repo: add path.superproject-root with absolute and relative suffixes

Scripts working in multi-repository setups often need to identify the
top-level working tree of a superproject from within a submodule.
Currently, this is only exposed via `git rev-parse
--show-superproject-working-tree`.

Introduce `path.superproject-root.absolute` and
`path.superproject-root.relative` keys to `git repo info`.
This exposes the core submodule context via a scriptable config-like key
using standard format rules.

If requested when not inside a submodule, the command returns an empty
string.

Mentored-by: Justin Tobler <jltobler@gmail.com>
Mentored-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-repo.adoc

@@ -119,6 +119,16 @@ values that they return:
 `path.gitdir.relative`::
 	The path to the Git repository directory relative to the current working directory.
 
+`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
+	string if not in a submodule.
+
+`path.superproject-root.relative`::
+	The path to the working tree root of the superproject relative to the
+	current working directory if the current repository is an initialized
+	submodule. Outputs an empty string if not in a submodule.
+
 `path.toplevel.absolute`::
 	The canonical absolute path to the top-level directory of the
 	repository's working tree. Outputs an empty string if the repository

builtin/repo.c

@@ -18,6 +18,7 @@
 #include "strbuf.h"
 #include "string-list.h"
 #include "shallow.h"
+#include "submodule.h"
 #include "tree.h"
 #include "tree-walk.h"
 #include "utf8.h"
@@ -121,6 +122,34 @@ static int get_path_gitdir_relative(struct repository *repo, struct strbuf *buf)
 	return 0;
 }
 
+static int get_path_superproject_absolute(struct repository *repo UNUSED, struct strbuf *buf)
+{
+	struct strbuf superproject = STRBUF_INIT;
+
+	if (!get_superproject_working_tree(&superproject)) {
+		strbuf_release(&superproject);
+		return 0;
+	}
+
+	format_path(buf, superproject.buf, "", PATH_FORMAT_CANONICAL);
+	strbuf_release(&superproject);
+	return 0;
+}
+
+static int get_path_superproject_relative(struct repository *repo, struct strbuf *buf)
+{
+	struct strbuf superproject = STRBUF_INIT;
+
+	if (!get_superproject_working_tree(&superproject)) {
+		strbuf_release(&superproject);
+		return 0;
+	}
+
+	format_path(buf, superproject.buf, repo->prefix, PATH_FORMAT_RELATIVE);
+	strbuf_release(&superproject);
+	return 0;
+}
+
 static int get_path_toplevel_absolute(struct repository *repo, struct strbuf *buf)
 {
 	const char *work_tree = repo_get_work_tree(repo);
@@ -159,6 +188,8 @@ static const struct repo_info_field repo_info_field[] = {
 	{ "path.commondir.relative", get_path_commondir_relative },
 	{ "path.gitdir.absolute", get_path_gitdir_absolute },
 	{ "path.gitdir.relative", get_path_gitdir_relative },
+	{ "path.superproject-root.absolute", get_path_superproject_absolute },
+	{ "path.superproject-root.relative", get_path_superproject_relative },
 	{ "path.toplevel.absolute", get_path_toplevel_absolute },
 	{ "path.toplevel.relative", get_path_toplevel_relative },
 	{ "references.format", get_references_format },

t/t1900-repo-info.sh

@@ -213,6 +213,45 @@ test_repo_info_path 'gitdir with explicit GIT_DIR' 'gitdir' \
 	'.git' \
 	'GIT_DIR="../.git" && export GIT_DIR'
 
+test_expect_success 'path.superproject-root absolute and relative' '
+	test_when_finished "rm -rf sub super" &&
+	git init sub &&
+	test_commit -C sub initial &&
+	git init super &&
+	(
+		cd super &&
+		git -c protocol.file.allow=always submodule add "../sub" sub &&
+		git commit -m "add submodule" &&
+
+		cd sub &&
+		ROOT="$(test-tool path-utils real_path ..)" &&
+
+		echo "path.superproject-root.absolute=$ROOT" >expect.abs &&
+		git repo info path.superproject-root.absolute >actual.abs &&
+		test_cmp expect.abs actual.abs &&
+
+		echo "path.superproject-root.relative=../" >expect.rel &&
+		git repo info path.superproject-root.relative >actual.rel &&
+		test_cmp expect.rel actual.rel
+	)
+'
+
+test_expect_success 'path.superproject-root returns empty when not in a submodule' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		echo "path.superproject-root.absolute=" >expect.abs &&
+		git repo info path.superproject-root.absolute >actual.abs &&
+		test_cmp expect.abs actual.abs &&
+
+		echo "path.superproject-root.relative=" >expect.rel &&
+		git repo info path.superproject-root.relative >actual.rel &&
+		test_cmp expect.rel actual.rel
+	)
+'
+
 test_expect_success 'path.toplevel absolute and relative' '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&