[PATCH v9 10/17] rebase: introduce the --rebase-merges option

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Apr 25, 2018, 12:29 UTC
Message-ID: <922adf04f1cbb04e83f075cb99c6673d2fee5026.1524659287.git.johannes.schindelin@gmx.de>
In-Reply-To: <cover.1524659287.git.johannes.schindelin@gmx.de>

Once upon a time, this here developer thought: wouldn't it be nice if, say, Git for Windows' patches on top of core Git could be represented as a thicket of branches, and be rebased on top of core Git in order to maintain a cherry-pick'able set of patch series?

The original attempt to answer this was: git rebase --preserve-merges.

However, that experiment was never intended as an interactive option, and it only piggy-backed on git rebase --interactive because that command's implementation looked already very, very familiar: it was designed by the same person who designed --preserve-merges: yours truly.

Some time later, some other developer (I am looking at you, Andreas! ;-)) decided that it would be a good idea to allow --preserve-merges to be combined with --interactive (with caveats!) and the Git maintainer (well, the interim Git maintainer during Junio's absence, that is) agreed, and that is when the glamor of the --preserve-merges design started to fall apart rather quickly and unglamorously.

The reason? In --preserve-merges mode, the parents of a merge commit (or for that matter, of *any* commit) were not stated explicitly, but were *implied* by the commit name passed to the `pick` command.

This made it impossible, for example, to reorder commits. Not to mention to move commits between branches or, deity forbid, to split topic branches into two.

Alas, these shortcomings also prevented that mode (whose original purpose was to serve Git for Windows' needs, with the additional hope that it may be useful to others, too) from serving Git for Windows' needs.

Five years later, when it became really untenable to have one unwieldy, big hodge-podge patch series of partly related, partly unrelated patches in Git for Windows that was rebased onto core Git's tags from time to time (earning the undeserved wrath of the developer of the ill-fated git-remote-hg series that first obsoleted Git for Windows' competing approach, only to be abandoned without maintainer later) was really untenable, the "Git garden shears" were born [*1*/*2*]: a script, piggy-backing on top of the interactive rebase, that would first determine the branch topology of the patches to be rebased, create a pseudo todo list for further editing, transform the result into a real todo list (making heavy use of the `exec` command to "implement" the missing todo list commands) and finally recreate the patch series on top of the new base commit.

That was in 2013. And it took about three weeks to come up with the design and implement it as an out-of-tree script. Needless to say, the implementation needed quite a few years to stabilize, all the while the design itself proved itself sound.

With this patch, the goodness of the Git garden shears comes to `git rebase -i` itself. Passing the `--rebase-merges` option will generate a todo list that can be understood readily, and where it is obvious how to reorder commits. New branches can be introduced by inserting `label` commands and calling `merge <label>`. And once this mode will have become stable and universally accepted, we can deprecate the design mistake that was `--preserve-merges`.

Link *1*: https://github.com/msysgit/msysgit/blob/master/share/msysGit/shears.sh Link *2*: https://github.com/git-for-windows/build-extra/blob/master/shears.sh

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 Documentation/git-rebase.txt           |  21 ++-
 contrib/completion/git-completion.bash |   2 +-
 git-rebase--interactive.sh             |   1 +
 git-rebase.sh                          |   6 +
 t/t3430-rebase-merges.sh               | 179 +++++++++++++++++++++++++
 5 files changed, 207 insertions(+), 2 deletions(-)
 create mode 100755 t/t3430-rebase-merges.sh

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index dd852068b1d..7f1756f1eba 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -379,6 +379,24 @@ 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.
 
