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| <div id="header"> | |
| <h1>How to revert a faulty merge</h1> | |
| </div> | |
| <div id="content"> | |
| <div id="preamble"> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>Alan <<a href="mailto:alan@clueserver.org">alan@clueserver.org</a>> said:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>I have a master branch. We have a branch off of that that some | |
| developers are doing work on. They claim it is ready. We merge it | |
| into the master branch. It breaks something so we revert the merge. | |
| They make changes to the code. they get it to a point where they say | |
| it is ok and we merge again.</code></pre> | |
| </div></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>When examined, we find that code changes made before the revert are | |
| not in the master branch, but code changes after are in the master | |
| branch.</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>and asked for help recovering from this situation.</p></div> | |
| <div class="paragraph"><p>The history immediately after the "revert of the merge" would look like | |
| this:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>---o---o---o---M---x---x---W | |
| / | |
| ---A---B</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>where A and B are on the side development that was not so good, M is the | |
| merge that brings these premature changes into the mainline, x are changes | |
| unrelated to what the side branch did and already made on the mainline, | |
| and W is the "revert of the merge M" (doesn’t W look M upside down?). | |
| IOW, <code>"diff W^..W"</code> is similar to <code>"diff -R M^..M"</code>.</p></div> | |
| <div class="paragraph"><p>Such a "revert" of a merge can be made with:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>$ git revert -m 1 M</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>After the developers of the side branch fix their mistakes, the history | |
| may look like this:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>---o---o---o---M---x---x---W---x | |
| / | |
| ---A---B-------------------C---D</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>where C and D are to fix what was broken in A and B, and you may already | |
| have some other changes on the mainline after W.</p></div> | |
| <div class="paragraph"><p>If you merge the updated side branch (with D at its tip), none of the | |
| changes made in A or B will be in the result, because they were reverted | |
| by W. That is what Alan saw.</p></div> | |
| <div class="paragraph"><p>Linus explains the situation:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>Reverting a regular commit just effectively undoes what that commit | |
| did, and is fairly straightforward. But reverting a merge commit also | |
| undoes the _data_ that the commit changed, but it does absolutely | |
| nothing to the effects on _history_ that the merge had.</code></pre> | |
| </div></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>So the merge will still exist, and it will still be seen as joining | |
| the two branches together, and future merges will see that merge as | |
| the last shared state - and the revert that reverted the merge brought | |
| in will not affect that at all.</code></pre> | |
| </div></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>So a "revert" undoes the data changes, but it's very much _not_ an | |
| "undo" in the sense that it doesn't undo the effects of a commit on | |
| the repository history.</code></pre> | |
| </div></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>So if you think of "revert" as "undo", then you're going to always | |
| miss this part of reverts. Yes, it undoes the data, but no, it doesn't | |
| undo history.</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>In such a situation, you would want to first revert the previous revert, | |
| which would make the history look like this:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>---o---o---o---M---x---x---W---x---Y | |
| / | |
| ---A---B-------------------C---D</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>where Y is the revert of W. Such a "revert of the revert" can be done | |
| with:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>$ git revert W</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>This history would (ignoring possible conflicts between what W and W..Y | |
| changed) be equivalent to not having W or Y at all in the history:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>---o---o---o---M---x---x-------x---- | |
| / | |
| ---A---B-------------------C---D</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>and merging the side branch again will not have conflict arising from an | |
| earlier revert and revert of the revert.</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>---o---o---o---M---x---x-------x-------* | |
| / / | |
| ---A---B-------------------C---D</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>Of course the changes made in C and D still can conflict with what was | |
| done by any of the x, but that is just a normal merge conflict.</p></div> | |
| <div class="paragraph"><p>On the other hand, if the developers of the side branch discarded their | |
| faulty A and B, and redone the changes on top of the updated mainline | |
| after the revert, the history would have looked like this:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>---o---o---o---M---x---x---W---x---x | |
| / \ | |
| ---A---B A'--B'--C'</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>If you reverted the revert in such a case as in the previous example:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>---o---o---o---M---x---x---W---x---x---Y---* | |
| / \ / | |
| ---A---B A'--B'--C'</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>where Y is the revert of W, A' and B' are rerolled A and B, and there may | |
| also be a further fix-up C' on the side branch. <code>"diff Y^..Y"</code> is similar | |
| to <code>"diff -R W^..W"</code> (which in turn means it is similar to <code>"diff M^..M"</code>), | |
| and <code>"diff A'^..C'"</code> by definition would be similar but different from that, | |
| because it is a rerolled series of the earlier change. There will be a | |
| lot of overlapping changes that result in conflicts. So do not do "revert | |
| of revert" blindly without thinking..</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>---o---o---o---M---x---x---W---x---x | |
| / \ | |
| ---A---B A'--B'--C'</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>In the history with rebased side branch, W (and M) are behind the merge | |
| base of the updated branch and the tip of the mainline, and they should | |
| merge without the past faulty merge and its revert getting in the way.</p></div> | |
| <div class="paragraph"><p>To recap, these are two very different scenarios, and they want two very | |
| different resolution strategies:</p></div> | |
| <div class="ulist"><ul> | |
| <li> | |
| <p> | |
| If the faulty side branch was fixed by adding corrections on top, then | |
| doing a revert of the previous revert would be the right thing to do. | |
