273 lines
11 KiB
Text
273 lines
11 KiB
Text
Date: Fri, 19 Dec 2008 00:45:19 -0800
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From: Linus Torvalds <torvalds@linux-foundation.org>, Junio C Hamano <gitster@pobox.com>
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Subject: Re: Odd merge behaviour involving reverts
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Abstract: Sometimes a branch that was already merged to the mainline
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is later found to be faulty. Linus and Junio give guidance on
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recovering from such a premature merge and continuing development
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after the offending branch is fixed.
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Message-ID: <7vocz8a6zk.fsf@gitster.siamese.dyndns.org>
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References: <alpine.LFD.2.00.0812181949450.14014@localhost.localdomain>
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Content-type: text/asciidoc
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How to revert a faulty merge
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============================
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Alan <alan@clueserver.org> said:
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I have a master branch. We have a branch off of that that some
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developers are doing work on. They claim it is ready. We merge it
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into the master branch. It breaks something so we revert the merge.
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They make changes to the code. they get it to a point where they say
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it is ok and we merge again.
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When examined, we find that code changes made before the revert are
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not in the master branch, but code changes after are in the master
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branch.
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and asked for help recovering from this situation.
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The history immediately after the "revert of the merge" would look like
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this:
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---o---o---o---M---x---x---W
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/
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---A---B
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where A and B are on the side development that was not so good, M is the
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merge that brings these premature changes into the mainline, x are changes
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unrelated to what the side branch did and already made on the mainline,
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and W is the "revert of the merge M" (doesn't W look M upside down?).
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IOW, `"diff W^..W"` is similar to `"diff -R M^..M"`.
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Such a "revert" of a merge can be made with:
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$ git revert -m 1 M
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After the developers of the side branch fix their mistakes, the history
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may look like this:
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---o---o---o---M---x---x---W---x
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/
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---A---B-------------------C---D
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where C and D are to fix what was broken in A and B, and you may already
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have some other changes on the mainline after W.
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If you merge the updated side branch (with D at its tip), none of the
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changes made in A or B will be in the result, because they were reverted
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by W. That is what Alan saw.
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Linus explains the situation:
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Reverting a regular commit just effectively undoes what that commit
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did, and is fairly straightforward. But reverting a merge commit also
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undoes the _data_ that the commit changed, but it does absolutely
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nothing to the effects on _history_ that the merge had.
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So the merge will still exist, and it will still be seen as joining
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the two branches together, and future merges will see that merge as
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the last shared state - and the revert that reverted the merge brought
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in will not affect that at all.
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So a "revert" undoes the data changes, but it's very much _not_ an
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"undo" in the sense that it doesn't undo the effects of a commit on
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the repository history.
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So if you think of "revert" as "undo", then you're going to always
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miss this part of reverts. Yes, it undoes the data, but no, it doesn't
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undo history.
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In such a situation, you would want to first revert the previous revert,
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which would make the history look like this:
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---o---o---o---M---x---x---W---x---Y
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/
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---A---B-------------------C---D
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where Y is the revert of W. Such a "revert of the revert" can be done
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with:
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$ git revert W
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This history would (ignoring possible conflicts between what W and W..Y
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changed) be equivalent to not having W or Y at all in the history:
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---o---o---o---M---x---x-------x----
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/
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---A---B-------------------C---D
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and merging the side branch again will not have conflict arising from an
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earlier revert and revert of the revert.
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---o---o---o---M---x---x-------x-------*
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/ /
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---A---B-------------------C---D
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Of course the changes made in C and D still can conflict with what was
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done by any of the x, but that is just a normal merge conflict.
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On the other hand, if the developers of the side branch discarded their
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faulty A and B, and redone the changes on top of the updated mainline
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after the revert, the history would have looked like this:
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---o---o---o---M---x---x---W---x---x
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/ \
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---A---B A'--B'--C'
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If you reverted the revert in such a case as in the previous example:
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---o---o---o---M---x---x---W---x---x---Y---*
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/ \ /
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---A---B A'--B'--C'
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where Y is the revert of W, A' and B' are rerolled A and B, and there may
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also be a further fix-up C' on the side branch. `"diff Y^..Y"` is similar
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to `"diff -R W^..W"` (which in turn means it is similar to `"diff M^..M"`),
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and `"diff A'^..C'"` by definition would be similar but different from that,
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because it is a rerolled series of the earlier change. There will be a
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lot of overlapping changes that result in conflicts. So do not do "revert
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of revert" blindly without thinking..
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---o---o---o---M---x---x---W---x---x
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/ \
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---A---B A'--B'--C'
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In the history with rebased side branch, W (and M) are behind the merge
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base of the updated branch and the tip of the mainline, and they should
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merge without the past faulty merge and its revert getting in the way.
