setup: improve error diagnosis for invalid .git files

'read_gitfile_gently()' treats any non-regular file as
'READ_GITFILE_ERR_NOT_A_FILE' and fails to discern between 'ENOENT'
and other stat failures. This flawed error reporting is noted by two
'NEEDSWORK' comments.

Address these comments by introducing two new error codes:
'READ_GITFILE_ERR_MISSING'(which groups the "file missing" scenarios
together) and 'READ_GITFILE_ERR_IS_A_DIR':

1. Update 'read_gitfile_error_die()' to treat 'IS_A_DIR', 'MISSING',
'NOT_A_FILE' and 'STAT_FAILED' as non-fatal no-ops. This accommodates
intentional non-repo scenarios (e.g., GIT_DIR=/dev/null).

2. Explicitly catch 'NOT_A_FILE' and 'STAT_FAILED' during
discovery and call 'die()' if 'die_on_error' is set.

3. Unconditionally pass '&error_code' to 'read_gitfile_gently()'.

4. Only invoke 'is_git_directory()' when we explicitly receive
   'READ_GITFILE_ERR_IS_A_DIR', avoiding redundant checks.

Additionally, audit external callers of 'read_gitfile_gently()' in
'submodule.c' and 'worktree.c' to accommodate the refined error codes.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 77 insertions, 0 deletions

diff --git a/t/t0009-git-dir-validation.sh b/t/t0009-git-dir-validation.sh
new file mode 100755
index 0000000000..33d21ed9ea
--- /dev/null
+++ b/t/t0009-git-dir-validation.sh
@@ -0,0 +1,77 @@
+#!/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' '
+	test_when_finished "rm -rf parent/link-to-dir" &&
+	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' '
+	test_when_finished "rm -rf parent/fifo-trap" &&
+	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' '
+	test_when_finished "rm -rf parent/symlink-fifo-trap" &&
+	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' '
+	test_when_finished "rm -rf parent/garbage-trap" &&
+	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' '
+	test_when_finished "rm -rf parent/empty-dir" &&
+	mkdir -p parent/empty-dir &&
+	(
+		cd parent/empty-dir &&
+		git rev-parse --git-dir >expect &&
+		mkdir .git &&
+		git rev-parse --git-dir >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_done