+-r::
+--rebase-merges::
+	By default, a rebase will simply drop merge commits from the todo
+	list, and put the rebased commits into a single, linear branch.
+	With `--rebase-merges`, the rebase will instead try to preserve
+	the branching structure within the commits that are to be rebased,
+	by recreating the merge commits. Any resolved merge conflicts or
+	manual amendments in these merge commits will have to be
+	resolved/re-applied manually.
++
+This mode is similar in spirit to `--preserve-merges`, but in contrast to
+that option works well in interactive rebases: commits can be reordered,
+inserted and dropped at will.
++
+It is currently only possible to recreate the merge commits using the
+`recursive` merge strategy; Different merge strategies can be used only via
+explicit `exec git merge -s <strategy> [...]` commands.
+
 -p::
 --preserve-merges::
 	Recreate merge commits instead of flattening the history by replaying
@@ -781,7 +799,8 @@ BUGS
 The todo list presented by `--preserve-merges --interactive` does not
 represent the topology of the revision graph.  Editing commits and
 rewording their commit messages should work fine, but attempts to
-reorder commits tend to produce counterintuitive results.
+reorder commits tend to produce counterintuitive results. Use
+`--rebase-merges` in such scenarios instead.
 
 For example, an attempt to rearrange
 ------------
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 01dd9ff07a2..e6469004099 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -1944,7 +1944,7 @@ _git_rebase ()
 	--*)
 		__gitcomp "
 			--onto --merge --strategy --interactive
-			--preserve-merges --stat --no-stat
+			--rebase-merges --preserve-merges --stat --no-stat
 			--committer-date-is-author-date --ignore-date
 			--ignore-whitespace --whitespace=
 			--autosquash --no-autosquash
diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index acb4bfd3fc8..e29da634339 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -970,6 +970,7 @@ git_rebase__interactive () {
 	init_revisions_and_shortrevisions
 
 	git rebase--helper --make-script ${keep_empty:+--keep-empty} \
+		${rebase_merges:+--rebase-merges} \
 		$revisions ${restrict_revision+^$restrict_revision} >"$todo" ||
 	die "$(gettext "Could not generate todo list")"
 
