Re: [PATCH 1/6] t1092: use ORT merge strategy

[Derrick Stolee <stolee@gmail.com>]

On 8/18/2021 2:10 PM, Junio C Hamano wrote:
> "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
>> From: Derrick Stolee <dstolee@microsoft.com>
>>
>> The sparse index will be compatible with the ORT merge strategy, so
>> let's use it explicitly in our tests.
> 
> Unless you mean that the sparse index will only be compatible with
> ort, but will never be with recursive, I do not quite see why this
> is taking us into a good direction.  Is this because we want to gain
> test coverage for ort early, before we flip the default to ort [*1*]?

The sparse index will _work_ with the recursive merge strategy, it will
just continue to be slow, and likely slower than if we had a full index.
This is because the recursive merge algorithm will expand a sparse index
into a full one before doing any of its logic (hence my confidence that
it will work).

The main point why ORT is the focus is that the ORT strategy is required
so the sparse index can get the intended performance gains (i.e. it does
not expand in most cases). The ORT algorithm can resolve conflicts
outside the sparse-checkout cone without needing the index as a data
structure and instead the resulting tree is recorded in the correct
sparse directory entry.

> [Footnote]
> 
> *1* If the answer is "no, it is because sparse index will not work
>     with recursive", the please disregard the rest, but just in
>     case it is not...
> 
>     It seems to me that it would let us live in the future in a more
>     comprehensive way if we tweaked merge_recursive() and/or
>     merge_recursive_generic() so that all internal callers, not just
>     builtin/merge.c, would redirect to the ort machinery when say
>     GIT_TEST_REPLACE_RECURSIVE_WITH_ORT environment exists, and
>     doing it that way we do not need to sprinkle "-srecursive" and
>     "-sort" everywhere in our tests at randomly chosen places to
>     give test coverage to both strategies.

I could also change this patch to stop using ORT _all the time_ and
instead let the GIT_TEST_MERGE_ALGORITHM decide which is tested.

That is, except for the final tests that check that the index is not
expanded. Those tests must specify the ORT strategy explicitly.

I think I started playing with the GIT_TEST_MERGE_ALGORITHM because
it appears to override the command-line option, but I will need to
double-check that.

Thanks,
-Stolee