[PATCH 8/8] rebase -i: introduce --recreate-merges=no-rebase-cousins

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This one is a bit tricky to explain, so let's try with a diagram:

        C
      /   \
A - B - E - F
  \   /
    D

To illustrate what this new mode is all about, let's consider what
happens upon `git rebase -i --recreate-merges B`, in particular to
the commit `D`. In the default mode, the new branch structure is:

       --- C' --
      /         \
A - B ------ E' - F'
      \    /
        D'

This is not really preserving the branch topology from before! The
reason is that the commit `D` does not have `B` as ancestor, and
therefore it gets rebased onto `B`.

However, when recreating branch structure, there are legitimate use
cases where one might want to preserve the branch points of commits that
do not descend from the <upstream> commit that was passed to the rebase
command, e.g. when a branch from core Git's `next` was merged into Git
for Windows' master we will not want to rebase those commits on top of a
Windows-specific commit. In the example above, the desired outcome would
look like this:

       --- C' --
      /         \
A - B ------ E' - F'
  \        /
   -- D' --

Let's introduce the term "cousins" for such commits ("D" in the
example), and the "no-rebase-cousins" mode of the merge-recreating
rebase, to help those use cases.

Signed-off-by: Johannes Schindelin <johannes.schindelin@xxxxxx>
---
 Documentation/git-rebase.txt      |  7 ++++++-
 builtin/rebase--helper.c          |  9 ++++++++-
 git-rebase--interactive.sh        |  1 +
 git-rebase.sh                     | 12 +++++++++++-
 sequencer.c                       |  4 ++++
 sequencer.h                       |  8 ++++++++
 t/t3430-rebase-recreate-merges.sh | 23 +++++++++++++++++++++++
 7 files changed, 61 insertions(+), 3 deletions(-)

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 1d061373288..ac07a5c3fc9 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -368,10 +368,15 @@ The commit list format can be changed by setting the configuration option
 rebase.instructionFormat.  A customized instruction format will automatically
 have the long commit hash prepended to the format.
 
---recreate-merges::
+--recreate-merges[=(rebase-cousins|no-rebase-cousins)]::
 	Recreate merge commits instead of flattening the history by replaying
 	merges. Merge conflict resolutions or manual amendments to merge
 	commits are not preserved.
++
+By default, or when `rebase-cousins` was specified, commits which do not have
+`<upstream>` as direct ancestor are rebased onto `<upstream>` (or `<onto>`,
+if specified). If the `rebase-cousins` mode is turned off, such commits will
+retain their original branch point.
 
 -p::
 --preserve-merges::
diff --git a/builtin/rebase--helper.c b/builtin/rebase--helper.c
index a34ab5c0655..ef08fef4d14 100644
--- a/builtin/rebase--helper.c
+++ b/builtin/rebase--helper.c
@@ -13,7 +13,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 {
 	struct replay_opts opts = REPLAY_OPTS_INIT;
 	unsigned flags = 0, keep_empty = 0, recreate_merges = 0;
-	int abbreviate_commands = 0;
+	int abbreviate_commands = 0, no_rebase_cousins = -1;
 	enum {
 		CONTINUE = 1, ABORT, MAKE_SCRIPT, SHORTEN_OIDS, EXPAND_OIDS,
 		CHECK_TODO_LIST, SKIP_UNNECESSARY_PICKS, REARRANGE_SQUASH,
@@ -23,6 +23,8 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 		OPT_BOOL(0, "ff", &opts.allow_ff, N_("allow fast-forward")),
 		OPT_BOOL(0, "keep-empty", &keep_empty, N_("keep empty commits")),
 		OPT_BOOL(0, "recreate-merges", &recreate_merges, N_("recreate merge commits")),
+		OPT_BOOL(0, "no-rebase-cousins", &no_rebase_cousins,
+			 N_("keep original branch points of cousins")),
 		OPT_CMDMODE(0, "continue", &command, N_("continue rebase"),
 				CONTINUE),
 		OPT_CMDMODE(0, "abort", &command, N_("abort rebase"),
@@ -57,8 +59,13 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 	flags |= keep_empty ? TODO_LIST_KEEP_EMPTY : 0;
 	flags |= abbreviate_commands ? TODO_LIST_ABBREVIATE_CMDS : 0;
 	flags |= recreate_merges ? TODO_LIST_RECREATE_MERGES : 0;
+	flags |= no_rebase_cousins > 0 ? TODO_LIST_NO_REBASE_COUSINS : 0;
 	flags |= command == SHORTEN_OIDS ? TODO_LIST_SHORTEN_IDS : 0;
 