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To recap, these are two very different scenarios, and they want two very
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different resolution strategies:
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- If the faulty side branch was fixed by adding corrections on top, then
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doing a revert of the previous revert would be the right thing to do.
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- If the faulty side branch whose effects were discarded by an earlier
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revert of a merge was rebuilt from scratch (i.e. rebasing and fixing,
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as you seem to have interpreted), then re-merging the result without
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doing anything else fancy would be the right thing to do.
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(See the ADDENDUM below for how to rebuild a branch from scratch
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without changing its original branching-off point.)
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However, there are things to keep in mind when reverting a merge (and
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reverting such a revert).
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For example, think about what reverting a merge (and then reverting the
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revert) does to bisectability. Ignore the fact that the revert of a revert
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is undoing it - just think of it as a "single commit that does a lot".
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Because that is what it does.
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When you have a problem you are chasing down, and you hit a "revert this
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merge", what you're hitting is essentially a single commit that contains
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all the changes (but obviously in reverse) of all the commits that got
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merged. So it's debugging hell, because now you don't have lots of small
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changes that you can try to pinpoint which _part_ of it changes.
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But does it all work? Sure it does. You can revert a merge, and from a
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purely technical angle, Git did it very naturally and had no real
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troubles. It just considered it a change from "state before merge" to
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"state after merge", and that was it. Nothing complicated, nothing odd,
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nothing really dangerous. Git will do it without even thinking about it.
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So from a technical angle, there's nothing wrong with reverting a merge,
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but from a workflow angle it's something that you generally should try to
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avoid.
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If at all possible, for example, if you find a problem that got merged
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into the main tree, rather than revert the merge, try _really_ hard to
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bisect the problem down into the branch you merged, and just fix it, or
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try to revert the individual commit that caused it.
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Yes, it's more complex, and no, it's not always going to work (sometimes
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the answer is: "oops, I really shouldn't have merged it, because it wasn't
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ready yet, and I really need to undo _all_ of the merge"). So then you
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really should revert the merge, but when you want to re-do the merge, you
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now need to do it by reverting the revert.
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ADDENDUM
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Sometimes you have to rewrite one of a topic branch's commits *and* you can't
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change the topic's branching-off point. Consider the following situation:
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P---o---o---M---x---x---W---x
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\ /
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A---B---C
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where commit W reverted commit M because it turned out that commit B was wrong
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and needs to be rewritten, but you need the rewritten topic to still branch
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from commit P (perhaps P is a branching-off point for yet another branch, and
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you want be able to merge the topic into both branches).
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The natural thing to do in this case is to checkout the A-B-C branch and use
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"rebase -i P" to change commit B. However this does not rewrite commit A,
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because "rebase -i" by default fast-forwards over any initial commits selected
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with the "pick" command. So you end up with this:
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P---o---o---M---x---x---W---x
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\ /
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A---B---C <-- old branch
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\
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B'---C' <-- naively rewritten branch
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To merge A-B'-C' into the mainline branch you would still have to first revert
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commit W in order to pick up the changes in A, but then it's likely that the
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changes in B' will conflict with the original B changes re-introduced by the
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reversion of W.
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However, you can avoid these problems if you recreate the entire branch,
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including commit A:
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A'---B'---C' <-- completely rewritten branch
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/
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P---o---o---M---x---x---W---x
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\ /
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A---B---C
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You can merge A'-B'-C' into the mainline branch without worrying about first
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reverting W. Mainline's history would look like this:
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A'---B'---C'------------------
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/ \
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P---o---o---M---x---x---W---x---M2
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\ /
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A---B---C
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But if you don't actually need to change commit A, then you need some way to
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recreate it as a new commit with the same changes in it. The rebase command's
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--no-ff option provides a way to do this:
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$ git rebase [-i] --no-ff P
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The --no-ff option creates a new branch A'-B'-C' with all-new commits (all the
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SHA IDs will be different) even if in the interactive case you only actually
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modify commit B. You can then merge this new branch directly into the mainline
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branch and be sure you'll get all of the branch's changes.
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You can also use --no-ff in cases where you just add extra commits to the topic
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to fix it up. Let's revisit the situation discussed at the start of this howto:
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P---o---o---M---x---x---W---x
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\ /
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A---B---C----------------D---E <-- fixed-up topic branch
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At this point, you can use --no-ff to recreate the topic branch:
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$ git checkout E
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$ git rebase --no-ff P
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yielding
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A'---B'---C'------------D'---E' <-- recreated topic branch
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/
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P---o---o---M---x---x---W---x
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\ /
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A---B---C----------------D---E
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You can merge the recreated branch into the mainline without reverting commit W,
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and mainline's history will look like this:
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A'---B'---C'------------D'---E'
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/ \
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P---o---o---M---x---x---W---x---M2
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\ /
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A---B---C
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