[v5,00/20] rebase: make the default backend configurable and change the default
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Message ID pull.679.v5.git.git.1581802602.gitgitgadget@gmail.com
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Elijah Newren via GitGitGadget Feb. 15, 2020, 9:36 p.m. UTC
This is v5 of en/rebase-backend based on v2.25.0. This series does a lot of
work around making the default rebase backend configurable, and switches the
default from the apply backend to the merge one.

Changes since v4:

 * Added a note about an unfortunately-timed Ctrl-C to the docs, as
   suggested by SZEDER.
 * Addressed feedback from Phillip
 * Added a new patch at the end renaming the two backends. PLEASE take a
   look at its commit message for rationale and let me know if you agree
   with it.

Note that in particular, the last patch at the end could have been partially
squashed into other patches, but if people objected to the patch in general
I wanted Junio to be able to take the rest of the series and just drop that
patch.

As with v4, there are some things mentioned earlier that are not addressed:

 * I did not remove the --am (now --apply) option as suggested by Phillip,
   since Junio and Phillip were still discussing whether it is
   wanted/needed. I kind of like having it, but have no strong opinion.
 * I noted the post-commit hook in the differences between backends. Emily
   is investigating what changes need to happen there, so I merely
   documented the existing differences.
 * I did not address the last two items Jonathan brought up as I couldn't
   find enough information to reproduce or understand the problems.

Elijah Newren (20):
  git-rebase.txt: update description of --allow-empty-message
  t3404: directly test the behavior of interest
  rebase (interactive-backend): make --keep-empty the default
  rebase (interactive-backend): fix handling of commits that become
    empty
  t3406: simplify an already simple test
  rebase, sequencer: remove the broken GIT_QUIET handling
  rebase: make sure to pass along the quiet flag to the sequencer
  rebase: fix handling of restrict_revision
  t3432: make these tests work with either am or merge backends
  rebase: allow more types of rebases to fast-forward
  git-rebase.txt: add more details about behavioral differences of
    backends
  rebase: move incompatibility checks between backend options a bit
    earlier
  rebase: add an --am option
  git-prompt: change the prompt for interactive-based rebases
  rebase: drop '-i' from the reflog for interactive-based rebases
  rebase tests: mark tests specific to the am-backend with --am
  rebase tests: repeat some tests using the merge backend instead of am
  rebase: make the backend configurable via config setting
  rebase: change the default backend from "am" to "merge"
  rebase: rename the two primary rebase backends

 Documentation/config/rebase.txt        |   6 +
 Documentation/git-rebase.txt           | 170 +++++++++++++---
 builtin/rebase.c                       | 257 +++++++++++++++++--------
 contrib/completion/git-prompt.sh       |   6 +-
 rebase-interactive.c                   |   7 +-
 rebase-interactive.h                   |   2 +-
 sequencer.c                            |  82 +++++---
 sequencer.h                            |   3 +-
 t/t3400-rebase.sh                      |  40 +++-
 t/t3401-rebase-and-am-rename.sh        |   4 +-
 t/t3404-rebase-interactive.sh          |  19 +-
 t/t3406-rebase-message.sh              |  19 +-
 t/t3407-rebase-abort.sh                |   6 +-
 t/t3420-rebase-autostash.sh            |  22 +--
 t/t3421-rebase-topology-linear.sh      |  48 ++---
 t/t3424-rebase-empty.sh                | 126 ++++++++++++
 t/t3425-rebase-topology-merges.sh      |   8 +-
 t/t3427-rebase-subtree.sh              |  12 +-
 t/t3432-rebase-fast-forward.sh         |  54 +++---
 t/t5407-post-rewrite-hook.sh           |  16 +-
 t/t5520-pull.sh                        |  27 ++-
 t/t6047-diff3-conflict-markers.sh      |  13 +-
 t/t7512-status-help.sh                 |  12 +-
 t/t9106-git-svn-commit-diff-clobber.sh |   3 +-
 t/t9903-bash-prompt.sh                 |   8 +-
 25 files changed, 693 insertions(+), 277 deletions(-)
 create mode 100755 t/t3424-rebase-empty.sh


base-commit: d0654dc308b0ba76dd8ed7bbb33c8d8f7aacd783
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-679%2Fnewren%2Frebase-fixes-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-679/newren/rebase-fixes-v5
Pull-Request: https://github.com/git/git/pull/679

