[PATCH v3 7/7] repo: add path.git-prefix path key

Scripts and command-line prompt integrations frequently need to know their
relative depth inside a repository working tree layout. Currently, this
is retrieved using `git rev-parse --show-prefix`.

Introduce the `path.git-prefix` key to `git repo info`. This mirrors the
prefix location tracking framework as a standalone key, returning the
exact relative path offset complete with a trailing slash, or an empty
string if run directly at the repository working tree root.

Mentored-by: Justin Tobler <[email protected]>
Mentored-by: Lucas Seiki Oshiro <[email protected]>
Signed-off-by: K Jayatheerth <[email protected]>
---
 Documentation/git-repo.adoc |  5 +++++
 builtin/repo.c              | 11 +++++++++++
 t/t1900-repo-info.sh        | 19 +++++++++++++++++++
 3 files changed, 35 insertions(+)

diff --git a/Documentation/git-repo.adoc b/Documentation/git-repo.adoc
index def888da91..98282ea668 100644
--- a/Documentation/git-repo.adoc
+++ b/Documentation/git-repo.adoc
@@ -113,6 +113,11 @@ values that they return:
 	The path to the Git repository's common directory relative to
 	the current working directory.
 
+`path.git-prefix`::
+	The relative path from the top-level directory of the working tree to
+	the current working directory (including a trailing slash). Outputs an
+	empty string if executed at the root of the working tree.
+
 `path.gitdir.absolute`::
 	The canonical absolute path to the Git repository directory (the `.git` directory).
 
diff --git a/builtin/repo.c b/builtin/repo.c
index a97ad71649..b93c140c74 100644
--- a/builtin/repo.c
+++ b/builtin/repo.c
@@ -100,6 +100,16 @@ static int get_path_commondir_relative(struct repository *repo, struct strbuf *b
 	return 0;
 }
 
+static int get_path_git_prefix(struct repository *repo UNUSED, struct strbuf *buf)
+{
+	/*
+	 * startup_info->prefix is NULL if we are at the working tree root.
+	 * We add an empty string to ensure the buffer is cleanly initialized.
+	 */
+	strbuf_addstr(buf, startup_info->prefix ? startup_info->prefix : "");
+	return 0;
+}
+
 static int get_path_gitdir_absolute(struct repository *repo, struct strbuf *buf)
 {
 	const char *git_dir = repo_get_git_dir(repo);
@@ -278,6 +288,7 @@ static const struct repo_info_field repo_info_field[] = {
 	{ "object.format", get_object_format },
 	{ "path.commondir.absolute", get_path_commondir_absolute },
 	{ "path.commondir.relative", get_path_commondir_relative },
+	{ "path.git-prefix", get_path_git_prefix },
 	{ "path.gitdir.absolute", get_path_gitdir_absolute },
 	{ "path.gitdir.relative", get_path_gitdir_relative },
 	{ "path.grafts.absolute", get_path_grafts_absolute },
diff --git a/t/t1900-repo-info.sh b/t/t1900-repo-info.sh
index 15a4ec9b78..2879e54668 100755
--- a/t/t1900-repo-info.sh
+++ b/t/t1900-repo-info.sh
@@ -207,6 +207,25 @@ test_repo_info_path 'commondir with only GIT_DIR' 'commondir' \
 	'.git' \
 	'GIT_DIR="../.git" && export GIT_DIR'
 
+test_expect_success 'path.git-prefix at root and in a subdirectory' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		echo "path.git-prefix=" >expect.root &&
+		git repo info path.git-prefix >actual.root &&
+		test_cmp expect.root actual.root &&
+
+		mkdir -p sub/dir &&
+		cd sub/dir &&
+
+		echo "path.git-prefix=sub/dir/" >expect.sub &&
+		git repo info path.git-prefix >actual.sub &&
+		test_cmp expect.sub actual.sub
+	)
+'
+
 test_repo_info_path 'gitdir standard' 'gitdir' '.git'
 
 test_repo_info_path 'gitdir with explicit GIT_DIR' 'gitdir' \
-- 
2.55.GIT