[PATCH v6 2/2] setup: allow cwd/.git to be a symlink to a directory

[Tian Yuchen <a3205153416@gmail.com>]

Strictly enforcing 'lstat()' prevents valid '.git' symlinks.

Rely on 'stat()' (via 'read_gitfile_gently()') to handle filesystem
resolution, instead of blocking it with strict logic checks in
'setup_git_directory_gently_1()'.

To ensure safety and correctness, we unconditionally delegate benign
cases ('ENOENT', 'IS_A_DIR') and security risks ('NOT_A_FILE') to
'read_gitfile_error_die()'.

For other errors (like invalid format), we only invoke the handler if
'die_on_error' is true; otherwise, we return the error code to respect
the gentle fallback behavior.

Add 't/t0009-git-dir-validation.sh' to verify symlink support and FIFO
rejection, and register it in 't/meson.build'.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
---
 setup.c                       | 21 ++++++----
 t/meson.build                 |  1 +
 t/t0009-git-dir-validation.sh | 72 +++++++++++++++++++++++++++++++++++
 3 files changed, 86 insertions(+), 8 deletions(-)
 create mode 100755 t/t0009-git-dir-validation.sh

diff --git a/setup.c b/setup.c
index d48b6a3a3d..f4b9d41f78 100644
--- a/setup.c
+++ b/setup.c
@@ -1590,15 +1590,20 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 		gitdirenv = read_gitfile_gently(dir->buf, die_on_error ?
 						NULL : &error_code);
 		if (!gitdirenv) {
-			if (die_on_error ||
-			    error_code == READ_GITFILE_ERR_NOT_A_FILE) {
-				/* NEEDSWORK: fail if .git is not file nor dir */
-				if (is_git_directory(dir->buf)) {
-					gitdirenv = DEFAULT_GIT_DIR_ENVIRONMENT;
-					gitdir_path = xstrdup(dir->buf);
+			if (error_code){
+				if (error_code == READ_GITFILE_ERR_STAT_ENOENT ||
+				error_code == READ_GITFILE_ERR_IS_A_DIR ||
+				error_code == READ_GITFILE_ERR_NOT_A_FILE ||
+				die_on_error) {
+					read_gitfile_error_die(error_code, dir->buf, NULL);
+				} else {
+					return GIT_DIR_INVALID_GITFILE;
 				}
-			} else if (error_code != READ_GITFILE_ERR_STAT_FAILED)
-				return GIT_DIR_INVALID_GITFILE;
+			}
+			if (is_git_directory(dir->buf)) {
+				gitdirenv = DEFAULT_GIT_DIR_ENVIRONMENT;
+				gitdir_path = xstrdup(dir->buf);
+			}
 		} else
 			gitfile = xstrdup(dir->buf);
 		/*
diff --git a/t/meson.build b/t/meson.build
index f80e366cff..c4afaacee5 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -80,6 +80,7 @@ integration_tests = [
   't0006-date.sh',
   't0007-git-var.sh',
   't0008-ignores.sh',
+  't0009-git-dir-validation.sh',
   't0010-racy-git.sh',
   't0012-help.sh',
   't0013-sha1dc.sh',
diff --git a/t/t0009-git-dir-validation.sh b/t/t0009-git-dir-validation.sh
new file mode 100755
index 0000000000..9b3925c85f
--- /dev/null
+++ b/t/t0009-git-dir-validation.sh
@@ -0,0 +1,72 @@
+#!/bin/sh
+
+test_description='setup: validation of .git file/directory types
+
+Verify that setup_git_directory() correctly handles:
+1. Valid .git directories (including symlinks to them).
+2. Invalid .git files (FIFOs, sockets) by erroring out.
+3. Invalid .git files (garbage) by erroring out.
+'
+
+. ./test-lib.sh
+
+test_expect_success 'setup: create parent git repository' '
+	git init parent &&
+	test_commit -C parent "root-commit"
+'
+
+test_expect_success SYMLINKS 'setup: .git as a symlink to a directory is valid' '
+	mkdir -p parent/link-to-dir &&
+	(
+		cd parent/link-to-dir &&
+		git init real-repo &&
+		ln -s real-repo/.git .git &&
+		git rev-parse --git-dir >actual &&
+		echo .git >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success PIPE 'setup: .git as a FIFO (named pipe) is rejected' '
+	mkdir -p parent/fifo-trap &&
+	(
+		cd parent/fifo-trap &&
+		mkfifo .git &&
+		test_must_fail git rev-parse --git-dir 2>stderr &&
+		grep "not a regular file" stderr
+	)
+'
+
+test_expect_success SYMLINKS,PIPE 'setup: .git as a symlink to a FIFO is rejected' '
+	mkdir -p parent/symlink-fifo-trap &&
+	(
+		cd parent/symlink-fifo-trap &&
+		mkfifo target-fifo &&
+		ln -s target-fifo .git &&
+		test_must_fail git rev-parse --git-dir 2>stderr &&
+		grep "not a regular file" stderr
+	)
+'
+
+test_expect_success 'setup: .git with garbage content is rejected' '
+	mkdir -p parent/garbage-trap &&
+	(
+		cd parent/garbage-trap &&
+		echo "garbage" >.git &&
+		test_must_fail git rev-parse --git-dir 2>stderr &&
+		grep "invalid gitfile format" stderr
+	)
+'
+
+test_expect_success 'setup: .git as an empty directory is ignored' '
+	mkdir -p parent/empty-dir &&
+	(
+		cd parent/empty-dir &&
+		mkdir .git &&
+		git rev-parse --git-dir >actual &&
+		echo "$TRASH_DIRECTORY/parent/.git" >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_done
-- 
2.43.0