Range-diff vs v4:

  1:  3ea48d53940 =  1:  3ea48d53940 git-rebase.txt: update description of --allow-empty-message
  2:  10fdd162a05 =  2:  10fdd162a05 t3404: directly test the behavior of interest
  3:  179f82ab83e !  3:  74cab9a32b3 rebase (interactive-backend): make --keep-empty the default
     @@ -4,13 +4,13 @@
      
          Different rebase backends have different treatment for commits which
          start empty (i.e. have no changes relative to their parent), and the
     -    --keep-empty option was added at some point to allow adjusting behavior
     -    for the interactive backend.  The handling of commits which start empty
     -    is actually quite similar to commit b00bf1c9a8dd (git-rebase: make
     -    --allow-empty-message the default, 2018-06-27), which pointed out that
     -    the behavior for various backends is often more happenstance than
     -    design.  The specific change made in that commit is actually quite
     -    relevant as well and much of the logic there directly applies here.
     +    --keep-empty option was added at some point to allow adjusting behavior.
     +    The handling of commits which start empty is actually quite similar to
     +    commit b00bf1c9a8dd (git-rebase: make --allow-empty-message the default,
     +    2018-06-27), which pointed out that the behavior for various backends is
     +    often more happenstance than design.  The specific change made in that
     +    commit is actually quite relevant as well and much of the logic there
     +    directly applies here.
      
          It makes a lot of sense in 'git commit' to error out on the creation of
          empty commits, unless an override flag is provided.  However, once
     @@ -405,8 +405,6 @@
      +	test_cmp expect actual
      +'
      +
     -+GIT_SEQUENCE_EDITOR=: && export GIT_SEQUENCE_EDITOR
     -+
      +test_expect_success 'rebase --interactive with a variety of empty commits' '
      +	git checkout -B testing localmods &&
      +	test_must_fail git rebase --interactive upstream &&
  4:  c9542a2abe0 !  4:  54b745c4ab5 rebase (interactive-backend): fix handling of commits that become empty
     @@ -98,6 +98,8 @@
      +	With ask (implied by --interactive), the rebase will halt when
      +	an empty commit is applied allowing you to choose whether to
      +	drop it, edit files more, or just commit the empty changes.
     ++	Other options, like --exec, will use the default of drop unless
     ++	-i/--interactive is explicitly specified.
      ++
      +Note that commits which start empty are kept, and commits which are
      +clean cherry-picks (as determined by `git log --cherry-mark ...`) are
     @@ -224,7 +226,7 @@
       			      REBASE_PRESERVE_MERGES, PARSE_OPT_HIDDEN),
       		OPT_RERERE_AUTOUPDATE(&options.allow_rerere_autoupdate),
      +		OPT_CALLBACK_F(0, "empty", &options, N_("{drop,keep,ask}"),
     -+			       N_("how to handle empty commits"),
     ++			       N_("how to handle commits that become empty"),
      +			       PARSE_OPT_NONEG, parse_opt_empty),
       		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,
       			N_("(DEPRECATED) keep empty commits"),
     @@ -342,8 +344,10 @@
      -	if (!opts->no_commit) {
      +	} else if (allow == 2) {
      +		drop_commit = 1;
     -+		fprintf(stderr, _("No changes -- Patch already applied.\n"));
     -+	} // else allow == 0 and there's nothing special to do
     ++		fprintf(stderr,
     ++			_("dropping %s %s -- patch contents already upstream\n"),
     ++			oid_to_hex(&commit->object.oid), msg.subject);
     ++	} /* else allow == 0 and there's nothing special to do */
      +	if (!opts->no_commit && !drop_commit) {
       		if (author || command == TODO_REVERT || (flags & AMEND_MSG))
       			res = do_commit(r, msg_file, author, opts, flags);
     @@ -404,9 +408,7 @@
      -test_expect_failure 'rebase --merge with a variety of empty commits' '
      -	test_when_finished "git rebase --abort" &&
      +test_expect_success 'rebase --merge --empty=drop' '
     - 	git checkout -B testing localmods &&
     --	# rebase --merge should not halt on the commit that becomes empty
     --	git rebase --merge upstream &&
     ++	git checkout -B testing localmods &&
      +	git rebase --merge --empty=drop upstream &&
      +
      +	test_write_lines D C B A >expect &&
     @@ -414,6 +416,17 @@
      +	test_cmp expect actual
      +'
      +
     ++test_expect_success 'rebase --merge uses default of --empty=drop' '
     + 	git checkout -B testing localmods &&
     +-	# rebase --merge should not halt on the commit that becomes empty
     + 	git rebase --merge upstream &&
     + 
     + 	test_write_lines D C B A >expect &&
     +@@
     + 	test_cmp expect actual
     + '
     + 
     +-test_expect_success 'rebase --interactive with a variety of empty commits' '
      +test_expect_success 'rebase --merge --empty=keep' '
      +	git checkout -B testing localmods &&
      +	git rebase --merge --empty=keep upstream &&
     @@ -428,14 +441,12 @@
      +	test_must_fail git rebase --merge --empty=ask upstream &&
      +
      +	git rebase --skip &&
     - 
     - 	test_write_lines D C B A >expect &&
     - 	git log --format=%s >actual &&
     -@@
     - 
     - GIT_SEQUENCE_EDITOR=: && export GIT_SEQUENCE_EDITOR
     - 
     --test_expect_success 'rebase --interactive with a variety of empty commits' '
     ++
     ++	test_write_lines D C B A >expect &&
     ++	git log --format=%s >actual &&
     ++	test_cmp expect actual
     ++'
     ++
      +test_expect_success 'rebase --interactive --empty=drop' '
      +	git checkout -B testing localmods &&
      +	git rebase --interactive --empty=drop upstream &&
     @@ -455,11 +466,19 @@
      +'
      +
      +test_expect_success 'rebase --interactive --empty=ask' '
     - 	git checkout -B testing localmods &&
     --	test_must_fail git rebase --interactive upstream &&
     ++	git checkout -B testing localmods &&
      +	test_must_fail git rebase --interactive --empty=ask upstream &&
     - 
     - 	git rebase --skip &&
     ++
     ++	git rebase --skip &&
     ++
     ++	test_write_lines D C B A >expect &&
     ++	git log --format=%s >actual &&
     ++	test_cmp expect actual
     ++'
     ++
     ++test_expect_success 'rebase --interactive uses default of --empty=ask' '
     + 	git checkout -B testing localmods &&
     + 	test_must_fail git rebase --interactive upstream &&
       
