Re: [PATCH] commit: refuse to amend during conflict resolution

> From: Elijah Newren <newren@gmail.com>
> 
> Running `git commit --amend` during conflict resolution is an ugly
> foot-gun.  For many years, we have rejected amending during conflict
> resolution in the middle of
>    - a merge
>    - a cherry-pick
> However, this was never extended to other operations that can also
> produce conflicts:
>    - an `am` operation
>    - a revert
>    - a rebase
> 
> Extend it to handle these other cases now.

> Extending to `am`, revert, and the apply backend of rebase are fairly
> straightforward.  However, with the merge backend of rebase we have to
> be more careful, since it powers interactive rebases and
>    - the interactive machinery internally uses `git commit --amend` for
>      `squash` and `reword` directives
>    - users are expected to `git commit --amend` after hitting an `edit`
>      or `break` directive
> So, we need to be careful with rebase to only reject amending when doing
> conflict resolution.
> 
> A few files under the rebase-merge/ directory provide us the necessary
> information:
> 
>    - stopped-sha is written only when the rebase stops and hands control
>      back to the user, so its presence marks a genuine stop -- as opposed
>      to the sequencer's own internal `git commit --amend` while applying
>      a squash, fixup, or reword, during which no stopped-sha exists.
> 
>    - amend is written only when the rebase stops with HEAD already
>      pointing at the commit the user is meant to amend: a clean `edit`,
>      or a fast-forward `reword`. 

>      Its absence at a stop therefore means
>      the commit did not apply, so HEAD is the previously-applied commit
>      rather than the one being rebased -- exactly the case we refuse.
> 
> So for the merge backend we die when stopped-sha exists and amend does
> not.  This covers a plain conflicted pick as well as a conflicted `edit`
> (both leave HEAD on the previously-applied commit), while still allowing
> a clean `edit` or `reword` stop and a `break` stop (no stopped-sha).
> stopped-sha is unlinked at the start of the resume loop, so a resumed
> squash's internal amend is unaffected.
> 
> Signed-off-by: Elijah Newren <newren@gmail.com>
> ---
>      commit: refuse to amend during conflict resolution
> 
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2389%2Fnewren%2Frefuse-amend-during-conflicts-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2389/newren/refuse-amend-during-conflicts-v1
> Pull-Request: https://github.com/git/git/pull/2389
> 
>   builtin/commit.c                | 41 ++++++++++++++++
>   t/t3404-rebase-interactive.sh   | 87 +++++++++++++++++++++++++++++++++
>   t/t3507-cherry-pick-conflict.sh | 11 +++++
>   t/t4151-am-abort.sh             | 11 +++++
>   4 files changed, 150 insertions(+)
> 
> diff --git a/builtin/commit.c b/builtin/commit.c
> index 28f6174503..a9fd04366e 100644
> --- a/builtin/commit.c
> +++ b/builtin/commit.c
> @@ -30,6 +30,7 @@
>   #include "path.h"
>   #include "preload-index.h"
>   #include "read-cache.h"
> +#include "refs.h"
>   #include "repository.h"
>   #include "string-list.h"
>   #include "rerere.h"
> @@ -1336,6 +1337,46 @@ static int parse_and_validate_options(int argc, const char *argv[],
>   		else if (whence == FROM_REBASE_PICK)
>   			die(_("You are in the middle of a rebase -- cannot amend."));
>   	}
> +	if (amend && whence == FROM_COMMIT) {
> +		char *applying, *apply_dir, *stopped_sha, *amend_marker;
> +		int in_am, conflicted_stop;
> +
> +		/* Check middle of revert */
> +		if (refs_ref_exists(get_main_ref_store(the_repository),
> +				    "REVERT_HEAD"))
> +			die(_("You are in the middle of a revert -- cannot amend."));