+	if (no_rebase_cousins >= 0&& !recreate_merges)
+		warning(_("--[no-]rebase-cousins has no effect without "
+			  "--recreate-merges"));
+
 	if (command == CONTINUE && argc == 1)
 		return !!sequencer_continue(&opts);
 	if (command == ABORT && argc == 1)
diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 3459ec5a018..23184c77e88 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -901,6 +901,7 @@ if test t != "$preserve_merges"
 then
 	git rebase--helper --make-script ${keep_empty:+--keep-empty} \
 		${recreate_merges:+--recreate-merges} \
+		${no_rebase_cousins:+--no-rebase-cousins} \
 		$revisions ${restrict_revision+^$restrict_revision} >"$todo" ||
 	die "$(gettext "Could not generate todo list")"
 else
diff --git a/git-rebase.sh b/git-rebase.sh
index d69bc7d0e0d..3403b1416a8 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -17,7 +17,7 @@ q,quiet!           be quiet. implies --no-stat
 autostash          automatically stash/stash pop before and after
 fork-point         use 'merge-base --fork-point' to refine upstream
 onto=!             rebase onto given branch instead of upstream
-recreate-merges!   try to recreate merges instead of skipping them
+recreate-merges?   try to recreate merges instead of skipping them
 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
@@ -88,6 +88,7 @@ state_dir=
 # One of {'', continue, skip, abort}, as parsed from command line
 action=
 recreate_merges=
+no_rebase_cousins=
 preserve_merges=
 autosquash=
 keep_empty=
@@ -268,6 +269,15 @@ do
 		recreate_merges=t
 		test -z "$interactive_rebase" && interactive_rebase=implied
 		;;
+	--recreate-merges=*)
+		recreate_merges=t
+		case "${1#*=}" in
+		rebase-cousins) no_rebase_cousins=;;
+		no-rebase-cousins) no_rebase_cousins=t;;
+		*) die "Unknown mode: $1";;
+		esac
+		test -z "$interactive_rebase" && interactive_rebase=implied
+		;;
 	--preserve-merges)
 		preserve_merges=t
 		test -z "$interactive_rebase" && interactive_rebase=implied
diff --git a/sequencer.c b/sequencer.c
index b63bfb9a141..2b4e6b12321 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2905,6 +2905,7 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 				   unsigned flags)
 {
 	int keep_empty = flags & TODO_LIST_KEEP_EMPTY;
+	int no_rebase_cousins = flags & TODO_LIST_NO_REBASE_COUSINS;
 	struct strbuf buf = STRBUF_INIT, oneline = STRBUF_INIT;
 	struct strbuf label = STRBUF_INIT;
 	struct commit_list *commits = NULL, **tail = &commits, *iter;
@@ -3078,6 +3079,9 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 					   &commit->object.oid);
 			if (entry)
 				to = entry->string;
+			else if (no_rebase_cousins)
+				to = label_oid(&commit->object.oid, NULL,
+					       &state);
 
 			if (!to || !strcmp("onto", to))
 				fprintf(out, "%s\n", b);
diff --git a/sequencer.h b/sequencer.h
index 11d1ac925ef..9530dba3cba 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -49,6 +49,14 @@ int sequencer_remove_state(struct replay_opts *opts);
 #define TODO_LIST_SHORTEN_IDS (1U << 1)
 #define TODO_LIST_ABBREVIATE_CMDS (1U << 2)
 #define TODO_LIST_RECREATE_MERGES (1U << 3)
+/*
+ * When recreating merges, commits that do have the base commit as ancestor
+ * ("cousins") are rebased onto the new base by default. If those commits
+ * should keep their original branch point, this flag needs to be passed.
+ *
+ * This flag only makes sense when <base> and <onto> are different.
+ */
+#define TODO_LIST_NO_REBASE_COUSINS (1U << 4)
 int sequencer_make_script(FILE *out, int argc, const char **argv,
 			  unsigned flags);
 
diff --git a/t/t3430-rebase-recreate-merges.sh b/t/t3430-rebase-recreate-merges.sh
index 76e615bd7c1..22930e470a4 100755
--- a/t/t3430-rebase-recreate-merges.sh
+++ b/t/t3430-rebase-recreate-merges.sh
@@ -143,6 +143,29 @@ test_expect_success 'with a branch tip that was cherry-picked already' '
 	EOF
 '
 
+test_expect_success 'rebase cousins unless told not to' '
+	write_script copy-editor.sh <<-\EOF &&
+	cp "$1" "$(git rev-parse --git-path ORIGINAL-TODO)"
+	EOF
+
+	test_config sequence.editor \""$PWD"/copy-editor.sh\" &&
+	git checkout -b cousins master &&
+	before="$(git rev-parse --verify HEAD)" &&
+	test_tick &&
+	git rebase -i --recreate-merges=no-rebase-cousins HEAD^ &&
+	test_cmp_rev HEAD $before &&
+	test_tick &&
+	git rebase -i --recreate-merges HEAD^ &&
+	test_cmp_graph HEAD^.. <<-\EOF
+	*   Merge the topic branch '\''onebranch'\''
+	|\
+	| * D
+	| * G
+	|/
+	o H
+	EOF
+'
+
 test_expect_success 'refs/rewritten/* is worktree-local' '
 	git worktree add wt &&
 	cat >wt/script-from-scratch <<-\EOF &&
-- 
2.15.1.windows.2.1430.ga56c4f9e2a9



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