Re: [PATCH 17/32] read-cache: split-index mode

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Nguyễn Thái Ngọc Duy  <pclouds@xxxxxxxxx> writes:

> diff --git a/cache.h b/cache.h
> index 0f6247c..90a5998 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -135,6 +135,7 @@ struct cache_entry {
>  	unsigned int ce_mode;
>  	unsigned int ce_flags;
>  	unsigned int ce_namelen;
> +	unsigned int index;	/* for link extension */
>  	unsigned char sha1[20];
>  	char name[FLEX_ARRAY]; /* more */
>  };

I am not sure if we want to keep an otherwise unused 8-byte around
per cache entry (especially for a large project where the split
index mode should matter) after we read the index.

I expected to see some code where entries in this incremental index
are used to override the entries from the base/shared index, but
merge_base_index() seems to do just memcpy() to discard the former
and replace them with the latter.  Is this step meant to work at
all, or is it a smaller step meant to be completed in later patches?

I do think it is sensible to keep two arrays of "struct cache_entry"
around (one for base and one for incremental changes) inside
index_state, and the patch seems to do so via "struct split_index"
that does have a copy of saved_cache.  If the write-out codepath
walks these two sorted arrays in parallel, shouldn't it be able to
figure out which entry is added, deleted and modified without
fattening this structure?

Maybe it is too early for me to be asking these questions and it may
be better if I read the whole series twice and wait it to become
clear to me why this field is necessary.  I dunno.
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