> +		/* Check middle of `am` */
> +		applying = repo_git_path(the_repository,
> +					 "rebase-apply/applying");
> +		in_am = file_exists(applying);
> +
> +		free(applying);
> +		if (in_am)
> +			die(_("You are in the middle of an am session -- cannot amend."));
> +
> +		/* Check middle of rebase specifically stopped for conflicts */
> +		apply_dir = repo_git_path(the_repository,
> +					  "rebase-apply");
> +		stopped_sha = repo_git_path(the_repository,
> +					    "rebase-merge/stopped-sha");
> +		amend_marker = repo_git_path(the_repository,
> +					     "rebase-merge/amend");
> +		/*
> +		 * The apply backend only ever stops for conflicts; the
> +		 * merge backend writes stopped-sha but omits `amend`,
> +		 * which it writes only at a clean edit/reword stop.
> +		 */
> +		conflicted_stop =
> +			file_exists(apply_dir) ||
> +			(file_exists(stopped_sha) && !file_exists(amend_marker));
> +
> +		free(apply_dir);
> +		free(stopped_sha);
> +		free(amend_marker);
> +		if (conflicted_stop)
> +			die(_("You are resolving conflicts during a rebase -- cannot amend."));
> +	}
>   	if (fixup_message && squash_message)
>   		die(_("options '%s' and '%s' cannot be used together"), "--squash", "--fixup");
>   	die_for_incompatible_opt4(!!use_message, "-C",
> diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
> index ff11abb2f2..01d4735b3b 100755
> --- a/t/t3404-rebase-interactive.sh
> +++ b/t/t3404-rebase-interactive.sh
> @@ -1884,6 +1884,93 @@ test_expect_success 'correct error message for commit --amend after empty pick'
>   	test_grep "middle of a rebase -- cannot amend." err
>   '
>   
> +test_expect_success 'commit --amend is refused at a rebase conflict stop' '
> +	test_when_finished "git rebase --abort" &&
> +	git checkout --detach conflict-branch &&
> +	(
> +		set_fake_editor &&
> +		FAKE_LINES="1 3" &&
> +		export FAKE_LINES &&
> +		test_must_fail git rebase -i A
> +	) &&
> +	test_path_is_file .git/rebase-merge/patch &&
> +	test_path_is_missing .git/rebase-merge/amend &&
> +	echo resolved >conflict &&
> +	git add conflict &&
> +	test_must_fail git commit --amend --no-edit 2>err &&
> +	test_grep "You are resolving conflicts during a rebase -- cannot amend" err
> +'
> +
> +test_expect_success 'commit --amend is refused when an "edit" pick conflicts' '
> +	test_when_finished "git rebase --abort" &&
> +	git checkout --detach conflict-branch &&
> +	(
> +		set_fake_editor &&
> +		FAKE_LINES="1 edit 3" &&
> +		export FAKE_LINES &&
> +		test_must_fail git rebase -i A
> +	) &&
> +	test_path_is_file .git/rebase-merge/patch &&
> +	test_path_is_missing .git/rebase-merge/amend &&
> +	echo resolved >conflict &&
> +	git add conflict &&
> +	test_must_fail git commit --amend --no-edit 2>err &&
> +	test_grep "You are resolving conflicts during a rebase -- cannot amend" err
> +'
> +
> +test_expect_success 'commit --amend is allowed at a rebase edit stop' '
> +	test_when_finished "git rebase --abort" &&
> +	git checkout --detach no-conflict-branch &&
> +	(
> +		set_fake_editor &&
> +		FAKE_LINES="edit 1 2 3 4" &&
> +		export FAKE_LINES &&
> +		git rebase -i A
> +	) &&
> +	test_path_is_file .git/rebase-merge/amend &&
> +	echo tweak >fileJ &&
> +	git add fileJ &&
> +	git commit --amend --no-edit
> +'
> +
> +test_expect_success 'commit --amend is allowed at a rebase break stop' '
> +	test_when_finished "git rebase --abort" &&
> +	git checkout --detach no-conflict-branch &&
> +	(
> +		set_fake_editor &&
> +		FAKE_LINES="break 1 2 3 4" &&
> +		export FAKE_LINES &&
> +		git rebase -i A
> +	) &&
> +	test_must_fail git rev-parse --verify REBASE_HEAD &&
> +	echo tweak >fileJ &&
> +	git add fileJ &&
> +	git commit --amend --no-edit
> +'
> +
> +test_expect_success 'commit --amend is refused at an apply-backend conflict stop' '
> +	test_when_finished "rm -rf apply-backend" &&
> +	test_create_repo apply-backend &&
> +	(
> +		cd apply-backend &&
> +		test_commit base file &&
> +		git branch -M mainline &&
> +		test_commit upstream file upstream &&
> +		git checkout -b side mainline~1 &&
> +		test_commit conflicting file side &&
> +		test_commit unrelated other &&
> +		test_must_fail git rebase --apply mainline &&
> +		# the apply backend only ever stops for conflicts, and
> +		# leaves HEAD on the previously-applied commit
> +		test_path_is_dir .git/rebase-apply &&
> +		test_path_is_missing .git/rebase-apply/applying &&
> +		echo resolved >file &&
> +		git add file &&
> +		test_must_fail git commit --amend --no-edit 2>err &&
> +		test_grep "You are resolving conflicts during a rebase -- cannot amend" err
> +	)
> +'
> +
>   test_expect_success 'todo has correct onto hash' '
>   	GIT_SEQUENCE_EDITOR=cat git rebase -i no-conflict-branch~4 no-conflict-branch >actual &&
>   	onto=$(git rev-parse --short HEAD~4) &&
> diff --git a/t/t3507-cherry-pick-conflict.sh b/t/t3507-cherry-pick-conflict.sh
> index 44596cb1e8..42de398f76 100755
> --- a/t/t3507-cherry-pick-conflict.sh
> +++ b/t/t3507-cherry-pick-conflict.sh
> @@ -364,6 +364,17 @@ test_expect_success 'failed revert sets REVERT_HEAD' '
>   	test_cmp_rev picked REVERT_HEAD
>   '
>   
> +test_expect_success 'commit --amend of revert fails' '
> +	pristine_detach initial &&
> +
> +	test_must_fail git revert picked &&
> +	echo resolved >foo &&
> +	git add foo &&
> +	test_must_fail git commit --amend 2>err &&
> +
> +	test_grep "in the middle of a revert -- cannot amend." err
> +'
> +
>   test_expect_success 'successful revert does not set REVERT_HEAD' '
>   	pristine_detach base &&
>   	git revert base &&
> diff --git a/t/t4151-am-abort.sh b/t/t4151-am-abort.sh
> index 8e1ecf8a68..9313a074b2 100755
> --- a/t/t4151-am-abort.sh
> +++ b/t/t4151-am-abort.sh
> @@ -63,6 +63,17 @@ do
>   
>   done
>   
> +test_expect_success 'commit --amend during a failed am fails' '
> +	git reset --hard initial &&
> +	cp file-2-expect file-2 &&
> +	test_must_fail git am 000[1245]-*.patch &&
> +	echo resolved >file-1 &&
> +	git add file-1 &&
> +	test_must_fail git commit --amend 2>err &&
> +	test_grep "in the middle of an am session -- cannot amend." err &&
> +	git am --abort
> +'
> +
>   test_expect_success 'am -3 --skip removes otherfile-4' '
>   	git reset --hard initial &&
>   	test_must_fail git am -3 0003-*.patch &&
> 
> base-commit: 2c3adbb2c475981e340c79fdc5e7f4f9b5d9054e