commit: refuse to amend during conflict resolution

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -1324,15 +1324,30 @@ static int parse_and_validate_options(int argc, const char *argv[],
 		use_editor = 0;
 
 	/* Sanity check options */
-	if (amend && !current_head)
-		die(_("You have nothing to amend."));
-	if (amend && whence != FROM_COMMIT) {
-		if (whence == FROM_MERGE)
+	if (amend) {
+		if (!current_head)
+			die(_("You have nothing to amend."));
+		/*
+		 * Refuse to amend in the middle of any operation that is
+		 * meant to record its result as a new commit on top of HEAD
+		 * rather than by rewriting HEAD.
+		 */
+		switch (sequencer_ongoing_operation(s->repo, whence)) {
+		case ONGOING_NONE:
+			break;
+		case ONGOING_MERGE:
 			die(_("You are in the middle of a merge -- cannot amend."));
-		else if (is_from_cherry_pick(whence))
+		case ONGOING_CHERRY_PICK:
 			die(_("You are in the middle of a cherry-pick -- cannot amend."));
-		else if (is_from_rebase_now_empty(whence))
+		case ONGOING_REBASE_NOW_EMPTY:
 			die(_("The now-empty commit has been dropped -- cannot amend."));
+		case ONGOING_REVERT:
+			die(_("You are in the middle of a revert -- cannot amend."));
+		case ONGOING_AM:
+			die(_("You are in the middle of an am session -- cannot amend."));
+		case ONGOING_REBASE_CONFLICT:
+			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");

sequencer.c

@@ -142,6 +142,13 @@ static GIT_PATH_FUNC(rebase_path_author_script, "rebase-merge/author-script")
  * command is processed, this file is deleted.
  */
 static GIT_PATH_FUNC(rebase_path_amend, "rebase-merge/amend")
+/*
+ * The apply ("am") backend keeps its state in the rebase-apply directory;
+ * the "applying" file within it marks a plain `git am` (as opposed to an
+ * apply-based rebase).
+ */
+static GIT_PATH_FUNC(apply_dir, "rebase-apply")
+static GIT_PATH_FUNC(apply_path_applying, "rebase-apply/applying")
 /*
  * When we stop at a given patch via the "edit" command, this file contains
  * the commit object name of the corresponding patch.
@@ -6865,6 +6872,56 @@ int sequencer_determine_whence(struct repository *r, enum commit_whence *whence)
 	return 0;
 }
 
+enum ongoing_operation sequencer_ongoing_operation(struct repository *r,
+						   enum commit_whence whence)
+{
+	/*
+	 * The merge, cherry-pick, and (empty) rebase-pick stops are already
+	 * distinguished by 'whence'.
+	 */
+	switch (whence) {
+	case FROM_MERGE:
+		return ONGOING_MERGE;
+	case FROM_CHERRY_PICK_SINGLE:
+	case FROM_CHERRY_PICK_MULTI:
+		return ONGOING_CHERRY_PICK;
+	case FROM_REBASE_NOW_EMPTY:
+		return ONGOING_REBASE_NOW_EMPTY;
+	case FROM_COMMIT:
+		break;
+	}
+
+	/*
+	 * 'whence' is FROM_COMMIT, but we may still be in the middle of an
+	 * operation that records its result on top of HEAD; detect those
+	 * from their on-disk state.
+	 */
+
+	/* In the middle of a revert? */
+	if (refs_ref_exists(get_main_ref_store(r), "REVERT_HEAD"))
+		return ONGOING_REVERT;
+
+	/* In the middle of an `am`? */
+	if (file_exists(apply_path_applying()))
+		return ONGOING_AM;
+
+	/*
+	 * In the middle of a rebase that stopped for conflict resolution?
+	 * The apply backend only ever stops for conflicts, so the presence
+	 * of its state directory is enough.  The merge backend writes
+	 * stopped-sha whenever it hands control back to the user, but omits
+	 * `amend` unless it stopped with HEAD already pointing at the commit
+	 * to be amended (a clean edit/reword stop); its absence therefore
+	 * marks a conflicted stop.
+	 */
+	if (file_exists(apply_dir()) ||
+	    (file_exists(rebase_path_stopped_sha()) &&
+	     !file_exists(rebase_path_amend())))
+		return ONGOING_REBASE_CONFLICT;
+
+	return ONGOING_NONE;
+}
+
 int sequencer_get_update_refs_state(const char *wt_dir,
 				    struct string_list *refs)
 {

sequencer.h

@@ -269,6 +269,29 @@ int sequencer_get_last_command(struct repository* r,
 			       enum replay_action *action);
 int sequencer_determine_whence(struct repository *r, enum commit_whence *whence);
 
+/*
+ * An in-progress operation that records its result (often a conflict
+ * resolution) as a new commit on top of HEAD, during which amending
+ * HEAD via "git commit --amend" is almost always a mistake.
+ */
+enum ongoing_operation {
+	ONGOING_NONE = 0,
+	ONGOING_MERGE,
+	ONGOING_CHERRY_PICK,
+	ONGOING_REBASE_NOW_EMPTY,
+	ONGOING_REVERT,
+	ONGOING_AM,
+	ONGOING_REBASE_CONFLICT
+};
+
+/*
+ * Return which in-progress operation, if any, is underway; see enum
+ * ongoing_operation.  'whence' is the origin already computed for the
+ * pending commit.
+ */
+enum ongoing_operation sequencer_ongoing_operation(struct repository *r,
+						   enum commit_whence whence);
+
 /**
  * Append the set of ref-OID pairs that are currently stored for the 'git
  * rebase --update-refs' feature if such a rebase is currently happening.

t/t3404-rebase-interactive.sh

@@ -1854,6 +1854,93 @@ test_expect_success 'correct error message for commit --amend after empty pick'
 	test_grep "now-empty commit has been dropped -- 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) &&

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 &&

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 &&