      
       diff --git a/t/t3427-rebase-subtree.sh b/t/t3427-rebase-subtree.sh
  5:  9f66229d5cc =  5:  f5170e71510 t3406: simplify an already simple test
  6:  8d731fa39c3 =  6:  2791b818c4d rebase, sequencer: remove the broken GIT_QUIET handling
  7:  b6b6597eef8 =  7:  3dccb58d066 rebase: make sure to pass along the quiet flag to the sequencer
  8:  0acefa988b1 =  8:  4e6f5ce8c46 rebase: fix handling of restrict_revision
  9:  8c5b5b51330 =  9:  bcd04ec4ded t3432: make these tests work with either am or merge backends
 10:  b8c087d6fb1 = 10:  45eb87b36b0 rebase: allow more types of rebases to fast-forward
 11:  b50a1741e0b = 11:  11e96b9a5fa git-rebase.txt: add more details about behavioral differences of backends
 12:  58e6e4ffb3f = 12:  0c921919dce rebase: move incompatibility checks between backend options a bit earlier
 13:  5478c730aca = 13:  de1b0aaca88 rebase: add an --am option
 14:  db5e29bd818 = 14:  e4c04d3cf8b git-prompt: change the prompt for interactive-based rebases
 15:  413e190ac9a ! 15:  f0f00c1e7fd rebase: drop '-i' from the reflog for interactive-based rebases
     @@ -54,17 +54,6 @@
       		 */
       		return error(_("%s: Unable to write new index file"),
       			_(action_name(opts)));
     -@@
     - 	int next = todo_list->current, offset, fd;
     - 
     - 	/*
     --	 * rebase -i writes "git-rebase-todo" without the currently executing
     --	 * command, appending it to "done" instead.
     -+	 * interactive backend writes "git-rebase-todo" without the currently
     -+	 * executing command, appending it to "done" instead.
     - 	 */
     - 	if (is_rebase_i(opts))
     - 		next++;
      @@
       		return error(_("illegal label name: '%.*s'"), len, name);
       
 16:  170be283a85 ! 16:  56486d4d694 rebase tests: mark tests specific to the am-backend with --am
     @@ -173,6 +173,163 @@
       testrebase " --interactive" .git/rebase-merge
       
