[GSoC Patch v8 5/7] repo: add path.index with absolute and relative suffixes

[K Jayatheerth <jayatheerthkulkarni2005@gmail.com>]

The repository index is a fundamental component used by Git and related
tooling to track the working tree state. Scripts that interact with the
index currently retrieve its location by invoking
`git rev-parse --git-path index`.

Introduce `path.index.absolute` and `path.index.relative` keys to
`git repo info`. This exposes the index file location as a scriptable
config-like key using standard format rules, allowing scripts to
retrieve it through the same interface as other repository path
information.

Mentored-by: Justin Tobler <jltobler@gmail.com>
Mentored-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
---
 Documentation/git-repo.adoc | 12 ++++++++++++
 builtin/repo.c              | 18 ++++++++++++++++++
 t/t1900-repo-info.sh        | 23 +++++++++++++++++++++++
 3 files changed, 53 insertions(+)

diff --git a/Documentation/git-repo.adoc b/Documentation/git-repo.adoc
index 526a4ccfbd..2240aae73e 100644
--- a/Documentation/git-repo.adoc
+++ b/Documentation/git-repo.adoc
@@ -129,6 +129,18 @@ values that they return:
 	working directory. Respects the `core.hooksPath` configuration. If
 	`core.hooksPath` is set to `/dev/null`, that value is returned unchanged.
 
+`path.index.absolute`::
+	The canonical absolute path to the repository's current index file.
+	Respects the `GIT_INDEX_FILE` environment override. Returns the
+	configured index path even if the repository is bare or the file does
+	not exist.
+
+`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.
+	Returns the configured index path even if the repository is bare or the
+	file does not exist.
+
 `path.superproject-root.absolute`::
 	The canonical absolute path to the working tree root of the superproject
 	if the current repository is an initialized submodule. Outputs an empty
diff --git a/builtin/repo.c b/builtin/repo.c
index 739ac2974c..859bd87617 100644
--- a/builtin/repo.c
+++ b/builtin/repo.c
@@ -150,6 +150,22 @@ 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);
+
+	format_path(buf, index_file, "", 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);
+
+	format_path(buf, index_file, repo->prefix, PATH_FORMAT_RELATIVE);
+	return 0;
+}
+
 static int get_path_superproject_absolute(struct repository *repo, struct strbuf *buf)
 {
 	struct strbuf superproject = STRBUF_INIT;
@@ -212,6 +228,8 @@ 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.superproject-root.absolute", get_path_superproject_absolute },
 	{ "path.superproject-root.relative", get_path_superproject_relative },
 	{ "path.toplevel.absolute", get_path_toplevel_absolute },
diff --git a/t/t1900-repo-info.sh b/t/t1900-repo-info.sh
index 5fefb0ca79..140dfbe68a 100755
--- a/t/t1900-repo-info.sh
+++ b/t/t1900-repo-info.sh
@@ -238,6 +238,29 @@ test_expect_success 'hooks with core.hooksPath=/dev/null' '
 	)
 '
 
+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_expect_success 'path.superproject-root absolute and relative' '
 	test_when_finished "rm -rf sub super" &&
 	git init sub &&
-- 
2.56.0-rc2