[PATCH v9 00/17] rebase -i: offer to recreate commit topology by rebasing merges

[Johannes Schindelin <johannes.schindelin@gmx.de>]

Junio, I think this is now ready for `next`. Thank you for your patience
and help with this.

Once upon a time, I dreamed of an interactive rebase that would not
linearize all patches and drop all merge commits, but instead recreate
the commit topology faithfully.

My original attempt was --preserve-merges, but that design was so
limited that I did not even enable it in interactive mode.

Subsequently, it *was* enabled in interactive mode, with the predictable
consequences: as the --preserve-merges design does not allow for
specifying the parents of merge commits explicitly, all the new commits'
parents are defined *implicitly* by the previous commit history, and
hence it is *not possible to even reorder commits*.

This design flaw cannot be fixed. Not without a complete re-design, at
least. This patch series offers such a re-design.

Think of --rebase-merges as "--preserve-merges done right". It
introduces new verbs for the todo list, `label`, `reset` and `merge`.
For a commit topology like this:

            A - B - C
              \   /
                D

the generated todo list would look like this:

            # branch D
            pick 0123 A
            label branch-point
            pick 1234 D
            label D

            reset branch-point
            pick 2345 B
            merge -C 3456 D # C

There are more patches in the pipeline, based on this patch series, but
left for later in the interest of reviewable patch series: one mini
series to use the sequencer even for `git rebase -i --root`, and another
one to add support for octopus merges to --rebase-merges. And then one
to allow for rebasing merge commits in a smarter way (this one will need
a bit more work, though, as it can result in very complicated, nested
merge conflicts *very* easily).

Changes since v8:

- Disentangled the patch introducing `label`/`reset` from the one
  introducing `merge` again (this was one stupid, tired `git commit
  --amend` too many).

- Augmented the commit message of "introduce the `merge` command" to
  describe what the `label onto` is all about.

- Fixed the error message when `reset` would overwrite untracked files to
  actually say that a "reset" failed (not a "merge").

- Clarified the rationale for `label onto` in the commit message of
  "rebase-helper --make-script: introduce a flag to rebase merges".

- Edited the description of `--rebase-merges` heavily, for clarity, in
  "rebase: introduce the --rebase-merges option".

- Edited the commit message of (and the documentation introduced by) " rebase
  -i: introduce --rebase-merges=[no-]rebase-cousins" for clarity (also
  mentioning the `--ancestry-path` option).

- When run_git_commit() fails after a successful merge, we now take pains
  not to reschedule the `merge` command.

- Rebased the patch series on top of current `master`, i.e. both
  `pw/rebase-keep-empty-fixes` and `pw/rebase-signoff`, to resolve merge
  conflicts myself.


Johannes Schindelin (15):
  sequencer: avoid using errno clobbered by rollback_lock_file()
  sequencer: make rearrange_squash() a bit more obvious
  sequencer: refactor how original todo list lines are accessed
  sequencer: offer helpful advice when a command was rescheduled
  sequencer: introduce new commands to reset the revision
  sequencer: introduce the `merge` command
  sequencer: fast-forward `merge` commands, if possible
  rebase-helper --make-script: introduce a flag to rebase merges
  rebase: introduce the --rebase-merges option
  sequencer: make refs generated by the `label` command worktree-local
  sequencer: handle post-rewrite for merge commands
  rebase --rebase-merges: avoid "empty merges"
  pull: accept --rebase=merges to recreate the branch topology
  rebase -i: introduce --rebase-merges=[no-]rebase-cousins
  rebase -i --rebase-merges: add a section to the man page