      
     + diff --git a/t/t3421-rebase-topology-linear.sh b/t/t3421-rebase-topology-linear.sh
     + --- a/t/t3421-rebase-topology-linear.sh
     + +++ b/t/t3421-rebase-topology-linear.sh
     +@@
     + 		test_linear_range 'd e' c..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 		test_cmp_rev e HEAD
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 		test_linear_range 'd e' b..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success --fork-point
     + test_run_rebase success -m
     + test_run_rebase success -i
     +@@
     + 		test_linear_range 'd e' branch-b..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success --fork-point
     + test_run_rebase success -m
     + test_run_rebase success -i
     +@@
     + 		test_cmp_rev e HEAD
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success --fork-point
     + test_run_rebase success -m
     + test_run_rebase success -i
     +@@
     + 		test_linear_range 'd i' h..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 		test_linear_range 'd' h..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 		test_linear_range 'd i' f..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 		test_linear_range 'd gp i' h..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 		test_linear_range 'j d k l' c..
     + 	"
     + }
     +-test_run_rebase failure ''
     ++test_run_rebase failure --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase failure -p
     +@@
     + 		test_linear_range 'd k l' c..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 		test_linear_range 'd k l' j..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 		test_linear_range 'x y' c..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 		test_linear_range 'x y' c..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase failure -p
     +@@
     + 		test_linear_range 'x y' m..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase success -p
     +@@
     + 	"
     + }
     + 
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase failure -p
     +@@
     + 		test_linear_range 'x y' m..
     + 	"
     + }
     +-test_run_rebase success ''
     ++test_run_rebase success --am
     + test_run_rebase success -m
     + test_run_rebase success -i
     + test_have_prereq !REBASE_P || test_run_rebase failure -p
     +
       diff --git a/t/t3425-rebase-topology-merges.sh b/t/t3425-rebase-topology-merges.sh
       --- a/t/t3425-rebase-topology-merges.sh
       +++ b/t/t3425-rebase-topology-merges.sh
 17:  1e3d4066c40 = 17:  dc12574a8b3 rebase tests: repeat some tests using the merge backend instead of am
 18:  9b4ac83d2d0 = 18:  ba1f2699b39 rebase: make the backend configurable via config setting
 19:  859a4a94d7f ! 19:  59faaa2f920 rebase: change the default backend from "am" to "merge"
     @@ -13,6 +13,7 @@
              patches incorrectly.[2]
            * lack of access to original commits means that conflict marker
              annotation has less information available.
     +      * the am backend has safety problems with an ill-timed interrupt.
      
          Also, the merge/interactive backend have far more abilities, appear to
          currently have a slight performance advantage[3] and have room for more
     @@ -38,6 +39,24 @@
       +
       Note that a rebase merge works by replaying each commit from the working
       branch on top of the <upstream> branch.  Because of this, when a merge
     +@@
     + should have the same behavior, though it is not clear which one is
     + correct.
     + 
     ++Interruptability
     ++~~~~~~~~~~~~~~~~
     ++
     ++The am backend has safety problems with an ill-timed interrupt; if the
     ++user presses Ctrl-C at the wrong time to try to abort the rebase, the
     ++rebase can enter a state where it cannot be aborted with a subsequent
     ++`git rebase --abort`.  The interactive backend does not appear to
     ++suffer from the same shortcoming.  (See
     ++https://lore.kernel.org/git/20200207132152.GC2868@szeder.dev/ for
     ++details.)
     ++
     + Miscellaneous differences
     + ~~~~~~~~~~~~~~~~~~~~~~~~~
     + 
      
       diff --git a/builtin/rebase.c b/builtin/rebase.c
       --- a/builtin/rebase.c
  -:  ----------- > 20:  ad8339aebf2 rebase: rename the two primary rebase backends

Comments

Phillip Wood Feb. 16, 2020, 3:01 p.m. UTC | #1
Hi Elijah

On 15/02/2020 21:36, Elijah Newren via GitGitGadget wrote:
> This is v5 of en/rebase-backend based on v2.25.0. This series does a lot of
> work around making the default rebase backend configurable, and switches the
> default from the apply backend to the merge one.
> 
> Changes since v4:
> 
>   * Added a note about an unfortunately-timed Ctrl-C to the docs, as
>     suggested by SZEDER.