diff --git a/git-rebase.sh b/git-rebase.sh
index ded5de085a8..a553f969d11 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -17,6 +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
+r,rebase-merges!   try to rebase 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
@@ -89,6 +90,7 @@ type=
 state_dir=
 # One of {'', continue, skip, abort}, as parsed from command line
 action=
+rebase_merges=
 preserve_merges=
 autosquash=
 keep_empty=
@@ -280,6 +282,10 @@ do
 	--no-keep-empty)
 		keep_empty=
 		;;
+	--rebase-merges)
+		rebase_merges=t
+		test -z "$interactive_rebase" && interactive_rebase=implied
+		;;
 	--preserve-merges)
 		preserve_merges=t
 		test -z "$interactive_rebase" && interactive_rebase=implied
diff --git a/t/t3430-rebase-merges.sh b/t/t3430-rebase-merges.sh
new file mode 100755
index 00000000000..5f0febb9970
--- /dev/null
+++ b/t/t3430-rebase-merges.sh
@@ -0,0 +1,179 @@
+#!/bin/sh
+#
+# Copyright (c) 2018 Johannes E. Schindelin
+#
+
+test_description='git rebase -i --rebase-merges
+
+This test runs git rebase "interactively", retaining the branch structure by
+recreating merge commits.
+
+Initial setup:
+
+    -- B --                   (first)
+   /       \
+ A - C - D - E - H            (master)
+       \       /
+         F - G                (second)
+'
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-rebase.sh
+
+test_cmp_graph () {
+	cat >expect &&
+	git log --graph --boundary --format=%s "$@" >output &&
+	sed "s/ *$//" <output >output.trimmed &&
+	test_cmp expect output.trimmed
+}
+
+test_expect_success 'setup' '
+	write_script replace-editor.sh <<-\EOF &&
+	mv "$1" "$(git rev-parse --git-path ORIGINAL-TODO)"
+	cp script-from-scratch "$1"
+	EOF
+
+	test_commit A &&
+	git checkout -b first &&
+	test_commit B &&
+	git checkout master &&
+	test_commit C &&
+	test_commit D &&
+	git merge --no-commit B &&
+	test_tick &&
+	git commit -m E &&
+	git tag -m E E &&
+	git checkout -b second C &&
+	test_commit F &&
+	test_commit G &&
+	git checkout master &&
+	git merge --no-commit G &&
+	test_tick &&
+	git commit -m H &&
+	git tag -m H H
+'
+
+test_expect_success 'create completely different structure' '
+	cat >script-from-scratch <<-\EOF &&
+	label onto
+
+	# onebranch
+	pick G
+	pick D
+	label onebranch
+
+	# second
+	reset onto
+	pick B
+	label second
+
+	reset onto
+	merge -C H second
+	merge onebranch # Merge the topic branch '\''onebranch'\''
+	EOF
+	test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
+	test_tick &&
+	git rebase -i -r A &&
+	test_cmp_graph <<-\EOF
+	*   Merge the topic branch '\''onebranch'\''
+	|\
+	| * D
+	| * G
+	* |   H
+	|\ \
+	| |/
+	|/|
+	| * B
+	|/
+	* A
+	EOF
+'
+
+test_expect_success 'generate correct todo list' '
+	cat >expect <<-\EOF &&
+	label onto
+
+	reset onto
+	pick d9df450 B
+	label E
+
+	reset onto
+	pick 5dee784 C
+	label branch-point
+	pick ca2c861 F
+	pick 088b00a G
+	label H
+
+	reset branch-point # C
+	pick 12bd07b D
+	merge -C 2051b56 E # E
+	merge -C 233d48a H # H
+
+	EOF
+
+	grep -v "^#" <.git/ORIGINAL-TODO >output &&
+	test_cmp expect output
+'
+
+test_expect_success '`reset` refuses to overwrite untracked files' '
+	git checkout -b refuse-to-reset &&
+	test_commit dont-overwrite-untracked &&
+	git checkout @{-1} &&
+	: >dont-overwrite-untracked.t &&
+	echo "reset refs/tags/dont-overwrite-untracked" >script-from-scratch &&
+	test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
+	test_must_fail git rebase -r HEAD &&
+	git rebase --abort
+'
+
+test_expect_success 'failed `merge` writes patch (may be rescheduled, too)' '
+	test_when_finished "test_might_fail git rebase --abort" &&
+	git checkout -b conflicting-merge A &&
+
+	: fail because of conflicting untracked file &&
+	>G.t &&
+	echo "merge -C H G" >script-from-scratch &&
+	test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
+	test_tick &&
+	test_must_fail git rebase -ir HEAD &&
+	grep "^merge -C .* G$" .git/rebase-merge/done &&
+	grep "^merge -C .* G$" .git/rebase-merge/git-rebase-todo &&
+	test_path_is_file .git/rebase-merge/patch &&
+
+	: fail because of merge conflict &&
+	rm G.t .git/rebase-merge/patch &&
+	git reset --hard &&
+	test_commit conflicting-G G.t not-G conflicting-G &&
+	test_must_fail git rebase --continue &&
+	! grep "^merge -C .* G$" .git/rebase-merge/git-rebase-todo &&
+	test_path_is_file .git/rebase-merge/patch
+'
+
+test_expect_success 'with a branch tip that was cherry-picked already' '
+	git checkout -b already-upstream master &&
+	base="$(git rev-parse --verify HEAD)" &&
+
+	test_commit A1 &&
+	test_commit A2 &&
+	git reset --hard $base &&
+	test_commit B1 &&
+	test_tick &&
+	git merge -m "Merge branch A" A2 &&
+
+	git checkout -b upstream-with-a2 $base &&
+	test_tick &&
+	git cherry-pick A2 &&
+
+	git checkout already-upstream &&
+	test_tick &&
+	git rebase -i -r upstream-with-a2 &&
+	test_cmp_graph upstream-with-a2.. <<-\EOF
+	*   Merge branch A
+	|\
+	| * A1
+	* | B1
+	|/
+	o A2
+	EOF
+'
+
+test_done

-- 
2.17.0.windows.1.33.gfcbb1fa0445