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| <title>Reverting an existing commit</title> | |
| </head> | |
| <body> | |
| <div id="header"> | |
| <h1>Reverting an existing commit</h1> | |
| </div> | |
| <div id="preamble"> | |
| <div class="sectionbody"> | |
| <p>One of the changes I pulled into the <em>master</em> branch turns out to | |
| break building GIT with GCC 2.95. While they were well intentioned | |
| portability fixes, keeping things working with gcc-2.95 was also | |
| important. Here is what I did to revert the change in the <em>master</em> | |
| branch and to adjust the <em>pu</em> branch, using core GIT tools and | |
| barebone Porcelain.</p> | |
| <p>First, prepare a throw-away branch in case I screw things up.</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git checkout -b revert-c99 master</tt></pre> | |
| </div></div> | |
| <p>Now I am on the <em>revert-c99</em> branch. Let's figure out which commit to | |
| revert. I happen to know that the top of the <em>master</em> branch is a | |
| merge, and its second parent (i.e. foreign commit I merged from) has | |
| the change I would want to undo. Further I happen to know that that | |
| merge introduced 5 commits or so:</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git show-branch --more=4 master master^2 | head | |
| * [master] Merge refs/heads/portable from http://www.cs.berkeley.... | |
| ! [master^2] Replace C99 array initializers with code. | |
| -- | |
| - [master] Merge refs/heads/portable from http://www.cs.berkeley.... | |
| *+ [master^2] Replace C99 array initializers with code. | |
| *+ [master^2~1] Replace unsetenv() and setenv() with older putenv(). | |
| *+ [master^2~2] Include sys/time.h in daemon.c. | |
| *+ [master^2~3] Fix ?: statements. | |
| *+ [master^2~4] Replace zero-length array decls with []. | |
| * [master~1] tutorial note about git branch</tt></pre> | |
| </div></div> | |
| <p>The <em>—more=4</em> above means "after we reach the merge base of refs, | |
| show until we display four more common commits". That last commit | |
| would have been where the "portable" branch was forked from the main | |
| git.git repository, so this would show everything on both branches | |
| since then. I just limited the output to the first handful using | |
| <em>head</em>.</p> | |
| <p>Now I know <em>master^2~4</em> (pronounce it as "find the second parent of | |
| the <em>master</em>, and then go four generations back following the first | |
| parent") is the one I would want to revert. Since I also want to say | |
| why I am reverting it, the <em>-n</em> flag is given to <em>git revert</em>. This | |
| prevents it from actually making a commit, and instead <em>git revert</em> | |
| leaves the commit log message it wanted to use in <em>.msg</em> file:</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git revert -n master^2~4 | |
| $ cat .msg | |
| Revert "Replace zero-length array decls with []." | |
| This reverts 6c5f9baa3bc0d63e141e0afc23110205379905a4 commit. | |
| $ git diff HEAD ;# to make sure what we are reverting makes sense. | |
| $ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage. | |
| $ make clean test ;# make sure it did not cause other breakage.</tt></pre> | |
| </div></div> | |
| <p>The reverted change makes sense (from reading the <em>diff</em> output), does | |
| fix the problem (from <em>make CC=gcc-2.95</em> test), and does not cause new | |
| breakage (from the last <em>make test</em>). I'm ready to commit:</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git commit -a -s ;# read .msg into the log, | |
| # and explain why I am reverting.</tt></pre> | |
| </div></div> | |
| <p>I could have screwed up in any of the above steps, but in the worst | |
| case I could just have done <em>git checkout master</em> to start over. | |
| Fortunately I did not have to; what I have in the current branch | |
| <em>revert-c99</em> is what I want. So merge that back into <em>master</em>:</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git checkout master | |
| $ git resolve master revert-c99 fast ;# this should be a fast forward | |
| Updating from 10d781b9caa4f71495c7b34963bef137216f86a8 to e3a693c... | |
| cache.h | 8 ++++---- | |
| commit.c | 2 +- | |
| ls-files.c | 2 +- | |
| receive-pack.c | 2 +- | |
| server-info.c | 2 +- | |
| 5 files changed, 8 insertions(+), 8 deletions(-)</tt></pre> | |
| </div></div> | |
| <p>The <em>fast</em> in the above <em>git resolve</em> is not a magic. I knew this | |