That's a good idea - I don't know why builtin/rebase.c only calls 
rebase_write_basic_state() which writes head-name after it has run 
format-patch and am. We write the patches to the state directory so it's 
not like it saves us from having to clean up.

>   * Addressed feedback from Phillip
>   * Added a new patch at the end renaming the two backends. PLEASE take a
>     look at its commit message for rationale and let me know if you agree
>     with it.

I had a look and I think it is a good idea - merge and apply make much 
more sense that interactive and am

> Note that in particular, the last patch at the end could have been partially
> squashed into other patches, but if people objected to the patch in general
> I wanted Junio to be able to take the rest of the series and just drop that
> patch.
> 
> As with v4, there are some things mentioned earlier that are not addressed:
> 
>   * I did not remove the --am (now --apply) option as suggested by Phillip,
>     since Junio and Phillip were still discussing whether it is
>     wanted/needed. I kind of like having it, but have no strong opinion.
>   * I noted the post-commit hook in the differences between backends. Emily
>     is investigating what changes need to happen there, so I merely
>     documented the existing differences.
>   * I did not address the last two items Jonathan brought up as I couldn't
>     find enough information to reproduce or understand the problems.
> 
> Elijah Newren (20):
>    git-rebase.txt: update description of --allow-empty-message
>    t3404: directly test the behavior of interest
>    rebase (interactive-backend): make --keep-empty the default
>    rebase (interactive-backend): fix handling of commits that become
>      empty
>    t3406: simplify an already simple test
>    rebase, sequencer: remove the broken GIT_QUIET handling
>    rebase: make sure to pass along the quiet flag to the sequencer
>    rebase: fix handling of restrict_revision
>    t3432: make these tests work with either am or merge backends
>    rebase: allow more types of rebases to fast-forward
>    git-rebase.txt: add more details about behavioral differences of
>      backends
>    rebase: move incompatibility checks between backend options a bit
>      earlier
>    rebase: add an --am option
>    git-prompt: change the prompt for interactive-based rebases
>    rebase: drop '-i' from the reflog for interactive-based rebases
>    rebase tests: mark tests specific to the am-backend with --am
>    rebase tests: repeat some tests using the merge backend instead of am
>    rebase: make the backend configurable via config setting
>    rebase: change the default backend from "am" to "merge"
>    rebase: rename the two primary rebase backends
> 
>   Documentation/config/rebase.txt        |   6 +
>   Documentation/git-rebase.txt           | 170 +++++++++++++---
>   builtin/rebase.c                       | 257 +++++++++++++++++--------
>   contrib/completion/git-prompt.sh       |   6 +-
>   rebase-interactive.c                   |   7 +-
>   rebase-interactive.h                   |   2 +-
>   sequencer.c                            |  82 +++++---
>   sequencer.h                            |   3 +-
>   t/t3400-rebase.sh                      |  40 +++-
>   t/t3401-rebase-and-am-rename.sh        |   4 +-
>   t/t3404-rebase-interactive.sh          |  19 +-
>   t/t3406-rebase-message.sh              |  19 +-
>   t/t3407-rebase-abort.sh                |   6 +-
>   t/t3420-rebase-autostash.sh            |  22 +--
>   t/t3421-rebase-topology-linear.sh      |  48 ++---
>   t/t3424-rebase-empty.sh                | 126 ++++++++++++
>   t/t3425-rebase-topology-merges.sh      |   8 +-
>   t/t3427-rebase-subtree.sh              |  12 +-
>   t/t3432-rebase-fast-forward.sh         |  54 +++---
>   t/t5407-post-rewrite-hook.sh           |  16 +-
>   t/t5520-pull.sh                        |  27 ++-
>   t/t6047-diff3-conflict-markers.sh      |  13 +-
>   t/t7512-status-help.sh                 |  12 +-
>   t/t9106-git-svn-commit-diff-clobber.sh |   3 +-
>   t/t9903-bash-prompt.sh                 |   8 +-
>   25 files changed, 693 insertions(+), 277 deletions(-)
>   create mode 100755 t/t3424-rebase-empty.sh
> 
> 
> base-commit: d0654dc308b0ba76dd8ed7bbb33c8d8f7aacd783
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-679%2Fnewren%2Frebase-fixes-v5
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-679/newren/rebase-fixes-v5
> Pull-Request: https://github.com/git/git/pull/679
> 
> Range-diff vs v4:
> 
>    1:  3ea48d53940 =  1:  3ea48d53940 git-rebase.txt: update description of --allow-empty-message
>    2:  10fdd162a05 =  2:  10fdd162a05 t3404: directly test the behavior of interest
>    3:  179f82ab83e !  3:  74cab9a32b3 rebase (interactive-backend): make --keep-empty the default
>       @@ -4,13 +4,13 @@
>        
>            Different rebase backends have different treatment for commits which
>            start empty (i.e. have no changes relative to their parent), and the
>       -    --keep-empty option was added at some point to allow adjusting behavior
>       -    for the interactive backend.  The handling of commits which start empty
>       -    is actually quite similar to commit b00bf1c9a8dd (git-rebase: make
>       -    --allow-empty-message the default, 2018-06-27), which pointed out that
>       -    the behavior for various backends is often more happenstance than
>       -    design.  The specific change made in that commit is actually quite
>       -    relevant as well and much of the logic there directly applies here.
>       +    --keep-empty option was added at some point to allow adjusting behavior.
>       +    The handling of commits which start empty is actually quite similar to
>       +    commit b00bf1c9a8dd (git-rebase: make --allow-empty-message the default,
>       +    2018-06-27), which pointed out that the behavior for various backends is
>       +    often more happenstance than design.  The specific change made in that
>       +    commit is actually quite relevant as well and much of the logic there
>       +    directly applies here.

