[GSoC PATCH v2 4/4] repo: add path.commondir with absolute and relative suffix formatting

[K Jayatheerth <jayatheerthkulkarni2005@gmail.com>]

In standard Git repositories, the Git directory and the common directory
are identical. However, in environments utilizing multiple worktrees, the
local working state ($GIT_DIR) is separated from the shared central data
($GIT_COMMON_DIR). Scripts require a reliable way to discover this shared
path.

Introduce `path.commondir.absolute` and `path.commondir.relative` keys
to `git repo info`. Similar to the `path.gitdir` keys, exposing explicit
format variants removes the ambiguity of default fallbacks. Both keys are
evaluated via the `format_path()` engine.

Insert the new keys into the `repo_info_field` array in lexicographical
order to maintain the integrity of binary search lookups.

Utilize the parameterized `test_repo_info_path` helper to validate the
worktree edge cases. This ensures that path resolution correctly respects
$GIT_COMMON_DIR when defined and safely falls back to $GIT_DIR otherwise.

Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Mentored-by: Justin Tobler <jltobler@gmail.com>
Mentored-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
---
 Documentation/git-repo.adoc |  9 +++++++++
 builtin/repo.c              | 24 ++++++++++++++++++++++++
 t/t1900-repo-info.sh        |  7 +++++++
 3 files changed, 40 insertions(+)

diff --git a/Documentation/git-repo.adoc b/Documentation/git-repo.adoc
index a0dca7ce88..ed7d80c690 100644
--- a/Documentation/git-repo.adoc
+++ b/Documentation/git-repo.adoc
@@ -104,6 +104,15 @@ values that they return:
 `object.format`::
 	The object format (hash algorithm) used in the repository.
 
+`path.commondir.absolute`::
+	The canonical absolute path to the Git repository's common
+	directory (the shared `.git` directory containing objects,
+	refs, and global configuration).
+
+`path.commondir.relative`::
+	The path to the Git repository's common directory relative to
+	the current working directory.
+
 `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 6e97f6a0e4..27c8caff38 100644
--- a/builtin/repo.c
+++ b/builtin/repo.c
@@ -77,6 +77,28 @@ static int get_object_format(struct repository *repo, struct strbuf *buf)
 	return 0;
 }
 
+static int get_path_commondir_absolute(struct repository *repo, struct strbuf *buf)
+{
+	const char *common_dir = repo_get_common_dir(repo);
+
+	if (!common_dir)
+		return error(_("unable to get common directory"));
+
+	format_path(buf, common_dir, startup_info->prefix, PATH_FORMAT_CANONICAL);
+	return 0;
+}
+
+static int get_path_commondir_relative(struct repository *repo, struct strbuf *buf)
+{
+	const char *common_dir = repo_get_common_dir(repo);
+
+	if (!common_dir)
+		return error(_("unable to get common directory"));
+
+	format_path(buf, common_dir, startup_info->prefix, PATH_FORMAT_RELATIVE);
+	return 0;
+}
+
 static int get_path_gitdir_absolute(struct repository *repo, struct strbuf *buf)
 {
 	const char *git_dir = repo_get_git_dir(repo);
@@ -111,6 +133,8 @@ static const struct repo_info_field repo_info_field[] = {
 	{ "layout.bare", get_layout_bare },
 	{ "layout.shallow", get_layout_shallow },
 	{ "object.format", get_object_format },
+	{ "path.commondir.absolute", get_path_commondir_absolute },
+	{ "path.commondir.relative", get_path_commondir_relative },
 	{ "path.gitdir.absolute", get_path_gitdir_absolute },
 	{ "path.gitdir.relative", get_path_gitdir_relative },
 	{ "references.format", get_references_format },
diff --git a/t/t1900-repo-info.sh b/t/t1900-repo-info.sh
index 0660b00bbc..21755d9d14 100755
--- a/t/t1900-repo-info.sh
+++ b/t/t1900-repo-info.sh
@@ -186,6 +186,13 @@ test_expect_success 'setup test repository layout for path fields' '
 	mkdir -p test-repo/sub
 '
 
+test_expect_success 'setup custom-common for commondir tests' '
+	git init --bare test-repo/custom-common
+'
+
+test_repo_info_path 'commondir' 'echo "$(cd .. && pwd)/.git"' '../.git'
+test_repo_info_path 'commondir' 'echo "$(cd .. && pwd)/custom-common"' '../custom-common' 'GIT_COMMON_DIR="$(cd .. && pwd)/custom-common" GIT_DIR=../.git'
+test_repo_info_path 'commondir' 'echo "$(cd .. && pwd)/.git"' '../.git' 'GIT_DIR=../.git'
 test_repo_info_path 'gitdir' 'echo "$(cd .. && pwd)/.git"' '../.git'
 
 test_done
-- 
2.54.0