Reverting an existing commit

One of the changes I pulled into the master 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 master +branch and to adjust the pu branch, using core GIT tools and +barebone Porcelain.

First, prepare a throw-away branch in case I screw things up.

$ git checkout -b revert-c99 master

Now I am on the revert-c99 branch. Let's figure out which commit to +revert. I happen to know that the top of the master 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:

$ 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

The —more=4 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 +head.

Now I know master^2~4 (pronounce it as "find the second parent of +the master, 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 -n flag is given to git revert. This +prevents it from actually making a commit, and instead git revert +leaves the commit log message it wanted to use in .msg file:

$ 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.

The reverted change makes sense (from reading the diff output), does +fix the problem (from make CC=gcc-2.95 test), and does not cause new +breakage (from the last make test). I'm ready to commit:

$ git commit -a -s ;# read .msg into the log,
+                    # and explain why I am reverting.

I could have screwed up in any of the above steps, but in the worst +case I could just have done git checkout master to start over. +Fortunately I did not have to; what I have in the current branch +revert-c99 is what I want. So merge that back into master:

$ 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(-)

The fast in the above git resolve is not a magic. I knew this +resolve would result in a fast forward merge, and if not, there is +something very wrong (so I would do git reset on the master branch +and examine the situation). When a fast forward merge is done, the +message parameter to git resolve is discarded, because no new commit +is created. You could have said junk or nothing there as well.

There is no need to redo the test at this point. We fast forwarded +and we know master matches revert-c99 exactly. In fact:

$ git diff master..revert-c99

says nothing.

Then we rebase the pu branch as usual.

$ 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.

The temporary tag pu-anchor is me just being careful, in case git +rebase screws up. After this, I can do these for sanity check:

$ 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.

Everything is in the good order. I do not need the temporary branch +nor tag anymore, so remove them:

$ rm -f .git/refs/tags/pu-anchor
+$ git branch -d revert-c99

It was an emergency fix, so we might as well merge it into the +release candidate branch, although I expect the next release would +be some days off:

$ 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(-)

And the final repository status looks like this:

$ 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....