Thanks for rewording this I think the new version is better for omitting 
the reference to the interactive backend. The interdiff for this patch 
and the next look good.

Thanks for working on this - it's turned into quite a large series

Best Wishes

Phillip

>            It makes a lot of sense in 'git commit' to error out on the creation of
>            empty commits, unless an override flag is provided.  However, once
>       @@ -405,8 +405,6 @@
>        +	test_cmp expect actual
>        +'
>        +
>       -+GIT_SEQUENCE_EDITOR=: && export GIT_SEQUENCE_EDITOR
>       -+
>        +test_expect_success 'rebase --interactive with a variety of empty commits' '
>        +	git checkout -B testing localmods &&
>        +	test_must_fail git rebase --interactive upstream &&
>    4:  c9542a2abe0 !  4:  54b745c4ab5 rebase (interactive-backend): fix handling of commits that become empty
>       @@ -98,6 +98,8 @@
>        +	With ask (implied by --interactive), the rebase will halt when
>        +	an empty commit is applied allowing you to choose whether to
>        +	drop it, edit files more, or just commit the empty changes.
>       ++	Other options, like --exec, will use the default of drop unless
>       ++	-i/--interactive is explicitly specified.
>        ++
>        +Note that commits which start empty are kept, and commits which are
>        +clean cherry-picks (as determined by `git log --cherry-mark ...`) are
>       @@ -224,7 +226,7 @@
>         			      REBASE_PRESERVE_MERGES, PARSE_OPT_HIDDEN),
>         		OPT_RERERE_AUTOUPDATE(&options.allow_rerere_autoupdate),
>        +		OPT_CALLBACK_F(0, "empty", &options, N_("{drop,keep,ask}"),
>       -+			       N_("how to handle empty commits"),
>       ++			       N_("how to handle commits that become empty"),
>        +			       PARSE_OPT_NONEG, parse_opt_empty),
>         		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,
>         			N_("(DEPRECATED) keep empty commits"),
>       @@ -342,8 +344,10 @@
>        -	if (!opts->no_commit) {
>        +	} else if (allow == 2) {
>        +		drop_commit = 1;
>       -+		fprintf(stderr, _("No changes -- Patch already applied.\n"));
>       -+	} // else allow == 0 and there's nothing special to do
>       ++		fprintf(stderr,
>       ++			_("dropping %s %s -- patch contents already upstream\n"),
>       ++			oid_to_hex(&commit->object.oid), msg.subject);
>       ++	} /* else allow == 0 and there's nothing special to do */
>        +	if (!opts->no_commit && !drop_commit) {
>         		if (author || command == TODO_REVERT || (flags & AMEND_MSG))
>         			res = do_commit(r, msg_file, author, opts, flags);
>       @@ -404,9 +408,7 @@
>        -test_expect_failure 'rebase --merge with a variety of empty commits' '
>        -	test_when_finished "git rebase --abort" &&
>        +test_expect_success 'rebase --merge --empty=drop' '
>       - 	git checkout -B testing localmods &&
>       --	# rebase --merge should not halt on the commit that becomes empty
>       --	git rebase --merge upstream &&
>       ++	git checkout -B testing localmods &&
>        +	git rebase --merge --empty=drop upstream &&
>        +
>        +	test_write_lines D C B A >expect &&
>       @@ -414,6 +416,17 @@
>        +	test_cmp expect actual
>        +'
>        +
>       ++test_expect_success 'rebase --merge uses default of --empty=drop' '
>       + 	git checkout -B testing localmods &&
>       +-	# rebase --merge should not halt on the commit that becomes empty
>       + 	git rebase --merge upstream &&
>       +
>       + 	test_write_lines D C B A >expect &&
>       +@@
>       + 	test_cmp expect actual
>       + '
>       +
>       +-test_expect_success 'rebase --interactive with a variety of empty commits' '
>        +test_expect_success 'rebase --merge --empty=keep' '
>        +	git checkout -B testing localmods &&
>        +	git rebase --merge --empty=keep upstream &&
>       @@ -428,14 +441,12 @@
>        +	test_must_fail git rebase --merge --empty=ask upstream &&
