Re: [PATCH v3 1/3] maintenance: add 'unregister --force'

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Tue, Sep 27 2022, Derrick Stolee wrote:

> On 9/27/2022 7:38 AM, Derrick Stolee wrote:
>> On 9/26/2022 5:55 PM, Junio C Hamano wrote:
>>> "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:
>>>
>>>> @@ -1538,11 +1546,23 @@ static int maintenance_unregister(int argc, const char **argv, const char *prefi
>>>>  		usage_with_options(builtin_maintenance_unregister_usage,
>>>>  				   options);
>>>>  
>>>> -	config_unset.git_cmd = 1;
>>>> -	strvec_pushl(&config_unset.args, "config", "--global", "--unset",
>>>> -		     "--fixed-value", "maintenance.repo", maintpath, NULL);
>>>> +	for_each_string_list_item(item, list) {
>>>> +		if (!strcmp(maintpath, item->string)) {
>>>> +			found = 1;
>>>> +			break;
>>>> +		}
>>>> +	}
>>>
>>> Just out of curiosity, I ran this in a fresh repository and got a
>>> segfault.
>> 
>> Yikes! Thanks for catching. I think there was another instance in
>> the 'register' code that I caught by tests, but I appreciate you
>> catching this one.
>> 
>>>  An attached patch obviously fixes it, but I am wondering
>>> if a better "fix" is to teach for_each_string_list_item() that it is
>>> perfectly reasonable to see a NULL passed to it as the list, which
>>> is a mere special case that the caller has a string list with 0
>>> items on it.
>>>
>>> Thoughts?
>> 
>> I agree that for_each_string_list_item() could handle NULL lists
>> better, especially because it looks like a method and hides some
>> details. Plus, wrapping the for-ish loop with an if statement is
>> particularly ugly.
> ...
>> I'll get a patch put together that changes the behavior of
>> for_each_string_list_item() and adds the missing 'unregister' test
>> so we can avoid this problem.
>
> Of course, there is a reason why we don't check for NULL here,
> and it's because -Werror=address complains when we use a non-pointer
> value in the macro:
>
> string-list.h:146:28: error: the address of ‘friendly_ref_names’ will always evaluate as ‘true’ [-Werror=address]
>   146 |         for (item = (list) ? (list)->items : NULL;      \
>       |
>
> I tried searching for a way to suppress this error in a particular
> case like this (perhaps using something like an attribute?), but I
> couldn't find anything.

We discussed this exact issue just a few months ago, see:
https://lore.kernel.org/git/220614.86czfcytlz.gmgdl@evledraar.gmail.com/

In general I don't think we should be teaching
for_each_string_list_item() to handle NULL.

Instead most callers that need to deal with a "NULL" list should
probably just use a list that's never NULL. See:
https://lore.kernel.org/git/220616.86bkuswuh5.gmgdl@evledraar.gmail.com/

In this case however it seems perfectly reasonable to return a valid
pointer or NULL, and the function documents as much:
	
	/**
	 * Finds and returns the value list, sorted in order of increasing priority
	 * for the configuration variable `key`. When the configuration variable
	 * `key` is not found, returns NULL. The caller should not free or modify
	 * the returned pointer, as it is owned by the cache.
	 */
	const struct string_list *git_config_get_value_multi(const char *key);

You also have code in 3/3 that uses that API in the correct way, I think
just adjusting this callsite in 1/3 would be the right move here.

This also gives the reader & compiler more information to e.g. eliminate
dead code. You're calling maintpath() unconditionally, but if you have
no config & the user provided --force we'll never end up using it, so we
can avoid allocating it in the first place.