Subject: Re: [PATCH v2 2/3] sparse-index: add ensure_correct_sparsity function

From: Junio C Hamano <[email-masked]>
Date: Thu, 21 Oct 2021 15:20:50 -0700

"Victoria Dye via GitGitGadget" <[email-masked]> writes:

> -int convert_to_sparse(struct index_state *istate, int flags)
> +static int can_convert_to_sparse(struct index_state *istate, int flags)
>  {
>  	int test_env;

May not be a problem with this patch, but this variable does not
need to be in this large a scope.

> -	if (istate->sparse_index || !istate->cache_nr ||
> -	    !core_apply_sparse_checkout || !core_sparse_checkout_cone)
> +	if (!core_apply_sparse_checkout || !core_sparse_checkout_cone)
>  		return 0;
>  
>  	if (!istate->repo)
> @@ -187,6 +186,30 @@ int convert_to_sparse(struct index_state *istate, int flags)
>  	if (cache_tree_update(istate, WRITE_TREE_MISSING_OK))
>  		return 0;
>  
> +	return 1;
> +}
> +
> +int convert_to_sparse(struct index_state *istate, int flags)
> +{
> +	int verify = flags & SPARSE_INDEX_VERIFY_ALLOWED;
> +
> +	/*
> +	 * If validating with strict checks against whether the sparse index is
> +	 * allowed, we want to check `can_convert_to_sparse` *before* exiting
> +	 * early due to an already sparse or empty index.
> +	 *
> +	 * If not performing strict validation, the order is reversed to avoid
> +	 * the more expensive checks in `can_convert_to_sparse` whenver possible.
> +	 */
> +	if (verify) {
> +		if (!can_convert_to_sparse(istate, flags))
> +			return -1;
> +		else if (istate->sparse_index || !istate->cache_nr)
> +			return 0;
> +	} else if (istate->sparse_index || !istate->cache_nr ||
> +		   !can_convert_to_sparse(istate, flags))
> +		return 0;
> +
>  	remove_fsmonitor(istate);
>  
>  	trace2_region_enter("index", "convert_to_sparse", istate->repo);
> @@ -313,6 +336,19 @@ void ensure_full_index(struct index_state *istate)
>  	trace2_region_leave("index", "ensure_full_index", istate->repo);
>  }
>  
> +void ensure_correct_sparsity(struct index_state *istate)
> +{
> +	/*
> +	 * First check whether the index can be converted to sparse by attempting
> +	 * to convert it with the SPARSE_INDEX_VERIFY_ALLOWED flag. If the
> +	 * SPARSE_INDEX_VERIFY_ALLOWED checks indicate that the index cannot
> +	 * be converted because repository settings and/or index properties
> +	 * do not allow it, expand the index to full.
> +	 */

The logic may be OK, but the need to give this long description is a
sign that the meaning of the value returned from the function is not
clear from the name of the function.

> +	if (convert_to_sparse(istate, SPARSE_INDEX_VERIFY_ALLOWED))
> +		ensure_full_index(istate);

It might make it more straight-forward to 

 (1) drop the "if (verify)" part in convert_to_sparse(), which would
     mean that for all callers convert_to_sparse() will retain the
     same behaviour as before;

 (2) make a call to can_convert_to_sparse() here, and if that
     returns true, make a call to ensure_full_index(); this would
     behave identically to what this patch does when can_convert is
     false; and

 (3) correct the can_convert_to_sparse() function to not blow away
     the cache_tree unconditionally and recompute, so that calling
     it twice in a row will not be costly.