Re: [PATCH v2 4/8] sha1_file: new object source for submodule's alt object database

> This patch separates submodule odb sources from ordinary alternate
> sources. The new sources can be accessed with ODB_EXTALT (e.g. via
> read_sha1_file_extended).
>
> ODB_EXTALT is only added to odb_default in certain cases. Basically:
>
>  - External commands do not access submodule odb by default
>  - unpack-objects, index-pack and rev-list do not
>  - All other builtin commands do
>
> unpack-objects, index-pack and rev-list take new objects from outside
> and have to make sure the repository is still in good state. They
> should not pay attention to submodule's odb, especially rev-list
> because it does connectivity check.
>
> External commands also do not have default access to submodule odb,
> simply because I see no reasons why the should. They don't usually

> play a big role in the user front, where submodule integration happens
> and requires looking into submodule odb.
>
> The die() in add_submodule_odb() may be too strong. There might be a
> use case where somebody wants to add_submodule_odb() and look some up
> with read_sha1_file_extended() even if odb_default does not contain
> ODB_EXTALT. Right now such a use case may need to work around die() by
> temporarily adding ODB_EXTALT to odb_default. Not nice, but as no such