| <em>resolve</em> would result in a fast forward merge, and if not, there is | |
| something very wrong (so I would do <em>git reset</em> on the <em>master</em> branch | |
| and examine the situation). When a fast forward merge is done, the | |
| message parameter to <em>git resolve</em> is discarded, because no new commit | |
| is created. You could have said <em>junk</em> or <em>nothing</em> there as well.</p> | |
| <p>There is no need to redo the test at this point. We fast forwarded | |
| and we know <em>master</em> matches <em>revert-c99</em> exactly. In fact:</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git diff master..revert-c99</tt></pre> | |
| </div></div> | |
| <p>says nothing.</p> | |
| <p>Then we rebase the <em>pu</em> branch as usual.</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git checkout pu | |
| $ git tag pu-anchor pu | |
| $ git rebase master | |
| * Applying: Redo "revert" using three-way merge machinery. | |
| First trying simple merge strategy to cherry-pick. | |
| Finished one cherry-pick. | |
| * Applying: Remove git-apply-patch-script. | |
| First trying simple merge strategy to cherry-pick. | |
| Simple cherry-pick fails; trying Automatic cherry-pick. | |
| Removing Documentation/git-apply-patch-script.txt | |
| Removing git-apply-patch-script | |
| Finished one cherry-pick. | |
| * Applying: Document "git cherry-pick" and "git revert" | |
| First trying simple merge strategy to cherry-pick. | |
| Finished one cherry-pick. | |
| * Applying: mailinfo and applymbox updates | |
| First trying simple merge strategy to cherry-pick. | |
| Finished one cherry-pick. | |
| * Applying: Show commits in topo order and name all commits. | |
| First trying simple merge strategy to cherry-pick. | |
| Finished one cherry-pick. | |
| * Applying: More documentation updates. | |
| First trying simple merge strategy to cherry-pick. | |
| Finished one cherry-pick.</tt></pre> | |
| </div></div> | |
| <p>The temporary tag <em>pu-anchor</em> is me just being careful, in case <em>git | |
| rebase</em> screws up. After this, I can do these for sanity check:</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git diff pu-anchor..pu ;# make sure we got the master fix. | |
| $ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage. | |
| $ make clean test ;# make sure it did not cause other breakage.</tt></pre> | |
| </div></div> | |
| <p>Everything is in the good order. I do not need the temporary branch | |
| nor tag anymore, so remove them:</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ rm -f .git/refs/tags/pu-anchor | |
| $ git branch -d revert-c99</tt></pre> | |
| </div></div> | |
| <p>It was an emergency fix, so we might as well merge it into the | |
| <em>release candidate</em> branch, although I expect the next release would | |
| be some days off:</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git checkout rc | |
| $ git pull . master | |
| Packing 0 objects | |
| Unpacking 0 objects | |
| * committish: e3a693c... refs/heads/master from . | |
| Trying to merge e3a693c... into 8c1f5f0... using 10d781b... | |
| Committed merge 7fb9b7262a1d1e0a47bbfdcbbcf50ce0635d3f8f | |
| cache.h | 8 ++++---- | |
| commit.c | 2 +- | |
| ls-files.c | 2 +- | |
| receive-pack.c | 2 +- | |
| server-info.c | 2 +- | |
| 5 files changed, 8 insertions(+), 8 deletions(-)</tt></pre> | |
| </div></div> | |
| <p>And the final repository status looks like this:</p> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><tt>$ git show-branch --more=1 master pu rc | |
| ! [master] Revert "Replace zero-length array decls with []." | |
| ! [pu] git-repack: Add option to repack all objects. | |
| * [rc] Merge refs/heads/master from . | |
| --- | |
| + [pu] git-repack: Add option to repack all objects. | |
| + [pu~1] More documentation updates. | |
| + [pu~2] Show commits in topo order and name all commits. | |
| + [pu~3] mailinfo and applymbox updates | |
| + [pu~4] Document "git cherry-pick" and "git revert" | |
| + [pu~5] Remove git-apply-patch-script. | |
| + [pu~6] Redo "revert" using three-way merge machinery. | |
| - [rc] Merge refs/heads/master from . | |
| ++* [master] Revert "Replace zero-length array decls with []." | |
| - [rc~1] Merge refs/heads/master from . | |
| ... [master~1] Merge refs/heads/portable from http://www.cs.berkeley....</tt></pre> | |
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| Last updated 03-Oct-2006 08:41:50 UTC | |
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