Re: [PATCH] read-cache.c: index format v5 -- 30% smaller/faster than v4

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

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

> @@ -1749,16 +1750,16 @@ static struct cache_entry *create_from_disk(struct mem_pool *ce_mem_pool,
>  	 * number of bytes to be stripped from the end of the previous name,
>  	 * and the bytes to append to the result, to come up with its name.
>  	 */
> -	int expand_name_field = version == 4;
> +	int expand_name_field = version >= 4;

The code can be lazy like this, insteasd of being more descriptive
to say "version 4 or 5", because we won't accept version 6 or later
anyway.  Which is OK, I guess.

>  	if (flags & CE_EXTENDED) {
> -		struct ondisk_cache_entry_extended *ondisk2;
> +		const struct ondisk_cache_entry_extended *ondisk2 = mmap;
>  		int extended_flags;
> -		ondisk2 = (struct ondisk_cache_entry_extended *)ondisk;
> +
>  		extended_flags = get_be16(&ondisk2->flags2) << 16;
>  		/* We do not yet understand any bit out of CE_EXTENDED_FLAGS */
>  		if (extended_flags & ~CE_EXTENDED_FLAGS)

This part may be a good clean-up regardless.

> @@ -1820,6 +1821,113 @@ static struct cache_entry *create_from_disk(struct mem_pool *ce_mem_pool,
>  	return ce;
>  }
>  
> +enum same_value_bit {
> +	DELTA_FORMAT = 1 << 0,
> +	SAME_CTIME   = 1 << 1, /* only covers sec, not nsec */
> +	SAME_MTIME   = 1 << 2, /* only covers sec, not nsec */
> +	SAME_DEV     = 1 << 3,
> +	SAME_INO     = 1 << 4,
> +	SAME_MODE    = 1 << 5,
> +	SAME_UID     = 1 << 6,
> +	SAME_GID     = 1 << 7,
> +	SAME_FLAGS   = 1 << 7
> +};

Hmph, really?

> +static struct cache_entry no_previous_ce;
> +
> +static uintmax_t decode_varoffset(const unsigned char **bufp, uintmax_t prev)
> +{
> +	uintmax_t val = decode_varint(bufp);

You'd need to make sure (1) !val, which indicates an overflow of the
varint, and (2) bufp after decoding haven't over-read the mmapped
index file.  We may want to improve decode_varint() API so that we
can detect truncated data (i.e. (2)) more reliably without first
reading too much.  Loose error checking like these would make good
targets for fuzz tests, I suspect.

> +	return val & 1 ? prev - (val >> 1) : prev + (val >> 1);
> +}

So, the LSB is used for sign, and the magnitude is shifted by one?
OK.

> +static uintmax_t decode_varoffset_same(const unsigned char **bufp, uintmax_t prev,
> +				       int same_flag)
> +{
> +	return same_flag ? prev : decode_varoffset(bufp, prev);
> +}
> +
> +static uintmax_t decode_varint_same(const unsigned char **bufp, uintmax_t prev,
> +				    int same_flag)
> +{
> +	return same_flag ? prev : decode_varint(bufp);
> +}

Likewise about two error conditions.

> @@ -1967,12 +2075,18 @@ static unsigned long load_cache_entry_block(struct index_state *istate,
>  	unsigned long src_offset = start_offset;
>  
>  	for (i = offset; i < offset + nr; i++) {
> -		struct ondisk_cache_entry *disk_ce;
>  		struct cache_entry *ce;
>  		unsigned long consumed;
>  
> -		disk_ce = (struct ondisk_cache_entry *)(mmap + src_offset);
> -		ce = create_from_disk(ce_mem_pool, istate->version, disk_ce, &consumed, previous_ce);
> +		if (istate->version <= 4)
> +			ce = create_from_disk(ce_mem_pool, istate->version,
> +					      mmap + src_offset, &consumed,
> +					      previous_ce);
> +		else
> +			ce = create_from_disk_v5(ce_mem_pool,
> +						 mmap + src_offset,
> +						 &consumed,
> +						 previous_ce);

This goes directly against the spirit of "create_from_disk()"
internal API, doesn't it?  It takes istate->version because it is an
implementation detail of that function how bytes at
&mmap[src_offset] are consumed, possibly using previous_ce
information.

IOW, I think the version dependent switch should go inside that
function and not in this loop.


> +static int ce_write_varint(git_hash_ctx *c, int fd, uintmax_t value)
> +{
> +	unsigned char varint[16];

We may want to do something about these "16".

> +static int ce_write_varoffset(git_hash_ctx *c, int fd, uintmax_t next, uintmax_t prev)
> +{
> +	unsigned char varint[16];