Re: [PATCH] Extend index to save more flags

[Junio C Hamano <gitster@pobox.com>]

Nguyễn Thái Ngọc Duy  <pclouds@gmail.com> writes:

> The on-disk format of index only saves 16 bit flags, nearly all have
> been used. The last bit (CE_EXTENDED) is used to for future extension.
>
> This patch extends index entry format to save more flags in future.
> The new entry format will be used when CE_EXTENDED bit is 1.
>
> Because older implementation may not understand CE_EXTENDED bit and
> misread the new format, if there is any extended entry in index, index
> header version will turn 3, which makes it incompatible for older git.
> If there is none, header version will return to 2 again.

I think this is a good change.

> diff --git a/cache.h b/cache.h
> index f725783..f8578d1 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -109,6 +109,26 @@ struct ondisk_cache_entry {
>  	char name[FLEX_ARRAY]; /* more */
>  };
>  
> +/*
> + * This struct is used when CE_EXTENDED bit is 1
> + * The struct must match ondisk_cache_entry exactly from
> + * ctime till flags
> + */
> +struct ondisk_cache_entry_extended {
> +	struct cache_time ctime;
> +	struct cache_time mtime;
> +	unsigned int dev;
> +	unsigned int ino;
> +	unsigned int mode;
> +	unsigned int uid;
> +	unsigned int gid;
> +	unsigned int size;
> +	unsigned char sha1[20];
> +	unsigned short flags;
> +	unsigned short flags2;
> +	char name[FLEX_ARRAY]; /* more */
> +};
> +

We should change these to more explicitly sized uint32_t both in the
original and this extended structure, but after this patch (or later
variant of it) lands in the tree.