>        +
>        +	git rebase --skip &&
>       -
>       - 	test_write_lines D C B A >expect &&
>       - 	git log --format=%s >actual &&
>       -@@
>       -
>       - GIT_SEQUENCE_EDITOR=: && export GIT_SEQUENCE_EDITOR
>       -
>       --test_expect_success 'rebase --interactive with a variety of empty commits' '
>       ++
>       ++	test_write_lines D C B A >expect &&
>       ++	git log --format=%s >actual &&
>       ++	test_cmp expect actual
>       ++'
>       ++
>        +test_expect_success 'rebase --interactive --empty=drop' '
>        +	git checkout -B testing localmods &&
>        +	git rebase --interactive --empty=drop upstream &&
>       @@ -455,11 +466,19 @@
>        +'
>        +
>        +test_expect_success 'rebase --interactive --empty=ask' '
>       - 	git checkout -B testing localmods &&
>       --	test_must_fail git rebase --interactive upstream &&
>       ++	git checkout -B testing localmods &&
>        +	test_must_fail git rebase --interactive --empty=ask upstream &&
>       -
>       - 	git rebase --skip &&
>       ++
>       ++	git rebase --skip &&
>       ++
>       ++	test_write_lines D C B A >expect &&
>       ++	git log --format=%s >actual &&
>       ++	test_cmp expect actual
>       ++'
>       ++
>       ++test_expect_success 'rebase --interactive uses default of --empty=ask' '
>       + 	git checkout -B testing localmods &&
>       + 	test_must_fail git rebase --interactive upstream &&
>         
>        
>         diff --git a/t/t3427-rebase-subtree.sh b/t/t3427-rebase-subtree.sh
>    5:  9f66229d5cc =  5:  f5170e71510 t3406: simplify an already simple test
>    6:  8d731fa39c3 =  6:  2791b818c4d rebase, sequencer: remove the broken GIT_QUIET handling
>    7:  b6b6597eef8 =  7:  3dccb58d066 rebase: make sure to pass along the quiet flag to the sequencer
>    8:  0acefa988b1 =  8:  4e6f5ce8c46 rebase: fix handling of restrict_revision
>    9:  8c5b5b51330 =  9:  bcd04ec4ded t3432: make these tests work with either am or merge backends
>   10:  b8c087d6fb1 = 10:  45eb87b36b0 rebase: allow more types of rebases to fast-forward
>   11:  b50a1741e0b = 11:  11e96b9a5fa git-rebase.txt: add more details about behavioral differences of backends
>   12:  58e6e4ffb3f = 12:  0c921919dce rebase: move incompatibility checks between backend options a bit earlier
>   13:  5478c730aca = 13:  de1b0aaca88 rebase: add an --am option
>   14:  db5e29bd818 = 14:  e4c04d3cf8b git-prompt: change the prompt for interactive-based rebases
>   15:  413e190ac9a ! 15:  f0f00c1e7fd rebase: drop '-i' from the reflog for interactive-based rebases
>       @@ -54,17 +54,6 @@
>         		 */
>         		return error(_("%s: Unable to write new index file"),
>         			_(action_name(opts)));
>       -@@
>       - 	int next = todo_list->current, offset, fd;
>       -
>       - 	/*
>       --	 * rebase -i writes "git-rebase-todo" without the currently executing
>       --	 * command, appending it to "done" instead.
>       -+	 * interactive backend writes "git-rebase-todo" without the currently
>       -+	 * executing command, appending it to "done" instead.
>       - 	 */
>       - 	if (is_rebase_i(opts))
>       - 		next++;
>        @@
>         		return error(_("illegal label name: '%.*s'"), len, name);
>         
>   16:  170be283a85 ! 16:  56486d4d694 rebase tests: mark tests specific to the am-backend with --am
>       @@ -173,6 +173,163 @@
>         testrebase " --interactive" .git/rebase-merge
>         
>        
>       + diff --git a/t/t3421-rebase-topology-linear.sh b/t/t3421-rebase-topology-linear.sh
>       + --- a/t/t3421-rebase-topology-linear.sh
>       + +++ b/t/t3421-rebase-topology-linear.sh
>       +@@
>       + 		test_linear_range 'd e' c..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 		test_cmp_rev e HEAD
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 		test_linear_range 'd e' b..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success --fork-point
