[PATCH v3 2/2] add-patch: classify '@' as a synonym for 'HEAD'

[Ghanshyam Thakkar <shyamthakkar001@gmail.com>]

Currently, (checkout, reset, restore) commands correctly take '@' as a
synonym for 'HEAD'. However, in patch mode (-p/--patch), for both '@' and
'HEAD', different prompts/messages are given by the commands mentioned
above. This is due to the literal and only string comparison with the word
'HEAD' in run_add_p(). Synonymity between '@' and 'HEAD' is obviously
desired, especially since '@' already resolves to 'HEAD'.

Therefore, make a new function user_meant_head() which takes the
revision string and compares it to 'HEAD' as well as '@'. However, in
builtin/checkout.c, there is a logic to convert all command line input
rev to the raw object name for underlying machinery (e.g., diff-index)
that does not recognize the <a>...<b> notation, but we'd need to leave
'HEAD' intact.  Now we need to teach that '@' is a synonym to 'HEAD'
 to that code and leave '@' intact, too.

There is one unintended side-effect/behavior change of this, even if
there exists a branch named '@', when providing '@' as a rev-source to
(checkout, reset, restore) commands in patch mode, it will consider it
as HEAD. This is due to the behavior of diff-index. However, naming a
branch '@' is an obvious foot-gun and there are many existing commands
which take '@' for 'HEAD' even if 'refs/heads/@' exists (e.g., 'git log
@', 'git push origin @' etc.). Therefore, this should be fine.

Also, add tests to check the above mentioned synonymity between 'HEAD'
and '@'.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
---
 add-patch.c               | 11 +++++++---
 builtin/checkout.c        | 11 +++++-----
 t/t2016-checkout-patch.sh | 46 ++++++++++++++++++++++-----------------
 t/t2071-restore-patch.sh  | 18 +++++++++------
 t/t7105-reset-patch.sh    | 10 +++++++++
 5 files changed, 61 insertions(+), 35 deletions(-)

diff --git a/add-patch.c b/add-patch.c
index 68f525b35c..7d565dcb33 100644
--- a/add-patch.c
+++ b/add-patch.c
@@ -378,6 +378,11 @@ static int parse_hunk_header(struct add_p_state *s, struct hunk *hunk)
 	return 0;
 }
 
+static inline int user_meant_head(const char *rev)
+{
+	return !strcmp(rev, "HEAD") || !strcmp(rev, "@");
+}
+
 static int is_octal(const char *p, size_t len)
 {
 	if (!len)
@@ -1729,21 +1734,21 @@ int run_add_p(struct repository *r, enum add_p_mode mode,
 	if (mode == ADD_P_STASH)
 		s.mode = &patch_mode_stash;
 	else if (mode == ADD_P_RESET) {
-		if (!revision || !strcmp(revision, "HEAD"))
+		if (!revision || user_meant_head(revision))
 			s.mode = &patch_mode_reset_head;
 		else
 			s.mode = &patch_mode_reset_nothead;
 	} else if (mode == ADD_P_CHECKOUT) {
 		if (!revision)
 			s.mode = &patch_mode_checkout_index;
-		else if (!strcmp(revision, "HEAD"))
+		else if (user_meant_head(revision))
 			s.mode = &patch_mode_checkout_head;
 		else
 			s.mode = &patch_mode_checkout_nothead;
 	} else if (mode == ADD_P_WORKTREE) {
 		if (!revision)
 			s.mode = &patch_mode_checkout_index;
-		else if (!strcmp(revision, "HEAD"))
+		else if (user_meant_head(revision))
 			s.mode = &patch_mode_worktree_head;
 		else
 			s.mode = &patch_mode_worktree_nothead;
diff --git a/builtin/checkout.c b/builtin/checkout.c
index a6e30931b5..79e208ee6d 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -539,12 +539,13 @@ static int checkout_paths(const struct checkout_opts *opts,
 		 * recognized by diff-index), we will always replace the name
 		 * with the hex of the commit (whether it's in `...` form or
 		 * not) for the run_add_interactive() machinery to work
-		 * properly. However, there is special logic for the HEAD case
-		 * so we mustn't replace that.  Also, when we were given a
-		 * tree-object, new_branch_info->commit would be NULL, but we
-		 * do not have to do any replacement, either.
+		 * properly. However, there is special logic for the 'HEAD' and
+		 * '@' case so we mustn't replace that.  Also, when we were
+		 * given a tree-object, new_branch_info->commit would be NULL,
+		 * but we do not have to do any replacement, either.
 		 */
-		if (rev && new_branch_info->commit && strcmp(rev, "HEAD"))
+		if (rev && new_branch_info->commit && strcmp(rev, "HEAD") &&
+		    strcmp(rev, "@"))
 			rev = oid_to_hex_r(rev_oid, &new_branch_info->commit->object.oid);
 
 		if (opts->checkout_index && opts->checkout_worktree)
diff --git a/t/t2016-checkout-patch.sh b/t/t2016-checkout-patch.sh
index 747eb5563e..c4f9bf09aa 100755
--- a/t/t2016-checkout-patch.sh
+++ b/t/t2016-checkout-patch.sh
@@ -38,26 +38,32 @@ test_expect_success 'git checkout -p with staged changes' '
 	verify_state dir/foo index index
 '
 
