Re: [PATCH 4/5] config: return an empty list, not NULL

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Sep 27, 2022, 19:18 UTC
Message-ID: <220927.86k05oy5oi.gmgdl@evledraar.gmail.com>
In-Reply-To: <b85d8a43-6640-aa9c-3103-0c3d43c2a479@github.com>

On Tue, Sep 27 2022, Derrick Stolee wrote:

Show 30 quoted lines

> On 9/27/2022 12:21 PM, Ævar Arnfjörð Bjarmason wrote:
>> 
>> On Tue, Sep 27 2022, Derrick Stolee via GitGitGadget wrote:
>
>>>  /**
>>>   * 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.
>>> + * `key` is not found, returns an empty list. 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);
>> 
>> Aside from the "DWIM API" aspect of this (which I don't mind) I think
>> this is really taking the low-level function in the wrong direction, and
>> that we should just add a new simple wrapper instead.
>> 
>> I.e. both the pre-image API docs & this series gloss over the fact that
>> we'd not just return NULL here if the config wasn't there, but also if
>> git_config_parse_key() failed.
>> 
>> So it seems to me that a better direction would be starting with
>> something like the WIP below (which doesn't compile the whole code, I
>> stopped at config.[ch] and pack-bitmap.c). I.e. the same "int" return
>> and "dest" pattern that most other things in the config API have.
>
> Do you have an example where a caller would benefit from this
> distinction? Without such an example, I don't think it is worth
> creating such a huge change for purity's sake alone.

Not initially, I started poking at this because the CL/series/commits says that we don't care about the case of non-existing keys, without being clear as to why we want to conflate that with other errors we might get from this API.

But after some digging I found:

	$ for k in a a.b. "'x.y"; do ./git for-each-repo --config=$k;  echo $?; done
	error: key does not contain a section: a
	0
	error: key does not contain variable name: a.b.
	0
	error: invalid key: 'x.y
	0
	
I.e. the repo_config_get_value_multi() you added in for-each-repo
doesn't distinguish between bad keys and non-existing keys, and returns
0 even though it printed an "error".

Show 7 quoted lines

> I'm pretty happy that the diff for this series is an overall
> reduction in code, while also not being too large in the interim:
>
>  12 files changed, 39 insertions(+), 57 deletions(-)
>
> If all callers that use the *_multi() methods would only use the
> wrapper, then what is the point of doing the low-level manipulations?

I hacked up something that's at least RFC-quality based on this approach, but CI is running etc., so not submitting it now:

	https://github.com/git/git/compare/master...avar:git:avar/have-git_configset_get_value-use-dest-and-int-pattern

I think the resulting diff is more idiomatic API use, i.e. you ended up with:

	        /* submodule.active is set */
	        sl = repo_config_get_value_multi(repo, "submodule.active");
	-       if (sl) {
	+       if (sl && sl->nr) {

But I ended up doing:

	        /* submodule.active is set */
	-       sl = repo_config_get_value_multi(repo, "submodule.active");
	-       if (sl) {
	+       if (!repo_config_get_const_value_multi(repo, "submodule.active", &sl)) {

Note the "const" in the function name, i.e. there's wrappers that handle the case where we have a hardcoded key name, in which case we can BUG() out if we'd return < 0, so all we have left is just "does key exist".

In any case, I'm all for having some simple wrapper for the common cases.

But I didn't find a single case where we actually needed this "never give me a non-NULL list" behavior, it could just be generalized to "let's have the API tell us if the key exist".

If you use the non-"const" API you can distinguish the err < 0 case, so for-each-repo can now error out appropriately:

	$ ./git for-each-repo --config=a; echo $?
	error: key does not contain a section: a
	fatal: got bad config --config=a
	
	usage: git for-each-repo --config=<config> <command-args>
	
	    --config <config>     config key storing a list of repository paths
	
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