826 lines
44 KiB
HTML
826 lines
44 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<meta name="generator" content="Asciidoctor 1.5.8">
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<title>How to revert a faulty merge</title>
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</head>
|
|
<body class="article">
|
|
<div id="header">
|
|
<h1>How to revert a faulty merge</h1>
|
|
</div>
|
|
<div id="content">
|
|
<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>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</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>---o---o---o---M---x---x---W
|
|
/
|
|
---A---B</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>$ git revert -m 1 M</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>---o---o---o---M---x---x---W---x
|
|
/
|
|
---A---B-------------------C---D</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>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</pre>
|
|
</div>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre>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.</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>---o---o---o---M---x---x---W---x---Y
|
|
/
|
|
---A---B-------------------C---D</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>$ git revert W</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>---o---o---o---M---x---x-------x----
|
|
/
|
|
---A---B-------------------C---D</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>---o---o---o---M---x---x-------x-------*
|
|
/ /
|
|
---A---B-------------------C---D</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>---o---o---o---M---x---x---W---x---x
|
|
/ \
|
|
---A---B A'--B'--C'</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>---o---o---o---M---x---x---W---x---x---Y---*
|
|
/ \ /
|
|
---A---B A'--B'--C'</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>---o---o---o---M---x---x---W---x---x
|
|
/ \
|
|
---A---B A'--B'--C'</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>P---o---o---M---x---x---W---x
|
|
\ /
|
|
A---B---C</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>P---o---o---M---x---x---W---x
|
|
\ /
|
|
A---B---C <-- old branch
|
|
\
|
|
B'---C' <-- naively rewritten branch</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> A'---B'---C' <-- completely rewritten branch
|
|
/
|
|
P---o---o---M---x---x---W---x
|
|
\ /
|
|
A---B---C</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> A'---B'---C'------------------
|
|
/ \
|
|
P---o---o---M---x---x---W---x---M2
|
|
\ /
|
|
A---B---C</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>$ git rebase [-i] --no-ff P</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>P---o---o---M---x---x---W---x
|
|
\ /
|
|
A---B---C----------------D---E <-- fixed-up topic branch</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>$ git checkout E
|
|
$ git rebase --no-ff P</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>yielding</p>
|
|
</div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre> A'---B'---C'------------D'---E' <-- recreated topic branch
|
|
/
|
|
P---o---o---M---x---x---W---x
|
|
\ /
|
|
A---B---C----------------D---E</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> A'---B'---C'------------D'---E'
|
|
/ \
|
|
P---o---o---M---x---x---W---x---M2
|
|
\ /
|
|
A---B---C</pre>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div id="footer">
|
|
<div id="footer-text">
|
|
Last updated 2019-02-26 19:31:11 UTC
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