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
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