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3459c2c4 JH |
1 | From: Junio C Hamano <junkio@cox.net> |
2 | To: git@vger.kernel.org | |
3 | Subject: [HOWTO] Reverting an existing commit | |
4 | Abstract: In this article, JC gives a small real-life example of using | |
5 | 'git revert' command, and using a temporary branch and tag for safety | |
6 | and easier sanity checking. | |
7 | Date: Mon, 29 Aug 2005 21:39:02 -0700 | |
8 | Content-type: text/asciidoc | |
9 | Message-ID: <7voe7g3uop.fsf@assigned-by-dhcp.cox.net> | |
10 | ||
11 | Reverting an existing commit | |
12 | ============================ | |
13 | ||
14 | One of the changes I pulled into the 'master' branch turns out to | |
15 | break building GIT with GCC 2.95. While they were well intentioned | |
16 | portability fixes, keeping things working with gcc-2.95 was also | |
17 | important. Here is what I did to revert the change in the 'master' | |
18 | branch and to adjust the 'pu' branch, using core GIT tools and | |
19 | barebone Porcelain. | |
20 | ||
21 | First, prepare a throw-away branch in case I screw things up. | |
22 | ||
23 | ------------------------------------------------ | |
24 | $ git checkout -b revert-c99 master | |
25 | ------------------------------------------------ | |
26 | ||
27 | Now I am on the 'revert-c99' branch. Let's figure out which commit to | |
28 | revert. I happen to know that the top of the 'master' branch is a | |
29 | merge, and its second parent (i.e. foreign commit I merged from) has | |
30 | the change I would want to undo. Further I happen to know that that | |
31 | merge introduced 5 commits or so: | |
32 | ||
33 | ------------------------------------------------ | |
34 | $ git show-branch --more=4 master master^2 | head | |
ebedc319 | 35 | * [master] Merge refs/heads/portable from http://www.cs.berkeley.... |
3459c2c4 JH |
36 | ! [master^2] Replace C99 array initializers with code. |
37 | -- | |
ebedc319 JH |
38 | - [master] Merge refs/heads/portable from http://www.cs.berkeley.... |
39 | *+ [master^2] Replace C99 array initializers with code. | |
40 | *+ [master^2~1] Replace unsetenv() and setenv() with older putenv(). | |
41 | *+ [master^2~2] Include sys/time.h in daemon.c. | |
42 | *+ [master^2~3] Fix ?: statements. | |
43 | *+ [master^2~4] Replace zero-length array decls with []. | |
44 | * [master~1] tutorial note about git branch | |
3459c2c4 JH |
45 | ------------------------------------------------ |
46 | ||
47 | The '--more=4' above means "after we reach the merge base of refs, | |
48 | show until we display four more common commits". That last commit | |
49 | would have been where the "portable" branch was forked from the main | |
50 | git.git repository, so this would show everything on both branches | |
51 | since then. I just limited the output to the first handful using | |
52 | 'head'. | |
53 | ||
54 | Now I know 'master^2~4' (pronounce it as "find the second parent of | |
55 | the 'master', and then go four generations back following the first | |
56 | parent") is the one I would want to revert. Since I also want to say | |
57 | why I am reverting it, the '-n' flag is given to 'git revert'. This | |
58 | prevents it from actually making a commit, and instead 'git revert' | |
59 | leaves the commit log message it wanted to use in '.msg' file: | |
60 | ||
61 | ------------------------------------------------ | |
62 | $ git revert -n master^2~4 | |
63 | $ cat .msg | |
64 | Revert "Replace zero-length array decls with []." | |
65 | ||
66 | This reverts 6c5f9baa3bc0d63e141e0afc23110205379905a4 commit. | |
67 | $ git diff HEAD ;# to make sure what we are reverting makes sense. | |
68 | $ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage. | |
69 | $ make clean test ;# make sure it did not cause other breakage. | |
70 | ------------------------------------------------ | |
71 | ||
72 | The reverted change makes sense (from reading the 'diff' output), does | |
73 | fix the problem (from 'make CC=gcc-2.95' test), and does not cause new | |
74 | breakage (from the last 'make test'). I'm ready to commit: | |
75 | ||
76 | ------------------------------------------------ | |
77 | $ git commit -a -s ;# read .msg into the log, | |
78 | # and explain why I am reverting. | |
79 | ------------------------------------------------ | |
80 | ||
81 | I could have screwed up in any of the above steps, but in the worst | |
82 | case I could just have done 'git checkout master' to start over. | |
83 | Fortunately I did not have to; what I have in the current branch | |
84 | 'revert-c99' is what I want. So merge that back into 'master': | |
85 | ||
86 | ------------------------------------------------ | |
87 | $ git checkout master | |
207dfa07 | 88 | $ git merge revert-c99 ;# this should be a fast forward |
3459c2c4 JH |
89 | Updating from 10d781b9caa4f71495c7b34963bef137216f86a8 to e3a693c... |
