[PATCH v7] setup: allow cwd/.git to be a symlink to a directory

[Tian Yuchen <a3205153416@gmail.com>]

Currently, `setup_git_directory_gently_1()` fails to recognize a `.git`
symlink pointing to a directory because `read_gitfile_gently()` strictly
expects a regular file and returns `READ_GITFILE_ERR_NOT_A_FILE` for
anything else, including valid directories.

Fix this by distinguishing directories from regular files and other
non-regular file types (like FIFOs or sockets) via newly introduced
error_code.

To preserve the original intent of the setup process:
1. Update `read_gitfile_error_die()` to treat `IS_A_DIR` as a no-op
   (like `ENOENT`), while still calling `die()` on true `NOT_A_FILE`
   errors.
2. Unconditionally pass `&error_code` to `read_gitfile_gently()`. This
   eliminates an uninitialized variable hazard that occurred when
   `die_on_error` was true and `NULL` was passed.
3. Only invoke `is_git_directory()` when we explicitly receive
   `READ_GITFILE_ERR_IS_A_DIR`, avoiding redundant filesystem checks.
4. Correctly return `GIT_DIR_INVALID_GITFILE` on unrecognized errors
   when `die_on_error` is false.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
---
Changes since v6:

 - Squashed into a single commit.
 - Fixed a hidden uninitialized variable trap. In v6:
   'setup_git_directory_gently_1()' passed 'die_on_error ? NULL : &error_code'
 to 'read_gitfile_gently()'. When 'NULL' was passed, the local 'error_code'
 remained uninitialized. The old code survived this because of short-circuit
 evaluation ('if (die_on_error || error_code == ...)'). 
   In this v7, I now unconditionally pass '&error_code'. This gives the caller
 explicit control over error routing.
   (Actually, I seem to have made the same modification back in v3 or v4, but I didn't
 realize at the time that the part I changed was originally a bug. So it was a 
 happy accident. :P
 - We now only invoke 'is_git_directory()' explicitly when 'error_code ==
 READ_GITFILE_ERR_IS_A_DIR'.

Thanks for your patience.

 setup.c                       | 44 +++++++++++++--------
 setup.h                       |  2 +
 t/meson.build                 |  1 +
 t/t0009-git-dir-validation.sh | 72 +++++++++++++++++++++++++++++++++++
 4 files changed, 104 insertions(+), 15 deletions(-)
 create mode 100755 t/t0009-git-dir-validation.sh

diff --git a/setup.c b/setup.c
index c8336eb20e..5a573e5865 100644
--- a/setup.c
+++ b/setup.c
@@ -897,10 +897,13 @@ int verify_repository_format(const struct repository_format *format,
 void read_gitfile_error_die(int error_code, const char *path, const char *dir)
 {
 	switch (error_code) {
+	case READ_GITFILE_ERR_STAT_ENOENT:
+	case READ_GITFILE_ERR_IS_A_DIR:
+		break;
 	case READ_GITFILE_ERR_STAT_FAILED:
+		die(_("error reading %s"), path);
 	case READ_GITFILE_ERR_NOT_A_FILE:
-		/* non-fatal; follow return path */
-		break;
+		die(_("not a regular file: %s"), path);
 	case READ_GITFILE_ERR_OPEN_FAILED:
 		die_errno(_("error opening '%s'"), path);
 	case READ_GITFILE_ERR_TOO_LARGE:
@@ -941,8 +944,14 @@ const char *read_gitfile_gently(const char *path, int *return_error_code)
 	static struct strbuf realpath = STRBUF_INIT;
 
 	if (stat(path, &st)) {
-		/* NEEDSWORK: discern between ENOENT vs other errors */
-		error_code = READ_GITFILE_ERR_STAT_FAILED;
+		if (errno == ENOENT)
+			error_code = READ_GITFILE_ERR_STAT_ENOENT;
+		else
+			error_code = READ_GITFILE_ERR_STAT_FAILED;
+		goto cleanup_return;
+	}
+	if (S_ISDIR(st.st_mode)) {
+		error_code = READ_GITFILE_ERR_IS_A_DIR;
 		goto cleanup_return;
 	}
 	if (!S_ISREG(st.st_mode)) {
@@ -1578,20 +1587,25 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 		if (offset > min_offset)
 			strbuf_addch(dir, '/');
 		strbuf_addstr(dir, DEFAULT_GIT_DIR_ENVIRONMENT);
-		gitdirenv = read_gitfile_gently(dir->buf, die_on_error ?
-						NULL : &error_code);
+		gitdirenv = read_gitfile_gently(dir->buf, &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 == READ_GITFILE_ERR_IS_A_DIR &&
+			is_git_directory(dir->buf)) {
+				gitdirenv = DEFAULT_GIT_DIR_ENVIRONMENT;
+				gitdir_path = xstrdup(dir->buf);
+			} else {
+				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;
-		} else
+			}
+		} else {
 			gitfile = xstrdup(dir->buf);
+		}
 		/*
 		 * Earlier, we tentatively added DEFAULT_GIT_DIR_ENVIRONMENT
 		 * to check that directory for a repository.
diff --git a/setup.h b/setup.h
index 0738dec244..ed4b13f061 100644
--- a/setup.h
+++ b/setup.h
@@ -36,6 +36,8 @@ int is_nonbare_repository_dir(struct strbuf *path);
 #define READ_GITFILE_ERR_NO_PATH 6
 #define READ_GITFILE_ERR_NOT_A_REPO 7
 #define READ_GITFILE_ERR_TOO_LARGE 8
+#define READ_GITFILE_ERR_STAT_ENOENT 9
+#define READ_GITFILE_ERR_IS_A_DIR 10
 void read_gitfile_error_die(int error_code, const char *path, const char *dir);
 const char *read_gitfile_gently(const char *path, int *return_error_code);
 #define read_gitfile(path) read_gitfile_gently((path), NULL)
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