[BUG?] submodule-config.c:config_from() has left me confused

[Atharva Raykar <raykar.ath@gmail.com>]

Hi Git Developers,

While I was working on converting submodule code from shell to C, I had
a question about the submodule-config API.

The usual way of obtaining the submodule configuration is by using the
'submodule_from_path()' or 'submodule_from_name()' functions. Taking the
example of the former, a call to it would look something like:

const struct submodule *sub = submodule_from_path(repo, oid, path-to-submod);

So my first question is what is the exact purpose of the second
argument?

The API docs say it should be a tree-ish:

   Given a tree-ish in the superproject and a path, return the submodule that
   is bound at the path in the named tree.

My guess is that the tree-ish that we pass lets us obtain the submodule
configuration for that specific commit or tree in the superproject.

Looking at the underlying implementation[1], it seems to me that the 'oid'
argument treats the 'null_oid()' specially to mean "get the configuration from
the latest revision".

While I had these questions, Christian noticed a possible bug in the
implementation[2]:

---8<------8<------8<------8<---

> It seems to me that config_from() in submodule-config.c accepts a path or a
> tree oid and submodule_from_path() just calls config_from().
> 
> By the way it seems to me that config_from() is buggy when it's passed a
> tree oid as it does:
> 
> ```
>         if (!gitmodule_oid_from_commit(treeish_name, &oid, &rev))
>                 goto out;
> ```
> 
> but gitmodule_oid_from_commit() returns 0 on success so instead of doing a
> `goto out` which will error out, it should continue to proceed using the oid
> it got. So I think it should be something like:
> 
> ```
>         if (gitmodule_oid_from_commit(treeish_name, &oid, &rev)) {
>                 switch (lookup_type) {
>                 case lookup_name:
>                         submodule = cache_lookup_name(cache, &oid, key);
>                         break;
>                 case lookup_path:
>                         submodule = cache_lookup_path(cache, &oid, key);
>                         break;
>                 }
>                 if (submodule)
>                         goto out;
>         }
> ```
> 
> Or maybe the gitmodule_oid_from_commit() call should be moved after the switch ().

---8<------8<------8<------8<---

I do agree that it looks wrong, but despite that, it still seems to pass 't7411' [3][4].

This should have failed according to my understanding:

cat >super/expect <<EOF
Submodule name: 'a' for path 'a'
Submodule name: 'a' for path 'b'
Submodule name: 'submodule' for path 'submodule'
Submodule name: 'submodule' for path 'submodule'
EOF

test_expect_success 'test parsing and lookup of submodule config by path' '
	(cd super &&
		test-tool submodule-config \
			HEAD^ a \
			HEAD b \
			HEAD^ submodule \
			HEAD submodule \
				>actual &&
		test_cmp expect actual
	)
'

Since HEAD and HEAD^ are valid tree-ish objects, the gitmodule_oid_from_commit()
line should have returned early with a NULL object, but that does not happen.
The output seems to match what I would expect from the API.

Applying Christian's suggestions pass the same test as well.

I also wonder what is the situation with the case where, the oid is non-null
and invalid?

If we look at the code again:

static int gitmodule_oid_from_commit(const struct object_id *treeish_name,
				     struct object_id *gitmodules_oid,
				     struct strbuf *rev)
{
	int ret = 0;

	if (is_null_oid(treeish_name)) {
		oidclr(gitmodules_oid);
		return 1;
	}

	strbuf_addf(rev, "%s:.gitmodules", oid_to_hex(treeish_name));
	if (get_oid(rev->buf, gitmodules_oid) >= 0)
		ret = 1;

	return ret;
}

...what happens to the value of gitmodules_oid when get_oid() fails?

*If* it is set to NULL, it would probably lead to undefined behaviour when
we try to hash the oid for the submodule cache lookup in [1]. If it is set
to 'null_oid()' or zero'd out (like with oidclr()), then it should return
the submodule from the latest revision, which does not seem desirable, as
the input is invalid.

Is this a bug in my understanding or in the program? Or is it a bit of both?

Footnotes:
---------

[1]: https://github.com/git/git/blob/daab8a564f8bbac55f70f8bf86c070e001a9b006/submodule-config.c#L545-L615
[2]: https://github.com/tfidfwastaken/git/commit/7ad7a9d1d03653aadfdc87b60e3a152b1cb37f22#r53637226
[3]: https://github.com/git/git/blob/master/t/t7411-submodule-config.sh
[4]: https://github.com/git/git/blob/master/t/helper/test-submodule-config.c

---
Atharva Raykar
ಅಥರ್ವ ರಾಯ್ಕರ್
अथर्व रायकर