Junio C Hamano
20 years ago
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From: Junio C Hamano <junkio@cox.net> |
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To: git@vger.kernel.org |
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Subject: [HOWTO] Reverting an existing commit |
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Abstract: In this article, JC gives a small real-life example of using |
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'git revert' command, and using a temporary branch and tag for safety |
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and easier sanity checking. |
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Date: Mon, 29 Aug 2005 21:39:02 -0700 |
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Content-type: text/asciidoc |
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Message-ID: <7voe7g3uop.fsf@assigned-by-dhcp.cox.net> |
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Reverting an existing commit |
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============================ |
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One of the changes I pulled into the 'master' branch turns out to |
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break building GIT with GCC 2.95. While they were well intentioned |
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portability fixes, keeping things working with gcc-2.95 was also |
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important. Here is what I did to revert the change in the 'master' |
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branch and to adjust the 'pu' branch, using core GIT tools and |
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barebone Porcelain. |
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First, prepare a throw-away branch in case I screw things up. |
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------------------------------------------------ |
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$ git checkout -b revert-c99 master |
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------------------------------------------------ |
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Now I am on the 'revert-c99' branch. Let's figure out which commit to |
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revert. I happen to know that the top of the 'master' branch is a |
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merge, and its second parent (i.e. foreign commit I merged from) has |
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the change I would want to undo. Further I happen to know that that |
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merge introduced 5 commits or so: |
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------------------------------------------------ |
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$ git show-branch --more=4 master master^2 | head |
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! [master] Merge refs/heads/portable from http://www.cs.berkeley.... |
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! [master^2] Replace C99 array initializers with code. |
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-- |
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+ [master] Merge refs/heads/portable from http://www.cs.berkeley.... |
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++ [master^2] Replace C99 array initializers with code. |
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++ [master^2~1] Replace unsetenv() and setenv() with older putenv(). |
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++ [master^2~2] Include sys/time.h in daemon.c. |
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++ [master^2~3] Fix ?: statements. |
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++ [master^2~4] Replace zero-length array decls with []. |
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+ [master~1] tutorial note about git branch |
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------------------------------------------------ |
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The '--more=4' above means "after we reach the merge base of refs, |
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show until we display four more common commits". That last commit |
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would have been where the "portable" branch was forked from the main |
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git.git repository, so this would show everything on both branches |
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since then. I just limited the output to the first handful using |
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'head'. |
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Now I know 'master^2~4' (pronounce it as "find the second parent of |
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the 'master', and then go four generations back following the first |
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parent") is the one I would want to revert. Since I also want to say |
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why I am reverting it, the '-n' flag is given to 'git revert'. This |
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prevents it from actually making a commit, and instead 'git revert' |
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leaves the commit log message it wanted to use in '.msg' file: |
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------------------------------------------------ |
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$ git revert -n master^2~4 |
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$ cat .msg |
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Revert "Replace zero-length array decls with []." |
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This reverts 6c5f9baa3bc0d63e141e0afc23110205379905a4 commit. |
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$ git diff HEAD ;# to make sure what we are reverting makes sense. |
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$ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage. |
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$ make clean test ;# make sure it did not cause other breakage. |
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------------------------------------------------ |
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The reverted change makes sense (from reading the 'diff' output), does |
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fix the problem (from 'make CC=gcc-2.95' test), and does not cause new |
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breakage (from the last 'make test'). I'm ready to commit: |
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------------------------------------------------ |
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$ git commit -a -s ;# read .msg into the log, |
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# and explain why I am reverting. |
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------------------------------------------------ |
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I could have screwed up in any of the above steps, but in the worst |
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case I could just have done 'git checkout master' to start over. |
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Fortunately I did not have to; what I have in the current branch |
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'revert-c99' is what I want. So merge that back into 'master': |
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------------------------------------------------ |
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$ git checkout master |
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$ git resolve master revert-c99 fast ;# this should be a fast forward |
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Updating from 10d781b9caa4f71495c7b34963bef137216f86a8 to e3a693c... |
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cache.h | 8 ++++---- |
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commit.c | 2 +- |
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ls-files.c | 2 +- |
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receive-pack.c | 2 +- |
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server-info.c | 2 +- |
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5 files changed, 8 insertions(+), 8 deletions(-) |
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------------------------------------------------ |
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The 'fast' in the above 'git resolve' is not a magic. I knew this |
