[PATCH 3/3] commit: give correct advice for empty commit during a rebase

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

In dcb500dc16c (cherry-pick/revert: advise using --skip, 2019-07-02),
`git commit` learned to suggest to run `git cherry-pick --skip` when
trying to cherry-pick an empty patch.

However, it was overlooked that there are more conditions than just a
`git cherry-pick` when this advice is printed (which originally
suggested the neutral `git reset`): the same can happen during a rebase.

Let's suggest the correct command, even during a rebase.

While at it, we adjust more places in `builtin/commit.c` that
incorrectly assumed that the presence of a `CHERRY_PICK_HEAD` meant that
surely this must be a `cherry-pick` in progress.

Note: we take pains to handle the situation when a user runs a `git
cherry-pick` _during_ a rebase. This is quite valid (e.g. in an `exec`
line in an interactive rebase). On the other hand, it is not possible to
run a rebase during a cherry-pick, meaning: if both `rebase-merge/` and
`sequencer/` exist, we still want to advise to use `git cherry-pick
--skip`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/commit.c       | 33 ++++++++++++++++++++++++---------
 t/t3403-rebase-skip.sh |  9 +++++++++
 2 files changed, 33 insertions(+), 9 deletions(-)

diff --git a/builtin/commit.c b/builtin/commit.c
index e588bc6ad3..2beae13620 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -59,6 +59,9 @@ N_("The previous cherry-pick is now empty, possibly due to conflict resolution.\
 "    git commit --allow-empty\n"
 "\n");
 
+static const char empty_rebase_advice[] =
+N_("Otherwise, please use 'git rebase --skip'\n");
+
 static const char empty_cherry_pick_advice_single[] =
 N_("Otherwise, please use 'git cherry-pick --skip'\n");
 
@@ -122,7 +125,7 @@ static enum commit_msg_cleanup_mode cleanup_mode;
 static const char *cleanup_arg;
 
 static enum commit_whence whence;
-static int sequencer_in_use;
+static int sequencer_in_use, rebase_in_progress;
 static int use_editor = 1, include_status = 1;
 static int have_option_m;
 static struct strbuf message = STRBUF_INIT;
@@ -183,6 +186,8 @@ static void determine_whence(struct wt_status *s)
 		whence = FROM_CHERRY_PICK;
 		if (file_exists(git_path_seq_dir()))
 			sequencer_in_use = 1;
+		if (file_exists(git_path_rebase_merge_dir()))
+			rebase_in_progress = 1;
 	}
 	else
 		whence = FROM_COMMIT;
@@ -453,8 +458,11 @@ static const char *prepare_index(int argc, const char **argv, const char *prefix
 	if (whence != FROM_COMMIT) {
 		if (whence == FROM_MERGE)
 			die(_("cannot do a partial commit during a merge."));
-		else if (whence == FROM_CHERRY_PICK)
+		else if (whence == FROM_CHERRY_PICK) {
+			if (rebase_in_progress && !sequencer_in_use)
+				die(_("cannot do a partial commit during a rebase."));
 			die(_("cannot do a partial commit during a cherry-pick."));
+		}
 	}
 
 	if (list_paths(&partial, !current_head ? NULL : "HEAD", &pathspec))
@@ -950,10 +958,12 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 			fputs(_(empty_amend_advice), stderr);
 		else if (whence == FROM_CHERRY_PICK) {
 			fputs(_(empty_cherry_pick_advice), stderr);
-			if (!sequencer_in_use)
-				fputs(_(empty_cherry_pick_advice_single), stderr);
-			else
+			if (sequencer_in_use)
 				fputs(_(empty_cherry_pick_advice_multi), stderr);
+			else if (rebase_in_progress)
+				fputs(_(empty_rebase_advice), stderr);
+			else
+				fputs(_(empty_cherry_pick_advice_single), stderr);
 		}
 		return 0;
 	}
@@ -1156,8 +1166,11 @@ static int parse_and_validate_options(int argc, const char *argv[],
 	if (amend && whence != FROM_COMMIT) {
 		if (whence == FROM_MERGE)
 			die(_("You are in the middle of a merge -- cannot amend."));
-		else if (whence == FROM_CHERRY_PICK)
+		else if (whence == FROM_CHERRY_PICK) {
+			if (rebase_in_progress && !sequencer_in_use)
+				die(_("You are in the middle of a rebase -- cannot amend."));
 			die(_("You are in the middle of a cherry-pick -- cannot amend."));
+		}
 	}
 	if (fixup_message && squash_message)
 		die(_("Options --squash and --fixup cannot be used together"));
@@ -1628,9 +1641,11 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 			reduce_heads_replace(&parents);
 	} else {
 		if (!reflog_msg)
-			reflog_msg = (whence == FROM_CHERRY_PICK)
-					? "commit (cherry-pick)"
-					: "commit";
+			reflog_msg = (whence != FROM_CHERRY_PICK)
+					? "commit"
+					: rebase_in_progress && !sequencer_in_use
+					? "commit (rebase)"
+					: "commit (cherry-pick)";
 		commit_list_insert(current_head, &parents);
 	}
 
diff --git a/t/t3403-rebase-skip.sh b/t/t3403-rebase-skip.sh
index 1f5122b632..77b03ac49f 100755
--- a/t/t3403-rebase-skip.sh
+++ b/t/t3403-rebase-skip.sh
@@ -76,4 +76,13 @@ test_expect_success 'moved back to branch correctly' '
 
 test_debug 'gitk --all & sleep 1'
 
+test_expect_success 'correct advice upon empty commit' '
+	git checkout -b rebase-skip &&
+	test_commit a1 &&
+	test_tick &&
+	git commit --amend -m amended --no-edit &&
+	test_must_fail git rebase -m --onto a1 HEAD^ 2>err &&
+	test_i18ngrep "git rebase --skip" err
+'
+
 test_done
-- 
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