Merge branch 'mb/rebase-i-no-ff'

* mb/rebase-i-no-ff:
  Teach rebase the --no-ff option.

Conflicts:
	git-rebase--interactive.sh
	t/t3404-rebase-interactive.sh

5 files changed, 152 insertions, 9 deletions

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 823f2a4638..0d07b1b207 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -274,9 +274,16 @@ which makes little sense.
 -f::
 --force-rebase::
 	Force the rebase even if the current branch is a descendant
-	of the commit you are rebasing onto.  Normally the command will
+	of the commit you are rebasing onto.  Normally non-interactive rebase will
 	exit with the message "Current branch is up to date" in such a
 	situation.
+	Incompatible with the --interactive option.
++
+You may find this (or --no-ff with an interactive rebase) helpful after
+reverting a topic branch merge, as this option recreates the topic branch with
+fresh commits so it can be remerged successfully without needing to "revert
+the reversion" (see the
+link:howto/revert-a-faulty-merge.txt[revert-a-faulty-merge How-To] for details).
 
 --ignore-whitespace::
 --whitespace=<option>::
@@ -316,7 +323,19 @@ which makes little sense.
 	commit to be modified, and change the action of the moved
 	commit from `pick` to `squash` (or `fixup`).
 +
-This option is only valid when '--interactive' option is used.
+This option is only valid when the '--interactive' option is used.
+
+--no-ff::
+	With --interactive, cherry-pick all rebased commits instead of
+	fast-forwarding over the unchanged ones.  This ensures that the
+	entire history of the rebased branch is composed of new commits.
++
+Without --interactive, this is a synonym for --force-rebase.
++
+You may find this helpful after reverting a topic branch merge, as this option
+recreates the topic branch with fresh commits so it can be remerged
+successfully without needing to "revert the reversion" (see the
+link:howto/revert-a-faulty-merge.txt[revert-a-faulty-merge How-To] for details).
 
 include::merge-strategies.txt[]
 
diff --git a/Documentation/howto/revert-a-faulty-merge.txt b/Documentation/howto/revert-a-faulty-merge.txt
index 3b4a390005..ff5c0bc27a 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
diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 2ff211cbaa..b817c4a76e 100755
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -20,6 +20,7 @@ v,verbose          display a diffstat of what changed upstream
 onto=              rebase onto given branch instead of upstream
 p,preserve-merges  try to recreate merges instead of ignoring them
 s,strategy=        use the given merge strategy
+no-ff              cherry-pick all commits, even if unchanged
 m,merge            always used (no-op)
 i,interactive      always used (no-op)
  Actions:
@@ -110,6 +111,7 @@ VERBOSE=
 OK_TO_SKIP_PRE_REBASE=
 REBASE_ROOT=
 AUTOSQUASH=
+NEVER_FF=
 
 GIT_CHERRY_PICK_HELP="  After resolving the conflicts,
 mark the corrected paths with 'git add <paths>', and
@@ -232,6 +234,7 @@ do_with_author () {
 pick_one () {
 	ff=--ff
 	case "$1" in -n) sha1=$2; ff= ;; *) sha1=$1 ;; esac
+	case "$NEVER_FF" in '') ;; ?*) ff= ;; esac
 	output git rev-parse --verify $sha1 || die "Invalid commit name: $sha1"
 	test -d "$REWRITTEN" &&
 		pick_one_preserving_merges "$@" && return
@@ -782,6 +785,9 @@ first and then run 'git rebase --continue' again."
 	-i)
 		# yeah, we know
 		;;
+	--no-ff)
+		NEVER_FF=t
+		;;
 	--root)
 		REBASE_ROOT=t
 		;;
@@ -965,7 +971,7 @@ EOF
 		has_action "$TODO" ||
 			die_abort "Nothing to do"
 
-		test -d "$REWRITTEN" || skip_unnecessary_picks
+		test -d "$REWRITTEN" || test -n "$NEVER_FF" || skip_unnecessary_picks
 
 		git update-ref ORIG_HEAD $HEAD
 		output git checkout $ONTO && do_rest
diff --git a/git-rebase.sh b/git-rebase.sh
index e0eb9568f3..44f5c65fdb 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -3,7 +3,7 @@
 # Copyright (c) 2005 Junio C Hamano.
 #
 
-USAGE='[--interactive | -i] [-v] [--force-rebase | -f] [--onto <newbase>] [<upstream>|--root] [<branch>] [--quiet | -q]'
+USAGE='[--interactive | -i] [-v] [--force-rebase | -f] [--no-ff] [--onto <newbase>] [<upstream>|--root] [<branch>] [--quiet | -q]'
 LONG_USAGE='git-rebase replaces <branch> with a new branch of the
 same name.  When the --onto option is provided the new branch starts
 out with a HEAD equal to <newbase>, otherwise it is equal to <upstream>
@@ -353,7 +353,7 @@ do
 	--root)
 		rebase_root=t
 		;;
-	-f|--f|--fo|--for|--forc|force|--force-r|--force-re|--force-reb|--force-reba|--force-rebas|--force-rebase)
+	-f|--f|--fo|--for|--forc|force|--force-r|--force-re|--force-reb|--force-reba|--force-rebas|--force-rebase|--no-ff)
 		force_rebase=t
 		;;
 	--rerere-autoupdate|--no-rerere-autoupdate)
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index b0b43c6d32..f20ea38411 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -22,12 +22,18 @@ set_fake_editor
 # | \
 # |   F - G - H                (branch1)
 # |     \
-#  \      I                    (branch2)
-#   \
-#     J - K - L - M            (no-conflict-branch)
+# |\      I                    (branch2)
+# | \
+# |   J - K - L - M            (no-conflict-branch)
+#  \
+#    N - O - P                 (no-ff-branch)
 #
 # where A, B, D and G all touch file1, and one, two, three, four all
 # touch file "conflict".
+#
+# WARNING: Modifications to the initial repository can change the SHA ID used
+# in the expect2 file for the 'stop on conflicting pick' test.
+
 
 test_expect_success 'setup' '
 	test_commit A file1 &&
@@ -50,6 +56,11 @@ test_expect_success 'setup' '
 	for n in J K L M
 	do
 		test_commit $n file$n
+	done &&
+	git checkout -b no-ff-branch A &&
+	for n in N O P
+	do
+		test_commit $n file$n
 	done
 '
 
@@ -113,7 +124,7 @@ cat > expect2 << EOF
 D
 =======
 G
->>>>>>> 51047de... G
+>>>>>>> 5d18e54... G
 EOF
 
 test_expect_success 'stop on conflicting pick' '
@@ -586,4 +597,21 @@ test_expect_success 'rebase while detaching HEAD' '
 	test_must_fail git symbolic-ref HEAD
 '
 
+test_tick # Ensure that the rebased commits get a different timestamp.
+test_expect_success 'always cherry-pick with --no-ff' '
+	git checkout no-ff-branch &&
+	git tag original-no-ff-branch &&
+	git rebase -i --no-ff A &&
+	touch empty &&
+	for p in 0 1 2
+	do
+		test ! $(git rev-parse HEAD~$p) = $(git rev-parse original-no-ff-branch~$p) &&
+		git diff HEAD~$p original-no-ff-branch~$p > out &&
+		test_cmp empty out
+	done &&
+	test $(git rev-parse HEAD~3) = $(git rev-parse original-no-ff-branch~3) &&
+	git diff HEAD~3 original-no-ff-branch~3 > out &&
+	test_cmp empty out
+'
+
 test_done