Re: [RFC WIP PATCH v8 13/13] WIP: can_fast_forward() support for --preserve-merges and --rebase-merges

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

[-- Attachment #1: Type: text/plain, Size: 6434 bytes --]

Hi Ævar,

On Wed, 8 May 2019, Ævar Arnfjörð Bjarmason wrote:

> This seems to work, needs more tests etc...

I can see how it works, but it is a bit limited, and at the same time
overzealous.

The reason why we do not enter the fast-forwarding block in the
interactive case would appear to me to be that the interactive rebase
*might* want to avoid fast-forwarding e.g. with --force-rebase.

However, that is not the only instance where we must not simply
fast-forward: Think `--exec`. There might be others, too. (I just saw that
`--signoff` sets `FORCE_REBASE`, but `--exec` does not, so you cannot even
use the `FORCE_REBASE` flag as indicator.)

Since `git rebase -rx "make -j8"` is something I use myself, this here
patch would therefore break *my* workflow.

This is the "overzealous" part. Now for the "limited" part:

Let's take a step back and ask how the interactive rebase handles these
potentially fast-forwarding cases? Via `skip_unnecessary_picks()`.

And that function is ill-prepared for rebasing merges (I specifically do
*not* think about `--preserve-merges` at this point, for all I care, it is
already deprecated *and* dropped).

Even if this function *was* well-prepared for rebasing merges, I think
that would miss the mark. Take this todo list, for example:

	label onto

	# Branch dscho
	reset onto
	pick a123 first
	label dscho

	# Branch avar
	reset onto
	pick b789 second
	label avar

	reset onto
	merge -C c124 dscho
	merge -C d314 avar

Two branches, both one patch deep, both merged, one after the other. Now,
if you insert `pick abc zeroth` before the first `pick`, obviously the
first branch would no longer be skippable, but the second one totally
would be!

This is the "limited" part.

To remedy this, I think what we would need is code in `pick_commits()`,
right where `TODO_RESET` is handled (or more toward the beginning of that
function), that would:

- parse the argument (this is currently done in `do_reset()` and would
  have to be refactored out) and pretend that it is `HEAD`,

- look at the following command: if it is

	- a `pick`, and if its parent agrees with `HEAD`, pretend that
	  the `pick` was actually a `reset`, update the pretended `HEAD`
	  and keep looking at the next command,

	- a `merge`, and if its option was `-C <orig-merge>` (not
	  lower-case `-c`!), and if its parent agrees with `HEAD`, and if
	  its merge head(s) agree with the original merge commit's (if
	  any), pretend that it was actually a `reset <orig-merge>`,
	  update the pretended `HEAD` and keep looking at the next
	  command,

	- a `label`, perform it, but with the pretended `HEAD`, and keep
	  looking for the next command,

	- a `reset`, update the `done` and `git-rebase-todo` files and
	  start the entire spiel from the top,

	- otherwise perform the reset.

- all while skipping, this code would need to take care of updating the
  `done` and `git-rebase-todo` files,

- if a `reset` is necessary, and if it fails, the `done` and
  `git-rebase-todo` files should *not* be updated, but the original
  `reset` should be re-scheduled, and

- since this adds quite a bit of code, it should probably be done in a
  separate function.

Instead of marking this as a left-over bit (which I would either forget,
or whose status would be hard to track), I decided to open a ticket:

	https://github.com/gitgitgadget/git/issues/209

(I opened GitGitGadget's issues for exactly this kind of use case, because
I recently tried to find some useful left-over bits as easy project
starters, and even *I* found it super hard to find those, let alone figure
out whether they are being/have been addressed already, a mailing list is
just not a good bug tracker, even if it is still better than trying to
report a bug on Twitter, where I could not even have written this
paragraph in a single Tweet.)

Ciao,
Dscho

> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
>  builtin/rebase.c               | 6 ++++++
>  t/t3432-rebase-fast-forward.sh | 7 +++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 167d4fcf67..de1c5cacb8 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -892,6 +892,12 @@ static void populate_merge_bases(struct commit *head, struct commit *onto,
>
>  static int should_fast_forward(struct rebase_options *opts)
>  {
> +	if (!(opts->flags & REBASE_INTERACTIVE_EXPLICIT)) {
> +		if (opts->rebase_merges)
> +			return 1;
> +		if (opts->type == REBASE_PRESERVE_MERGES)
> +			return 1;
> +	}
>  	return !is_interactive(opts);
>  }
>
> diff --git a/t/t3432-rebase-fast-forward.sh b/t/t3432-rebase-fast-forward.sh
> index e8a9bf42b6..d3e1815057 100755
> --- a/t/t3432-rebase-fast-forward.sh
> +++ b/t/t3432-rebase-fast-forward.sh
> @@ -44,12 +44,13 @@ test_rebase_same_head_ () {
>  	test_expect_$status "git rebase$flag $* with $changes is $what with $cmp HEAD" "
>  		oldhead=\$(git rev-parse HEAD) &&
>  		test_when_finished 'git reset --hard \$oldhead' &&
> -		git rebase$flag $* >stdout &&
> +		git rebase$flag $* >stdout 2>stderr &&
>  		if test $what = work
>  		then
>  			# Must check this case first, for 'is up to
>  			# date, rebase forced[...]rewinding head' cases
> -			test_i18ngrep 'rewinding head' stdout
> +			test_i18ngrep 'rewinding head' stdout ||
> +			test_i18ngrep 'is up to date, rebase forced' stdout
>  		elif test $what = noop
>  		then
>  			test_i18ngrep 'is up to date' stdout &&
> @@ -79,6 +80,8 @@ test_rebase_same_head success noop same success noop-force same --keep-base mast
>  test_rebase_same_head success noop same success noop-force same --keep-base
>  test_rebase_same_head success noop same success noop-force same --no-fork-point
>  test_rebase_same_head success noop same success noop-force same --keep-base --no-fork-point
> +test_rebase_same_head success noop same success noop-force same --preserve-merges
> +test_rebase_same_head success noop same success noop-force same --rebase-merges
>  test_rebase_same_head success noop same success work same --fork-point master
>  test_rebase_same_head success noop same success work diff --fork-point --onto B B
>  test_rebase_same_head success noop same success work diff --fork-point --onto B... B
> --
> 2.21.0.1020.gf2820cf01a
>
>