Phillip Wood (1):
  rebase --rebase-merges: add test for --keep-empty

Stefan Beller (1):
  git-rebase--interactive: clarify arguments

 Documentation/config.txt               |   8 +
 Documentation/git-pull.txt             |   6 +-
 Documentation/git-rebase.txt           | 163 ++++-
 builtin/pull.c                         |  14 +-
 builtin/rebase--helper.c               |  13 +-
 builtin/remote.c                       |  18 +-
 contrib/completion/git-completion.bash |   4 +-
 git-rebase--interactive.sh             |  22 +-
 git-rebase.sh                          |  16 +
 refs.c                                 |   3 +-
 sequencer.c                            | 892 ++++++++++++++++++++++++-
 sequencer.h                            |   7 +
 t/t3421-rebase-topology-linear.sh      |   1 +
 t/t3430-rebase-merges.sh               | 244 +++++++
 14 files changed, 1352 insertions(+), 59 deletions(-)
 create mode 100755 t/t3430-rebase-merges.sh


base-commit: 1f1cddd558b54bb0ce19c8ace353fd07b758510d
Published-As: https://github.com/dscho/git/releases/tag/recreate-merges-v9
Fetch-It-Via: git fetch https://github.com/dscho/git recreate-merges-v9

Interdiff vs v8:
 diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
 index e691b93e920..bd5ecff980e 100644
 --- a/Documentation/git-rebase.txt
 +++ b/Documentation/git-rebase.txt
 @@ -381,21 +381,24 @@ have the long commit hash prepended to the format.
  
  -r::
  --rebase-merges[=(rebase-cousins|no-rebase-cousins)]::
 -	By default, a rebase will simply drop merge commits and only rebase
 -	the non-merge commits. With this option, it will try to preserve
 +	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. If a merge commit resolved any merge
 -	or contained manual amendments, then they will have to be re-applied
 -	manually.
 +	by recreating the merge commits. Any resolved merge conflicts or
 +	manual amendments in these merge commits will have to be
 +	resolved/re-applied manually.
  +
  By default, or when `no-rebase-cousins` was specified, commits which do not
 -have `<upstream>` as direct ancestor will keep their original branch point.
 -If the `rebase-cousins` mode is turned on, such commits are instead rebased
 +have `<upstream>` as direct ancestor will keep their original branch point,
 +i.e. commits that would be excluded by gitlink:git-log[1]'s
 +`--ancestry-path` option will keep their original ancestry by default. If
 +the `rebase-cousins` mode is turned on, such commits are instead rebased
  onto `<upstream>` (or `<onto>`, if specified).
  +
 -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.
 +The `--rebase-merges` 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
 diff --git a/sequencer.c b/sequencer.c
 index b5715f69450..e2f83942843 100644
 --- a/sequencer.c
 +++ b/sequencer.c
 @@ -2635,6 +2635,7 @@ static int do_reset(const char *name, int len, struct replay_opts *opts)
  	}
  
  	memset(&unpack_tree_opts, 0, sizeof(unpack_tree_opts));
 +	setup_unpack_trees_porcelain(&unpack_tree_opts, "reset");
  	unpack_tree_opts.head_idx = 1;
  	unpack_tree_opts.src_index = &the_index;
  	unpack_tree_opts.dst_index = &the_index;
 @@ -2855,7 +2856,12 @@ static int do_merge(struct commit *commit, const char *arg, int arg_len,
  	if (ret)
  		rerere(opts->allow_rerere_auto);
  	else
 -		ret = run_git_commit(git_path_merge_msg(), opts,
 +		/*
 +		 * In case of problems, we now want to return a positive
 +		 * value (a negative one would indicate that the `merge`
 +		 * command needs to be rescheduled).
 +		 */
 +		ret = !!run_git_commit(git_path_merge_msg(), opts,
  				     run_commit_flags);
  
  leave_merge:
 @@ -3809,12 +3815,9 @@ int sequencer_make_script(FILE *out, int argc, const char **argv,
  
  	init_revisions(&revs, NULL);
  	revs.verbose_header = 1;
 -	if (rebase_merges)
 -		revs.cherry_mark = 1;
 -	else {
 +	if (!rebase_merges)
  		revs.max_parents = 1;
 -		revs.cherry_pick = 1;
 -	}
 +	revs.cherry_mark = 1;
  	revs.limited = 1;
  	revs.reverse = 1;
  	revs.right_only = 1;
-- 
2.17.0.windows.1.33.gfcbb1fa0445