submodule.c: use get_git_dir() instead of get_git_common_dir()

Ever since df56607dff (git-common-dir: make "modules/"
per-working-directory directory, 2014-11-30), submodules in linked worktrees
are cloned to $GIT_DIR/modules, i.e. $GIT_COMMON_DIR/worktrees/<name>/modules.

However, this convention was not followed when the worktree updater commands
checkout, reset and read-tree learned to recurse into submodules. Specifically,
submodule.c::submodule_move_head, introduced in 6e3c1595c6 (update submodules:
add submodule_move_head, 2017-03-14) and submodule.c::submodule_unset_core_worktree,
(re)introduced in 898c2e65b7 (submodule: unset core.worktree if no working tree
is present, 2018-12-14) use get_git_common_dir() instead of get_git_dir()
to get the path of the submodule repository.

This means that, for example, 'git checkout --recurse-submodules <branch>'
in a linked worktree will correctly checkout <branch>, detach the submodule's HEAD
at the commit recorded in <branch> and update the submodule working tree, but the
submodule HEAD that will be moved is the one in $GIT_COMMON_DIR/modules/<name>/,
i.e. the submodule repository of the main superproject working tree.
It will also rewrite the gitfile in the submodule working tree of the linked worktree
to point to $GIT_COMMON_DIR/modules/<name>/.
This leads to an incorrect (and confusing!) state in the submodule working tree
of the main superproject worktree.

Additionally, if switching to a commit where the submodule is not present,
submodule_unset_core_worktree will be called and will incorrectly remove
'core.wortree' from the config file of the submodule in the main superproject worktree,
$GIT_COMMON_DIR/modules/<name>/config.

Fix this by constructing the path to the submodule repository using get_git_dir()
in both submodule_move_head and submodule_unset_core_worktree.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -1811,7 +1811,7 @@ out:
 void submodule_unset_core_worktree(const struct submodule *sub)
 {
 	char *config_path = xstrfmt("%s/modules/%s/config",
-				    get_git_common_dir(), sub->name);
+				    get_git_dir(), sub->name);
 
 	if (git_config_set_in_file_gently(config_path, "core.worktree", NULL))
 		warning(_("Could not unset core.worktree setting in submodule '%s'"),
@@ -1914,7 +1914,7 @@ int submodule_move_head(const char *path,
 					ABSORB_GITDIR_RECURSE_SUBMODULES);
 		} else {
 			char *gitdir = xstrfmt("%s/modules/%s",
-				    get_git_common_dir(), sub->name);
+				    get_git_dir(), sub->name);
 			connect_work_tree_and_git_dir(path, gitdir, 0);
 			free(gitdir);
 
@@ -1924,7 +1924,7 @@ int submodule_move_head(const char *path,
 
 		if (old_head && (flags & SUBMODULE_MOVE_HEAD_FORCE)) {
 			char *gitdir = xstrfmt("%s/modules/%s",
-				    get_git_common_dir(), sub->name);
+				    get_git_dir(), sub->name);
 			connect_work_tree_and_git_dir(path, gitdir, 1);
 			free(gitdir);
 		}

t/t2405-worktree-submodule.sh

@@ -10,6 +10,7 @@ test_expect_success 'setup: create origin repos'  '
 	git init origin/sub &&
 	test_commit -C origin/sub file1 &&
 	git init origin/main &&
+	test_commit -C origin/main first &&
 	git -C origin/main submodule add ../sub &&
 	git -C origin/main commit -m "add sub" &&
 	test_commit -C origin/sub "file1 updated" file1 file1updated file1updated &&
@@ -54,4 +55,36 @@ test_expect_success 'submodule is checked out after manually adding submodule wo
 	grep "file1 updated" out
 '
 
+test_expect_success 'checkout --recurse-submodules uses $GIT_DIR for submodules in a linked worktree' '
+	git -C main worktree add "$base_path/checkout-recurse" --detach  &&
+	git -C checkout-recurse submodule update --init &&
+	echo "gitdir: ../../main/.git/worktrees/checkout-recurse/modules/sub" >expect-gitfile &&
+	cat checkout-recurse/sub/.git >actual-gitfile &&
+	test_cmp expect-gitfile actual-gitfile &&
+	git -C main/sub rev-parse HEAD >expect-head-main &&
+	git -C checkout-recurse checkout --recurse-submodules HEAD~1 &&
+	cat checkout-recurse/sub/.git >actual-gitfile &&
+	git -C main/sub rev-parse HEAD >actual-head-main &&
+	test_cmp expect-gitfile actual-gitfile &&
+	test_cmp expect-head-main actual-head-main
+'
+
+test_expect_success 'core.worktree is removed in $GIT_DIR/modules/<name>/config, not in $GIT_COMMON_DIR/modules/<name>/config' '
+	echo "../../../sub" >expect-main &&
+	git -C main/sub config --get core.worktree >actual-main &&
+	test_cmp expect-main actual-main &&
+	echo "../../../../../../checkout-recurse/sub" >expect-linked &&
+	git -C checkout-recurse/sub config --get core.worktree >actual-linked &&
+	test_cmp expect-linked actual-linked &&
+	git -C checkout-recurse checkout --recurse-submodules first &&
+	test_expect_code 1 git -C main/.git/worktrees/checkout-recurse/modules/sub config --get core.worktree >linked-config &&
+	test_must_be_empty linked-config &&
+	git -C main/sub config --get core.worktree >actual-main &&
+	test_cmp expect-main actual-main
+'
+
+test_expect_success 'unsetting core.worktree does not prevent running commands directly against the submodule repository' '
+	git -C main/.git/worktrees/checkout-recurse/modules/sub log
+'
+
 test_done