Re: [PATCH v11] 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'.
>
> To preserve the original intent of the setup process:
> 1. Update 'read_gitfile_error_die()' to treat both 'IS_A_DIR' and
>    'MISSING' as no-ops, while continuing to call 'die()' on true
>    'NOT_A_FILE' errors to prevent security hazards (like FIFOs).
> 2. Unconditionally pass '&error_code' to 'read_gitfile_gently()'.
> 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.
>
> 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>
> ---
>  setup.c                       | 45 ++++++++++++++------
>  setup.h                       |  2 +
>  submodule.c                   |  2 +-
>  t/meson.build                 |  1 +
>  t/t0009-git-dir-validation.sh | 77 +++++++++++++++++++++++++++++++++++
>  worktree.c                    |  6 ++-
>  6 files changed, 118 insertions(+), 15 deletions(-)
>  create mode 100755 t/t0009-git-dir-validation.sh