-test_expect_success 'git checkout -p HEAD with NO staged changes: abort' '
-	set_and_save_state dir/foo work head &&
-	test_write_lines n y n | git checkout -p HEAD &&
-	verify_saved_state bar &&
-	verify_saved_state dir/foo
-'
-
-test_expect_success 'git checkout -p HEAD with NO staged changes: apply' '
-	test_write_lines n y y | git checkout -p HEAD &&
-	verify_saved_state bar &&
-	verify_state dir/foo head head
-'
-
-test_expect_success 'git checkout -p HEAD with change already staged' '
-	set_state dir/foo index index &&
-	# the third n is to get out in case it mistakenly does not apply
-	test_write_lines n y n | git checkout -p HEAD &&
-	verify_saved_state bar &&
-	verify_state dir/foo head head
-'
+for opt in "HEAD" "@"
+do
+	test_expect_success "git checkout -p $opt with NO staged changes: abort" '
+		set_and_save_state dir/foo work head &&
+		test_write_lines n y n | git checkout -p $opt >output &&
+		verify_saved_state bar &&
+		verify_saved_state dir/foo &&
+		test_grep "Discard" output
+	'
+
+	test_expect_success "git checkout -p $opt with NO staged changes: apply" '
+		test_write_lines n y y | git checkout -p $opt >output &&
+		verify_saved_state bar &&
+		verify_state dir/foo head head &&
+		test_grep "Discard" output
+	'
+
+	test_expect_success "git checkout -p $opt with change already staged" '
+		set_state dir/foo index index &&
+		# the third n is to get out in case it mistakenly does not apply
+		test_write_lines n y n | git checkout -p $opt >output &&
+		verify_saved_state bar &&
+		verify_state dir/foo head head &&
+		test_grep "Discard" output
+	'
+done
 
 test_expect_success 'git checkout -p HEAD^...' '
 	# the third n is to get out in case it mistakenly does not apply
diff --git a/t/t2071-restore-patch.sh b/t/t2071-restore-patch.sh
index b5c5c0ff7e..3dc9184b4a 100755
--- a/t/t2071-restore-patch.sh
+++ b/t/t2071-restore-patch.sh
@@ -44,13 +44,17 @@ test_expect_success PERL 'git restore -p with staged changes' '
 	verify_state dir/foo index index
 '
 
-test_expect_success PERL 'git restore -p --source=HEAD' '
-	set_state dir/foo work index &&
-	# the third n is to get out in case it mistakenly does not apply
-	test_write_lines n y n | git restore -p --source=HEAD &&
-	verify_saved_state bar &&
-	verify_state dir/foo head index
-'
+for opt in "HEAD" "@"
+do
+	test_expect_success PERL "git restore -p --source=$opt" '
+		set_state dir/foo work index &&
+		# the third n is to get out in case it mistakenly does not apply
+		test_write_lines n y n | git restore -p --source=$opt >output &&
+		verify_saved_state bar &&
+		verify_state dir/foo head index &&
+		test_grep "Discard" output
+	'
+done
 
 test_expect_success PERL 'git restore -p --source=HEAD^' '
 	set_state dir/foo work index &&
diff --git a/t/t7105-reset-patch.sh b/t/t7105-reset-patch.sh
index 05079c7246..ec7f16dfb6 100755
--- a/t/t7105-reset-patch.sh
+++ b/t/t7105-reset-patch.sh
@@ -33,6 +33,16 @@ test_expect_success PERL 'git reset -p' '
 	test_grep "Unstage" output
 '
 
+for opt in "HEAD" "@"
+do
+	test_expect_success PERL "git reset -p $opt" '
+		test_write_lines n y | git reset -p $opt >output &&
+		verify_state dir/foo work head &&
+		verify_saved_state bar &&
+		test_grep "Unstage" output
+	'
+done
+
 test_expect_success PERL 'git reset -p HEAD^' '
 	test_write_lines n y | git reset -p HEAD^ >output &&
 	verify_state dir/foo work parent &&
-- 
2.43.0