[RFC PATCH 2/4] introduce submodule.superprojectGitDir cache

[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 cached value is only introduced during new submodule creation
via `git submodule add`, though, there is more work to do to allow the
cache to be created at other times.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
---
 builtin/submodule--helper.c |  4 ++++
 t/t7400-submodule-basic.sh  | 40 ++++++++++++++++++++-----------------
 2 files changed, 26 insertions(+), 18 deletions(-)

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 9d505a6329..3d6fd54c9b 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -1909,6 +1909,10 @@ 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 f5dc051a6e..e45f42588f 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
 }
 
 
@@ -139,7 +143,7 @@ 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
@@ -230,7 +234,7 @@ 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
@@ -246,7 +250,7 @@ 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
@@ -262,7 +266,7 @@ 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
@@ -278,7 +282,7 @@ 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
@@ -294,7 +298,7 @@ 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
@@ -310,7 +314,7 @@ 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
@@ -341,7 +345,7 @@ 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
@@ -482,7 +486,7 @@ 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
 '
 
@@ -541,7 +545,7 @@ test_expect_success 'update should checkout rev1' '
 	echo "$rev1" >expect &&
 
 	git submodule update init &&
-	inspect init &&
+	inspect init . &&
 
 	test_cmp expect head-sha1
 '
-- 
2.32.0.272.g935e593368-goog