90 | cache.h | 8 ++++---- | |
91 | commit.c | 2 +- | |
92 | ls-files.c | 2 +- | |
93 | receive-pack.c | 2 +- | |
94 | server-info.c | 2 +- | |
95 | 5 files changed, 8 insertions(+), 8 deletions(-) | |
96 | ------------------------------------------------ | |
97 | ||
3459c2c4 JH |
98 | There is no need to redo the test at this point. We fast forwarded |
99 | and we know 'master' matches 'revert-c99' exactly. In fact: | |
100 | ||
101 | ------------------------------------------------ | |
102 | $ git diff master..revert-c99 | |
103 | ------------------------------------------------ | |
104 | ||
105 | says nothing. | |
106 | ||
107 | Then we rebase the 'pu' branch as usual. | |
108 | ||
109 | ------------------------------------------------ | |
110 | $ git checkout pu | |
111 | $ git tag pu-anchor pu | |
112 | $ git rebase master | |
113 | * Applying: Redo "revert" using three-way merge machinery. | |
114 | First trying simple merge strategy to cherry-pick. | |
115 | Finished one cherry-pick. | |
116 | * Applying: Remove git-apply-patch-script. | |
117 | First trying simple merge strategy to cherry-pick. | |
118 | Simple cherry-pick fails; trying Automatic cherry-pick. | |
119 | Removing Documentation/git-apply-patch-script.txt | |
120 | Removing git-apply-patch-script | |
121 | Finished one cherry-pick. | |
122 | * Applying: Document "git cherry-pick" and "git revert" | |
123 | First trying simple merge strategy to cherry-pick. | |
124 | Finished one cherry-pick. | |
125 | * Applying: mailinfo and applymbox updates | |
126 | First trying simple merge strategy to cherry-pick. | |
127 | Finished one cherry-pick. | |
128 | * Applying: Show commits in topo order and name all commits. | |
129 | First trying simple merge strategy to cherry-pick. | |
130 | Finished one cherry-pick. | |
131 | * Applying: More documentation updates. | |
132 | First trying simple merge strategy to cherry-pick. | |
133 | Finished one cherry-pick. | |
134 | ------------------------------------------------ | |
135 | ||
136 | The temporary tag 'pu-anchor' is me just being careful, in case 'git | |
137 | rebase' screws up. After this, I can do these for sanity check: | |
138 | ||
139 | ------------------------------------------------ | |
140 | $ git diff pu-anchor..pu ;# make sure we got the master fix. | |
141 | $ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage. | |
142 | $ make clean test ;# make sure it did not cause other breakage. | |
143 | ------------------------------------------------ | |
144 | ||
145 | Everything is in the good order. I do not need the temporary branch | |
146 | nor tag anymore, so remove them: | |
147 | ||
148 | ------------------------------------------------ | |
a6080a0a | 149 | $ rm -f .git/refs/tags/pu-anchor |
e66ab03f | 150 | $ git branch -d revert-c99 |
3459c2c4 JH |
151 | ------------------------------------------------ |
152 | ||
153 | It was an emergency fix, so we might as well merge it into the | |
154 | 'release candidate' branch, although I expect the next release would | |
155 | be some days off: | |
156 | ||
157 | ------------------------------------------------ | |
158 | $ git checkout rc | |
159 | $ git pull . master | |
160 | Packing 0 objects | |
161 | Unpacking 0 objects | |
162 | ||
163 | * committish: e3a693c... refs/heads/master from . | |
164 | Trying to merge e3a693c... into 8c1f5f0... using 10d781b... | |
165 | Committed merge 7fb9b7262a1d1e0a47bbfdcbbcf50ce0635d3f8f | |
166 | cache.h | 8 ++++---- | |
167 | commit.c | 2 +- | |
168 | ls-files.c | 2 +- | |
169 | receive-pack.c | 2 +- | |
170 | server-info.c | 2 +- | |
171 | 5 files changed, 8 insertions(+), 8 deletions(-) | |
172 | ------------------------------------------------ | |
173 | ||
174 | And the final repository status looks like this: | |
175 | ||
176 | ------------------------------------------------ | |
177 | $ git show-branch --more=1 master pu rc | |
178 | ! [master] Revert "Replace zero-length array decls with []." | |
215a7ad1 | 179 | ! [pu] git-repack: Add option to repack all objects. |
3459c2c4 JH |
180 | * [rc] Merge refs/heads/master from . |
181 | --- | |
215a7ad1 | 182 | + [pu] git-repack: Add option to repack all objects. |
3459c2c4 JH |
183 | + [pu~1] More documentation updates. |
184 | + [pu~2] Show commits in topo order and name all commits. | |
185 | + [pu~3] mailinfo and applymbox updates | |
186 | + [pu~4] Document "git cherry-pick" and "git revert" | |
187 | + [pu~5] Remove git-apply-patch-script. | |
188 | + [pu~6] Redo "revert" using three-way merge machinery. | |
ebedc319 JH |
189 | - [rc] Merge refs/heads/master from . |
190 | ++* [master] Revert "Replace zero-length array decls with []." | |
191 | - [rc~1] Merge refs/heads/master from . | |
192 | ... [master~1] Merge refs/heads/portable from http://www.cs.berkeley.... | |
3459c2c4 | 193 | ------------------------------------------------ |