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'resolve' would result in a fast forward merge, and if not, there is |
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something very wrong (so I would do 'git reset' on the 'master' branch |
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and examine the situation). When a fast forward merge is done, the |
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message parameter to 'git resolve' is discarded, because no new commit |
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is created. You could have said 'junk' or 'nothing' there as well. |
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There is no need to redo the test at this point. We fast forwarded |
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and we know 'master' matches 'revert-c99' exactly. In fact: |
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------------------------------------------------ |
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$ git diff master..revert-c99 |
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------------------------------------------------ |
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says nothing. |
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Then we rebase the 'pu' branch as usual. |
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------------------------------------------------ |
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$ git checkout pu |
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$ git tag pu-anchor pu |
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$ git rebase master |
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* Applying: Redo "revert" using three-way merge machinery. |
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First trying simple merge strategy to cherry-pick. |
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Finished one cherry-pick. |
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* Applying: Remove git-apply-patch-script. |
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First trying simple merge strategy to cherry-pick. |
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Simple cherry-pick fails; trying Automatic cherry-pick. |
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Removing Documentation/git-apply-patch-script.txt |
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Removing git-apply-patch-script |
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Finished one cherry-pick. |
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* Applying: Document "git cherry-pick" and "git revert" |
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First trying simple merge strategy to cherry-pick. |
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Finished one cherry-pick. |
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* Applying: mailinfo and applymbox updates |
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First trying simple merge strategy to cherry-pick. |
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Finished one cherry-pick. |
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* Applying: Show commits in topo order and name all commits. |
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First trying simple merge strategy to cherry-pick. |
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Finished one cherry-pick. |
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* Applying: More documentation updates. |
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First trying simple merge strategy to cherry-pick. |
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Finished one cherry-pick. |
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------------------------------------------------ |
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The temporary tag 'pu-anchor' is me just being careful, in case 'git |
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rebase' screws up. After this, I can do these for sanity check: |
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------------------------------------------------ |
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$ git diff pu-anchor..pu ;# make sure we got the master fix. |
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$ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage. |
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$ make clean test ;# make sure it did not cause other breakage. |
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------------------------------------------------ |
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Everything is in the good order. I do not need the temporary branch |
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nor tag anymore, so remove them: |
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------------------------------------------------ |
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$ rm -f .git/refs/tags/pu-anchor .git/refs/heads/revert-c99 |
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------------------------------------------------ |
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It was an emergency fix, so we might as well merge it into the |
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'release candidate' branch, although I expect the next release would |
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be some days off: |
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------------------------------------------------ |
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$ git checkout rc |
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$ git pull . master |
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Packing 0 objects |
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Unpacking 0 objects |
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* committish: e3a693c... refs/heads/master from . |
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Trying to merge e3a693c... into 8c1f5f0... using 10d781b... |
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Committed merge 7fb9b7262a1d1e0a47bbfdcbbcf50ce0635d3f8f |
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cache.h | 8 ++++---- |
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commit.c | 2 +- |
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ls-files.c | 2 +- |
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receive-pack.c | 2 +- |
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server-info.c | 2 +- |
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5 files changed, 8 insertions(+), 8 deletions(-) |
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------------------------------------------------ |
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And the final repository status looks like this: |
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------------------------------------------------ |
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$ git show-branch --more=1 master pu rc |
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! [master] Revert "Replace zero-length array decls with []." |
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! [pu] git-repack-script: Add option to repack all objects. |
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* [rc] Merge refs/heads/master from . |
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--- |
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+ [pu] git-repack-script: Add option to repack all objects. |
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+ [pu~1] More documentation updates. |
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+ [pu~2] Show commits in topo order and name all commits. |
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+ [pu~3] mailinfo and applymbox updates |
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+ [pu~4] Document "git cherry-pick" and "git revert" |
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+ [pu~5] Remove git-apply-patch-script. |
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+ [pu~6] Redo "revert" using three-way merge machinery. |
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+ [rc] Merge refs/heads/master from . |
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+++ [master] Revert "Replace zero-length array decls with []." |
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+ [rc~1] Merge refs/heads/master from . |
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+++ [master~1] Merge refs/heads/portable from http://www.cs.berkeley.... |
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------------------------------------------------ |
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