| </p> | |
| </li> | |
| <li> | |
| <p> | |
| If the faulty side branch whose effects were discarded by an earlier | |
| revert of a merge was rebuilt from scratch (i.e. rebasing and fixing, | |
| as you seem to have interpreted), then re-merging the result without | |
| doing anything else fancy would be the right thing to do. | |
| (See the ADDENDUM below for how to rebuild a branch from scratch | |
| without changing its original branching-off point.) | |
| </p> | |
| </li> | |
| </ul></div> | |
| <div class="paragraph"><p>However, there are things to keep in mind when reverting a merge (and | |
| reverting such a revert).</p></div> | |
| <div class="paragraph"><p>For example, think about what reverting a merge (and then reverting the | |
| revert) does to bisectability. Ignore the fact that the revert of a revert | |
| is undoing it - just think of it as a "single commit that does a lot". | |
| Because that is what it does.</p></div> | |
| <div class="paragraph"><p>When you have a problem you are chasing down, and you hit a "revert this | |
| merge", what you’re hitting is essentially a single commit that contains | |
| all the changes (but obviously in reverse) of all the commits that got | |
| merged. So it’s debugging hell, because now you don’t have lots of small | |
| changes that you can try to pinpoint which <em>part</em> of it changes.</p></div> | |
| <div class="paragraph"><p>But does it all work? Sure it does. You can revert a merge, and from a | |
| purely technical angle, Git did it very naturally and had no real | |
| troubles. It just considered it a change from "state before merge" to | |
| "state after merge", and that was it. Nothing complicated, nothing odd, | |
| nothing really dangerous. Git will do it without even thinking about it.</p></div> | |
| <div class="paragraph"><p>So from a technical angle, there’s nothing wrong with reverting a merge, | |
| but from a workflow angle it’s something that you generally should try to | |
| avoid.</p></div> | |
| <div class="paragraph"><p>If at all possible, for example, if you find a problem that got merged | |
| into the main tree, rather than revert the merge, try <em>really</em> hard to | |
| bisect the problem down into the branch you merged, and just fix it, or | |
| try to revert the individual commit that caused it.</p></div> | |
| <div class="paragraph"><p>Yes, it’s more complex, and no, it’s not always going to work (sometimes | |
| the answer is: "oops, I really shouldn’t have merged it, because it wasn’t | |
| ready yet, and I really need to undo <em>all</em> of the merge"). So then you | |
| really should revert the merge, but when you want to re-do the merge, you | |
| now need to do it by reverting the revert.</p></div> | |
| <div class="paragraph"><p>ADDENDUM</p></div> | |
| <div class="paragraph"><p>Sometimes you have to rewrite one of a topic branch’s commits <strong>and</strong> you can’t | |
| change the topic’s branching-off point. Consider the following situation:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>P---o---o---M---x---x---W---x | |
| \ / | |
| A---B---C</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>where commit W reverted commit M because it turned out that commit B was wrong | |
| and needs to be rewritten, but you need the rewritten topic to still branch | |
| from commit P (perhaps P is a branching-off point for yet another branch, and | |
| you want be able to merge the topic into both branches).</p></div> | |
| <div class="paragraph"><p>The natural thing to do in this case is to checkout the A-B-C branch and use | |
| "rebase -i P" to change commit B. However this does not rewrite commit A, | |
| because "rebase -i" by default fast-forwards over any initial commits selected | |
| with the "pick" command. So you end up with this:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>P---o---o---M---x---x---W---x | |
| \ / | |
| A---B---C <-- old branch | |
| \ | |
| B'---C' <-- naively rewritten branch</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>To merge A-B'-C' into the mainline branch you would still have to first revert | |
| commit W in order to pick up the changes in A, but then it’s likely that the | |
| changes in B' will conflict with the original B changes re-introduced by the | |
| reversion of W.</p></div> | |
| <div class="paragraph"><p>However, you can avoid these problems if you recreate the entire branch, | |
| including commit A:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code> A'---B'---C' <-- completely rewritten branch | |
| / | |
| P---o---o---M---x---x---W---x | |
| \ / | |
| A---B---C</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>You can merge A'-B'-C' into the mainline branch without worrying about first | |
| reverting W. Mainline’s history would look like this:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code> A'---B'---C'------------------ | |
| / \ | |
| P---o---o---M---x---x---W---x---M2 | |
| \ / | |
| A---B---C</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>But if you don’t actually need to change commit A, then you need some way to | |
| recreate it as a new commit with the same changes in it. The rebase command’s | |
| --no-ff option provides a way to do this:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>$ git rebase [-i] --no-ff P</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>The --no-ff option creates a new branch A'-B'-C' with all-new commits (all the | |
| SHA IDs will be different) even if in the interactive case you only actually | |
| modify commit B. You can then merge this new branch directly into the mainline | |
| branch and be sure you’ll get all of the branch’s changes.</p></div> | |
| <div class="paragraph"><p>You can also use --no-ff in cases where you just add extra commits to the topic | |
| to fix it up. Let’s revisit the situation discussed at the start of this howto:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>P---o---o---M---x---x---W---x | |
| \ / | |
| A---B---C----------------D---E <-- fixed-up topic branch</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>At this point, you can use --no-ff to recreate the topic branch:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>$ git checkout E | |
| $ git rebase --no-ff P</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>yielding</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code> A'---B'---C'------------D'---E' <-- recreated topic branch | |
| / | |
| P---o---o---M---x---x---W---x | |
| \ / | |
| A---B---C----------------D---E</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>You can merge the recreated branch into the mainline without reverting commit W, | |
| and mainline’s history will look like this:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code> A'---B'---C'------------D'---E' | |
| / \ | |
| P---o---o---M---x---x---W---x---M2 | |
| \ / | |
| A---B---C</code></pre> | |
| </div></div> | |
| </div> | |
| </div> | |
| </div> | |
| <div id="footnotes"><hr /></div> | |
| <div id="footer"> | |
| <div id="footer-text"> | |
| Last updated | |
| 2019-02-24 08:32:59 PST | |
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