Re: [PATCH 2/2] sparse-index: update index read to consider index.sparse config

[Derrick Stolee <stolee@gmail.com>]

On 10/17/2021 1:58 AM, Junio C Hamano wrote:
> Derrick Stolee <stolee@gmail.com> writes:
> 
>> I think you are slightly mistaken here: If index.sparse=true, then a
>> full index will be converted to one on write, but not immediately upon
>> read. This means that subsequent commands will read a sparse index, and
>> they will either benefit from that or not depending on whether they are
>> integrated with the sparse index or not.
>>
>> The new behavior here is that if index.sparse=false, then we convert
>> a sparse index to a full one upon read, avoiding any chance that a
>> Git command is operating on a sparse index in-memory.
> 
> And if index.sparse=true, then we convert a full on-disk index to a
> sparse one in-core upon reading, right?  My comment was solely on
> that side of the picture, not on the "index.sparse is set to false
> so we automatically expand" case.
> 
>> The simplest summary I can say is here:
>>
>> * If index.sparse=false, then a sparse index will be converted to
>>   full upon read.
>>
>> * If index.sparse=true, then a full index will be converted to sparse
>>   on write.
> 
> Oh, I see, so yes I was very much misunderstanding what you guys are
> trying to do.  I somehow thought that sparse-to-full and
> full-to-sparse conversions (1) already happen on the write codepath,
> and (2) this patch makes them both happen also on the read codepath.
> 
> IOW:
> 
>     * If index.sparse=false, a sparse index will be written as full,
>       and if it is true, a non-sparse index will be written as
>       sparse, even before these patches.

This statement is true.

>     * In addition, with these patches, if index.sparse=false, a
>       sparse index will be expaned to full upon reading, and if it
>       is true, a non-sparse index will be shrunk to sparse upon
>       reading

This is only half true. If "index.sparse=false", then a sparse
index will be expanded to full upon reading. If "index.sparse=true"
then nothing special will happen to an index on reading.
> What your summary above is saying is very much different.

Yes, these are different things. It also gets around the concern that
"if we have a non-integrated command, then index.sparse=true would
convert a full index to a sparse one, only to be expanded again".
That doesn't happen if we only convert from full to sparse on write.

Thanks,
-Stolee