[PATCH v3 5/7] repo: add path.index with absolute and relative suffix formatting

External script workflows and formatting layers require straightforward
access to the location of the index staging file. Currently, tracking
this necessitates a legacy call to `git rev-parse --git-path index` or
`--show-toplevel` logic abstractions.

Introduce `path.index.absolute` and `path.index.relative` keys to
`git repo info`. This allows tooling utilities to discover the active
index context cleanly while scaling transparently with localized
`GIT_INDEX_FILE` environment overrides.

Mentored-by: Justin Tobler <[email protected]>
Mentored-by: Lucas Seiki Oshiro <[email protected]>
Signed-off-by: K Jayatheerth <[email protected]>
---
 Documentation/git-repo.adoc | 12 ++++++++++++
 builtin/repo.c              | 24 ++++++++++++++++++++++++
 t/t1900-repo-info.sh        | 23 +++++++++++++++++++++++
 3 files changed, 59 insertions(+)

diff --git a/Documentation/git-repo.adoc b/Documentation/git-repo.adoc
index 7bc1c51310..34c4f7d61c 100644
--- a/Documentation/git-repo.adoc
+++ b/Documentation/git-repo.adoc
@@ -127,6 +127,18 @@ values that they return:
 	The path to the repository's hooks directory relative to the current
 	working directory. Respects `core.hooksPath` configuration adjustments.
 
+`path.index.absolute`::
+	The canonical absolute path to the repository's current index file.
+	Respects the `GIT_INDEX_FILE` environment override. The returned path
+	reflects the configured/default index location regardless of whether the
+	repository is bare or whether the file currently exists.
+
+`path.index.relative`::
+	The path to the repository's current index file relative to the current
+	working directory. Respects the `GIT_INDEX_FILE` environment override.
+	The returned path reflects the configured/default index location regardless
+	of whether the repository is bare or whether the file currently exists.
+
 `path.objects.absolute`::
 	The canonical absolute path to the repository's object database directory.
 	Respects the `GIT_OBJECT_DIRECTORY` environment override.
diff --git a/builtin/repo.c b/builtin/repo.c
index c921de222d..66bf4c67cc 100644
--- a/builtin/repo.c
+++ b/builtin/repo.c
@@ -142,6 +142,28 @@ static int get_path_hooks_relative(struct repository *repo, struct strbuf *buf)
 	return 0;
 }
 
+static int get_path_index_absolute(struct repository *repo, struct strbuf *buf)
+{
+	const char *index_file = repo_get_index_file(repo);
+
+	if (!index_file)
+		return error(_("unable to get index file"));
+
+	format_path(buf, index_file, startup_info->prefix, PATH_FORMAT_CANONICAL);
+	return 0;
+}
+
+static int get_path_index_relative(struct repository *repo, struct strbuf *buf)
+{
+	const char *index_file = repo_get_index_file(repo);
+
+	if (!index_file)
+		return error(_("unable to get index file"));
+
+	format_path(buf, index_file, startup_info->prefix, PATH_FORMAT_RELATIVE);
+	return 0;
+}
+
 static int get_path_objects_absolute(struct repository *repo, struct strbuf *buf)
 {
 	const char *obj_dir = repo_get_object_directory(repo);
@@ -238,6 +260,8 @@ static const struct repo_info_field repo_info_field[] = {
 	{ "path.gitdir.relative", get_path_gitdir_relative },
 	{ "path.hooks.absolute", get_path_hooks_absolute },
 	{ "path.hooks.relative", get_path_hooks_relative },
+	{ "path.index.absolute", get_path_index_absolute },
+	{ "path.index.relative", get_path_index_relative },
 	{ "path.objects.absolute", get_path_objects_absolute },
 	{ "path.objects.relative", get_path_objects_relative },
 	{ "path.superproject-working-tree.absolute", get_path_superproject_absolute },
diff --git a/t/t1900-repo-info.sh b/t/t1900-repo-info.sh
index cd3f856d04..dee1db2a49 100755
--- a/t/t1900-repo-info.sh
+++ b/t/t1900-repo-info.sh
@@ -219,6 +219,29 @@ test_repo_info_path 'hooks with core.hooksPath override' 'hooks' \
 	'custom-hooks' \
 	'git config core.hooksPath "$ROOT/custom-hooks" && mkdir -p "$ROOT/custom-hooks"'
 
+test_repo_info_path 'index standard' 'index' '.git/index'
+
+test_repo_info_path 'index with GIT_INDEX_FILE override' 'index' \
+	'custom-index-file' \
+	'GIT_INDEX_FILE="$ROOT/custom-index-file" && export GIT_INDEX_FILE'
+
+test_expect_success 'path.index in a bare repository returns default index location' '
+	test_when_finished "rm -rf bare.git" &&
+	git init --bare bare.git &&
+	(
+		cd bare.git &&
+		ROOT="$(test-tool path-utils real_path .)" &&
+
+		echo "path.index.absolute=$ROOT/index" >expect.abs &&
+		git repo info path.index.absolute >actual.abs &&
+		test_cmp expect.abs actual.abs &&
+
+		echo "path.index.relative=index" >expect.rel &&
+		git repo info path.index.relative >actual.rel &&
+		test_cmp expect.rel actual.rel
+	)
+'
+
 test_repo_info_path 'objects standard' 'objects' '.git/objects'
 
 test_repo_info_path 'objects with GIT_OBJECT_DIRECTORY override' 'objects' \
-- 
2.55.GIT