X-Git-Url: https://git.tokkee.org/?a=blobdiff_plain;f=Documentation%2Fhowto%2Frevert-a-faulty-merge.txt;h=ff5c0bc27a416de16f106c15b9076082c6509e42;hb=b807c524df69fb44d7fe2a68a899985a7f7e6838;hp=3b4a390005b07c86ee320ee8ca1cf57e46458cb6;hpb=5e04a1ee33f82051668f09d73cf16348169e7f1f;p=git.git

diff --git a/Documentation/howto/revert-a-faulty-merge.txt b/Documentation/howto/revert-a-faulty-merge.txt
index 3b4a39000..ff5c0bc27 100644
--- a/Documentation/howto/revert-a-faulty-merge.txt
+++ b/Documentation/howto/revert-a-faulty-merge.txt
@@ -142,6 +142,8 @@ 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 +179,91 @@ 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 commmand'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