[PATCH v3 2/4] introduce submodule.superprojectGitDir record

[Emily Shaffer <emilyshaffer@google.com>]

Teach submodules a reference to their superproject's gitdir. This allows
us to A) know that we're running from a submodule, and B) have a
shortcut to the superproject's vitals, for example, configs.

By using a relative path instead of an absolute path, we can move the
superproject directory around on the filesystem without breaking the
submodule's cache.

Since this hint value is only introduced during new submodule creation
via `git submodule add`, though, there is more work to do to allow the
record to be created at other times.

If the new config is present, we can do some optional value-added
behavior, like letting "git status" print additional info about the
submodule's status in relation to its superproject, or like letting the
superproject and submodule share an additional config file separate from
either one's local config.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/config/submodule.txt | 15 +++++++++++
 builtin/submodule--helper.c        |  4 +++
 t/t7400-submodule-basic.sh         | 40 ++++++++++++++++--------------
 3 files changed, 41 insertions(+), 18 deletions(-)

diff --git a/Documentation/config/submodule.txt b/Documentation/config/submodule.txt
index d7a63c8c12..23e0a01d90 100644
--- a/Documentation/config/submodule.txt
+++ b/Documentation/config/submodule.txt
@@ -90,3 +90,18 @@ submodule.alternateErrorStrategy::
 	`ignore`, `info`, `die`. Default is `die`. Note that if set to `ignore`
 	or `info`, and if there is an error with the computed alternate, the
 	clone proceeds as if no alternate was specified.
+
+submodule.superprojectGitDir::
+	The relative path from the submodule's worktree to its superproject's
+	gitdir. When Git is run in a repository, it usually makes no difference
+	whether this repository is standalone or a submodule, but if this
+	configuration variable is present, additional behavior may be possible,
+	such as "git status" printing additional information about this
+	submodule's status with respect to its superproject. This config should
+	only be present in projects which are submodules, but is not guaranteed
+	to be present in every submodule, so only optional value-added behavior
+	should be linked to it. It is set automatically during
+	submodule creation.
++
+	Because of this configuration variable, it is forbidden to use the
+	same submodule worktree shared by multiple superprojects.
diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index d55f6262e9..d60fcd2c7d 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -1910,6 +1910,10 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 		git_config_set_in_file(p, "submodule.alternateErrorStrategy",
 					   error_strategy);
 
+	git_config_set_in_file(p, "submodule.superprojectGitdir",
+			       relative_path(absolute_path(get_git_dir()),
+					     path, &sb));
+
 	free(sm_alternate);
 	free(error_strategy);
 
diff --git a/t/t7400-submodule-basic.sh b/t/t7400-submodule-basic.sh
index 4bc6b6c886..e407329d81 100755
--- a/t/t7400-submodule-basic.sh
+++ b/t/t7400-submodule-basic.sh
@@ -108,14 +108,18 @@ test_expect_success 'setup - repository to add submodules to' '
 submodurl=$(pwd -P)
 
 inspect() {
-	dir=$1 &&
-
-	git -C "$dir" for-each-ref --format='%(refname)' 'refs/heads/*' >heads &&
-	{ git -C "$dir" symbolic-ref HEAD || :; } >head &&
-	git -C "$dir" rev-parse HEAD >head-sha1 &&
-	git -C "$dir" update-index --refresh &&
-	git -C "$dir" diff-files --exit-code &&
-	git -C "$dir" clean -n -d -x >untracked
+	sub_dir=$1 &&
+	super_dir=$2 &&
+
+	git -C "$sub_dir" for-each-ref --format='%(refname)' 'refs/heads/*' >heads &&
+	{ git -C "$sub_dir" symbolic-ref HEAD || :; } >head &&
+	git -C "$sub_dir" rev-parse HEAD >head-sha1 &&
+	git -C "$sub_dir" update-index --refresh &&
+	git -C "$sub_dir" diff-files --exit-code &&
+	cached_super_dir="$(git -C "$sub_dir" config --get submodule.superprojectGitDir)" &&
+	[ "$(git -C "$super_dir" rev-parse --absolute-git-dir)" \
+		-ef "$sub_dir/$cached_super_dir" ] &&
+	git -C "$sub_dir" clean -n -d -x >untracked
 }
 
 test_expect_success 'submodule add' '
@@ -138,7 +142,7 @@ test_expect_success 'submodule add' '
 	) &&
 
 	rm -f heads head untracked &&
-	inspect addtest/submod &&
+	inspect addtest/submod addtest &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
 	test_must_be_empty untracked
@@ -229,7 +233,7 @@ test_expect_success 'submodule add --branch' '
 	) &&
 
 	rm -f heads head untracked &&
-	inspect addtest/submod-branch &&
+	inspect addtest/submod-branch addtest &&
 	test_cmp expect-heads heads &&
 	test_cmp expect-head head &&
 	test_must_be_empty untracked
@@ -245,7 +249,7 @@ test_expect_success 'submodule add with ./ in path' '
 	) &&
 
 	rm -f heads head untracked &&
-	inspect addtest/dotsubmod/frotz &&
+	inspect addtest/dotsubmod/frotz addtest &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
 	test_must_be_empty untracked
@@ -261,7 +265,7 @@ test_expect_success 'submodule add with /././ in path' '
 	) &&
 
 	rm -f heads head untracked &&
-	inspect addtest/dotslashdotsubmod/frotz &&
+	inspect addtest/dotslashdotsubmod/frotz addtest &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
 	test_must_be_empty untracked
@@ -277,7 +281,7 @@ test_expect_success 'submodule add with // in path' '
 	) &&
 
 	rm -f heads head untracked &&
-	inspect addtest/slashslashsubmod/frotz &&
+	inspect addtest/slashslashsubmod/frotz addtest &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
 	test_must_be_empty untracked
@@ -293,7 +297,7 @@ test_expect_success 'submodule add with /.. in path' '
 	) &&
 
 	rm -f heads head untracked &&
-	inspect addtest/realsubmod &&
+	inspect addtest/realsubmod addtest &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
 	test_must_be_empty untracked
@@ -309,7 +313,7 @@ test_expect_success 'submodule add with ./, /.. and // in path' '
 	) &&
 
 	rm -f heads head untracked &&
-	inspect addtest/realsubmod2 &&
+	inspect addtest/realsubmod2 addtest &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
 	test_must_be_empty untracked
@@ -340,7 +344,7 @@ test_expect_success 'submodule add in subdirectory' '
 	) &&
 
 	rm -f heads head untracked &&
-	inspect addtest/realsubmod3 &&
+	inspect addtest/realsubmod3 addtest &&
 	test_cmp expect heads &&
 	test_cmp expect head &&
 	test_must_be_empty untracked
@@ -481,7 +485,7 @@ test_expect_success 'update should work when path is an empty dir' '
 	git submodule update -q >update.out &&
 	test_must_be_empty update.out &&
 
-	inspect init &&
+	inspect init . &&
 	test_cmp expect head-sha1
 '
 
@@ -540,7 +544,7 @@ test_expect_success 'update should checkout rev1' '
 	echo "$rev1" >expect &&
 
 	git submodule update init &&
-	inspect init &&
+	inspect init . &&
 
 	test_cmp expect head-sha1
 '
-- 
2.33.0.rc2.250.ged5fa647cd-goog