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       +@@
>       + 		test_linear_range 'd e' branch-b..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success --fork-point
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       +@@
>       + 		test_cmp_rev e HEAD
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success --fork-point
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       +@@
>       + 		test_linear_range 'd i' h..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 		test_linear_range 'd' h..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 		test_linear_range 'd i' f..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 		test_linear_range 'd gp i' h..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 		test_linear_range 'j d k l' c..
>       + 	"
>       + }
>       +-test_run_rebase failure ''
>       ++test_run_rebase failure --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase failure -p
>       +@@
>       + 		test_linear_range 'd k l' c..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 		test_linear_range 'd k l' j..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 		test_linear_range 'x y' c..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 		test_linear_range 'x y' c..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase failure -p
>       +@@
>       + 		test_linear_range 'x y' m..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase success -p
>       +@@
>       + 	"
>       + }
>       +
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase failure -p
>       +@@
>       + 		test_linear_range 'x y' m..
>       + 	"
>       + }
>       +-test_run_rebase success ''
>       ++test_run_rebase success --am
>       + test_run_rebase success -m
>       + test_run_rebase success -i
>       + test_have_prereq !REBASE_P || test_run_rebase failure -p
>       +
>         diff --git a/t/t3425-rebase-topology-merges.sh b/t/t3425-rebase-topology-merges.sh
>         --- a/t/t3425-rebase-topology-merges.sh
>         +++ b/t/t3425-rebase-topology-merges.sh
>   17:  1e3d4066c40 = 17:  dc12574a8b3 rebase tests: repeat some tests using the merge backend instead of am
>   18:  9b4ac83d2d0 = 18:  ba1f2699b39 rebase: make the backend configurable via config setting
>   19:  859a4a94d7f ! 19:  59faaa2f920 rebase: change the default backend from "am" to "merge"
>       @@ -13,6 +13,7 @@
>                patches incorrectly.[2]
>              * lack of access to original commits means that conflict marker
>                annotation has less information available.
>       +      * the am backend has safety problems with an ill-timed interrupt.
>        
>            Also, the merge/interactive backend have far more abilities, appear to
>            currently have a slight performance advantage[3] and have room for more
>       @@ -38,6 +39,24 @@
>         +
>         Note that a rebase merge works by replaying each commit from the working
>         branch on top of the <upstream> branch.  Because of this, when a merge
>       +@@
>       + should have the same behavior, though it is not clear which one is
>       + correct.
>       +
>       ++Interruptability
>       ++~~~~~~~~~~~~~~~~
>       ++
>       ++The am backend has safety problems with an ill-timed interrupt; if the
>       ++user presses Ctrl-C at the wrong time to try to abort the rebase, the
>       ++rebase can enter a state where it cannot be aborted with a subsequent
>       ++`git rebase --abort`.  The interactive backend does not appear to
>       ++suffer from the same shortcoming.  (See
>       ++https://lore.kernel.org/git/20200207132152.GC2868@szeder.dev/ for
>       ++details.)
>       ++
>       + Miscellaneous differences
>       + ~~~~~~~~~~~~~~~~~~~~~~~~~
>       +
>        
>         diff --git a/builtin/rebase.c b/builtin/rebase.c
>         --- a/builtin/rebase.c
>    -:  ----------- > 20:  ad8339aebf2 rebase: rename the two primary rebase backends
>