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Diffstat (limited to 'Documentation/howto/revert-a-faulty-merge.txt')
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diff --git a/Documentation/howto/revert-a-faulty-merge.txt b/Documentation/howto/revert-a-faulty-merge.txt index 3b4a390005..8a685483f4 100644 --- a/Documentation/howto/revert-a-faulty-merge.txt +++ b/Documentation/howto/revert-a-faulty-merge.txt @@ -7,6 +7,10 @@ Abstract: Sometimes a branch that was already merged to the mainline 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: @@ -142,6 +146,8 @@ different resolution strategies: 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). @@ -177,3 +183,91 @@ 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 |