
</em>. This
599 prevents it from actually making a commit, and instead
<em>git revert
</em>
600 leaves the commit log message it wanted to use in
<em>.msg
</em> file:
</p></div>
601 <div class=
"listingblock">
602 <div class=
"content">
603 <pre><tt>$ git revert -n master^
2~
4
605 Revert
"Replace zero-length array decls with []."
607 This reverts
6c5f9baa3bc0d63e141e0afc23110205379905a4 commit.
608 $ git diff HEAD ;# to make sure what we are reverting makes sense.
609 $ make CC=gcc-
2.95 clean test ;# make sure it fixed the breakage.
610 $ make clean test ;# make sure it did not cause other breakage.
</tt></pre>
612 <div class=
"paragraph"><p>The reverted change makes sense (from reading the
<em>diff
</em> output), does
613 fix the problem (from
<em>make CC=gcc-
2.95</em> test), and does not cause new
614 breakage (from the last
<em>make test
</em>). I
’m ready to commit:
</p></div>
615 <div class=
"listingblock">
616 <div class=
"content">
617 <pre><tt>$ git commit -a -s ;# read .msg into the log,
618 # and explain why I am reverting.
</tt></pre>
620 <div class=
"paragraph"><p>I could have screwed up in any of the above steps, but in the worst
621 case I could just have done
<em>git checkout master
</em> to start over.
622 Fortunately I did not have to; what I have in the current branch
623 <em>revert-c99
</em> is what I want. So merge that back into
<em>master
</em>:
</p></div>
624 <div class=
"listingblock">
625 <div class=
"content">
626 <pre><tt>$ git checkout master
627 $ git merge revert-c99 ;# this should be a fast-forward
628 Updating from
10d781b9caa4f71495c7b34963bef137216f86a8 to e3a693c...
632 receive-pack.c |
2 +-
634 5 files changed,
8 insertions(+),
8 deletions(-)
</tt></pre>
636 <div class=
"paragraph"><p>There is no need to redo the test at this point. We fast-forwarded
637 and we know
<em>master
</em> matches
<em>revert-c99
</em> exactly. In fact:
</p></div>
638 <div class=
"listingblock">
639 <div class=
"content">
640 <pre><tt>$ git diff master..revert-c99
</tt></pre>
642 <div class=
"paragraph"><p>says nothing.
</p></div>
643 <div class=
"paragraph"><p>Then we rebase the
<em>pu
</em> branch as usual.
</p></div>
644 <div class=
"listingblock">
645 <div class=
"content">
646 <pre><tt>$ git checkout pu
647 $ git tag pu-anchor pu
649 * Applying: Redo
"revert" using three-way merge machinery.
650 First trying simple merge strategy to cherry-pick.
651 * Applying: Remove git-apply-patch-script.
652 First trying simple merge strategy to cherry-pick.
653 Simple cherry-pick fails; trying Automatic cherry-pick.
654 Removing Documentation/git-apply-patch-script.txt
655 Removing git-apply-patch-script
656 * Applying: Document
"git cherry-pick" and
"git revert"
657 First trying simple merge strategy to cherry-pick.
658 * Applying: mailinfo and applymbox updates
659 First trying simple merge strategy to cherry-pick.
660 * Applying: Show commits in topo order and name all commits.
661 First trying simple merge strategy to cherry-pick.
662 * Applying: More documentation updates.
663 First trying simple merge strategy to cherry-pick.
</tt></pre>
665 <div class=
"paragraph"><p>The temporary tag
<em>pu-anchor
</em> is me just being careful, in case
<em>git
666 rebase
</em> screws up. After this, I can do these for sanity check:
</p></div>
667 <div class=
"listingblock">
668 <div class=
"content">
669 <pre><tt>$ git diff pu-anchor..pu ;# make sure we got the master fix.
670 $ make CC=gcc-
2.95 clean test ;# make sure it fixed the breakage.
671 $ make clean test ;# make sure it did not cause other breakage.
</tt></pre>
673 <div class=
"paragraph"><p>Everything is in the good order. I do not need the temporary branch
674 nor tag anymore, so remove them:
</p></div>
675 <div class=
"listingblock">
676 <div class=
"content">
677 <pre><tt>$ rm -f .git/refs/tags/pu-anchor
678 $ git branch -d revert-c99
</tt></pre>
680 <div class=
"paragraph"><p>It was an emergency fix, so we might as well merge it into the
681 <em>release candidate
</em> branch, although I expect the next release would
682 be some days off:
</p></div>
683 <div class=
"listingblock">
684 <div class=
"content">
685 <pre><tt>$ git checkout rc
690 * committish: e3a693c... refs/heads/master from .
691 Trying to merge e3a693c... into
8c1f5f0... using
10d781b...
692 Committed merge
7fb9b7262a1d1e0a47bbfdcbbcf50ce0635d3f8f
696 receive-pack.c |
2 +-
698 5 files changed,
8 insertions(+),
8 deletions(-)
</tt></pre>
700 <div class=
"paragraph"><p>And the final repository status looks like this:
</p></div>
701 <div class=
"listingblock">
702 <div class=
"content">
703 <pre><tt>$ git show-branch --more=
1 master pu rc
704 ! [master] Revert
"Replace zero-length array decls with []."
705 ! [pu] git-repack: Add option to repack all objects.
706 * [rc] Merge refs/heads/master from .
708 + [pu] git-repack: Add option to repack all objects.
709 + [pu~
1] More documentation updates.
710 + [pu~
2] Show commits in topo order and name all commits.
711 + [pu~
3] mailinfo and applymbox updates
712 + [pu~
4] Document
"git cherry-pick" and
"git revert"
713 + [pu~
5] Remove git-apply-patch-script.
714 + [pu~
6] Redo
"revert" using three-way merge machinery.
715 - [rc] Merge refs/heads/master from .
716 ++* [master] Revert
"Replace zero-length array decls with []."
717 - [rc~
1] Merge refs/heads/master from .
718 ... [master~
1] Merge refs/heads/portable from http://www.cs.berkeley....
</tt></pre>
723 <div id=
"footnotes"><hr /></div>
725 <div id=
"footer-text">
726 Last updated
2011-
09-
21 23:
17:
57 PDT