Re: [PATCH 0/5] [RFC] config API: return empty list, not NULL

> On 9/27/2022 10:40 PM, Junio C Hamano wrote:
>> "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:
>> 
>>> This work changes the behavior of asking for a multi-valued config key to
>>> return an empty list instead of a NULL value. This simplifies the handling
>>> of the result and is safer for development in the future.
>>>
>>> This is based on v4 of my unregister series [1]
>>>
>>> [1]
>>> https://lore.kernel.org/git/pull.1358.v4.git.1664287021.gitgitgadget@gmail.com/
>>>
>>> This idea came about due to a bug in the git maintenance unregister work
>>> where the result from git_config_get_value_multi() was sent directly to
>>> for_each_string_list_item() without checking for a NULL value first.
>>>
>>> I'm sending this as an RFC mostly because I'm not 100% sure this shift is
>>> worth the refactoring pain and effort. I personally think getting an empty
>>> list is a safer choice, but I can also understand if someone has a different
>>> opinion.
>> 
>> Thanks.
>> 
>> I actually am in favor of the idea that a NULL can be passed around
>> to signal the lack of a string_list (or the lack of a instance of
>> any "collection" type), and the current code is structured as such,
>> and it gives us extra flexibility.  Of course, we need to see if
>> that extra flexibility is worth it.
>> 
>> With a colleciton col, "if (col && col->nr)" checks if we have
>> something to work on.  But a code like this (which is a longhand for
>> the for_each_string_list_item() issue we just reencountered):
>> 
>>     col = git_get_some_collection(...);
>>     if (!col)
>> 	return; /* no collection */
>>     if (!col->nr)
>> 	git_add_to_some_collection(col, the default item);
>>     for (i = 0; i < col->nr; i++)
>> 	do things on col.stuff[i];
>> 
>> can react differently to cases where we have an empty collection
>> and where we do not have any collection to begin with.  
>> 
>> The other side of the coin is that it would make it harder to treat
>> the lack of collection itself and the collection being empty the
>> same way.  The above code might need to become
>> 
>>     col = git_get_some_collection(...);
>>     if (!col)
>> 	col = git_get_empty_collection();
>>     if (!col->nr)
>> 	git_add_to_some_collection(col, the default item);
>>     for (i = 0; i < col->nr; i++)
>> 	do things on col.stuff[i];
>> 
>> but if the "get the collection" thing returns an empty collection
>> when there is actually no collection, we can lose two lines from
>> there.
>
> I'm all for conveying more information when possible, but how can
> the config API provide a distinction between an empty list and a
> NULL list? The only thing I can think about is a case where the
> empty value clears the list and no new values are added, such as
>
> 	[bogus "key"]
> 		item = one
> 		item = two
> 		item =
>
> With this, the key exists in the config file, but the multi-valued
> list is empty.