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diff --git a/Documentation/howto/revert-a-faulty-merge.txt b/Documentation/howto/revert-a-faulty-merge.txt new file mode 100644 index 0000000..462255e --- /dev/null +++ b/Documentation/howto/revert-a-faulty-merge.txt @@ -0,0 +1,273 @@ +Date: Fri, 19 Dec 2008 00:45:19 -0800 +From: Linus Torvalds <torvalds@linux-foundation.org>, Junio C Hamano <gitster@pobox.com> +Subject: Re: Odd merge behaviour involving reverts +Abstract: Sometimes a branch that was already merged to the mainline + is later found to be faulty. Linus and Junio give guidance on + recovering from such a premature merge and continuing development + after the offending branch is fixed. +Message-ID: <7vocz8a6zk.fsf@gitster.siamese.dyndns.org> +References: <alpine.LFD.2.00.0812181949450.14014@localhost.localdomain> +Content-type: text/asciidoc + +How to revert a faulty merge +============================ + +Alan <alan@clueserver.org> said: + + 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. + + 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. + +and asked for help recovering from this situation. + +The history immediately after the "revert of the merge" would look like +this: + + ---o---o---o---M---x---x---W + / + ---A---B + +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, `"diff W^..W"` is similar to `"diff -R M^..M"`. + +Such a "revert" of a merge can be made with: + + $ git revert -m 1 M + +After the developers of the side branch fix their mistakes, the history +may look like this: + + ---o---o---o---M---x---x---W---x + / + ---A---B-------------------C---D + +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. + +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. + +Linus explains the situation: + + 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. + + 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. + + 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. + + 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. + +In such a situation, you would want to first revert the previous revert, +which would make the history look like this: + + ---o---o---o---M---x---x---W---x---Y + / + ---A---B-------------------C---D + +where Y is the revert of W. Such a "revert of the revert" can be done +with: + + $ git revert W + +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: + + ---o---o---o---M---x---x-------x---- + / + ---A---B-------------------C---D + +and merging the side branch again will not have conflict arising from an +earlier revert and revert of the revert. + + ---o---o---o---M---x---x-------x-------* + / / + ---A---B-------------------C---D + +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. + +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: + + ---o---o---o---M---x---x---W---x---x + / \ + ---A---B A'--B'--C' + +If you reverted the revert in such a case as in the previous example: + + ---o---o---o---M---x---x---W---x---x---Y---* + / \ / + ---A---B A'--B'--C' + +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. `"diff Y^..Y"` is similar +to `"diff -R W^..W"` (which in turn means it is similar to `"diff M^..M"`), +and `"diff A'^..C'"` 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.. + + ---o---o---o---M---x---x---W---x---x + / \ + ---A---B A'--B'--C' + +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. + +To recap, these are two very different scenarios, and they want two very +different resolution strategies: + + - 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. + + - 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.) + +However, there are things to keep in mind when reverting a merge (and +reverting such a revert). + +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. + +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 _part_ of it changes. + +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. + +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. + +If at all possible, for example, if you find a problem that got merged +into the main tree, rather than revert the merge, try _really_ 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. + +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 _all_ 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. + +ADDENDUM + +Sometimes you have to rewrite one of a topic branch's commits *and* you can't +change the topic's branching-off point. Consider the following situation: + + P---o---o---M---x---x---W---x + \ / + A---B---C + +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). + +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---o---o---M---x---x---W---x + \ / + A---B---C <-- old branch + \ + B'---C' <-- naively rewritten branch + +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. + +However, you can avoid these problems if you recreate the entire branch, +including commit A: + + A'---B'---C' <-- completely rewritten branch + / + P---o---o---M---x---x---W---x + \ / + A---B---C + +You can merge A'-B'-C' into the mainline branch without worrying about first +reverting W. Mainline's history would look like this: + + A'---B'---C'------------------ + / \ + P---o---o---M---x---x---W---x---M2 + \ / + A---B---C + +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: + + $ git rebase [-i] --no-ff 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. + +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---o---o---M---x---x---W---x + \ / + A---B---C----------------D---E <-- fixed-up topic branch + +At this point, you can use --no-ff to recreate the topic branch: + + $ git checkout E + $ git rebase --no-ff P + +yielding + + A'---B'---C'------------D'---E' <-- recreated topic branch + / + P---o---o---M---x---x---W---x + \ / + A---B---C----------------D---E + +You can merge the recreated branch into the mainline without reverting commit W, +and mainline's history will look like this: + + A'---B'---C'------------D'---E' + / \ + P---o---o---M---x---x---W---x---M2 + \ / + A---B---C |