Duy Nguyen <pclo...@gmail.com> writes:

> .... Even
> when cache-tree is not involved, I do not want the index to point to
> an non-existing SHA-1 ("git diff --cached" may fail next time, for
> example).

I think we have tests that explicitly add SHA-1 that names an object
that does not exist to the index and check failures; you may have to
think what to do with them.

>> This is a tangent, but in addition, you may want to add an even
>> narrower variant that checks the same but ignoring _all_ alternate
>> object databases, "external" or not:
>>         int has_sha1_file_local(const unsigned char *sha1);
>> That may be useful if we want to later make the alternate safer to
>> use; instead of letting the codepath to create an object ask
>> has_sha1_file() to see if it already exists and refrain from writing
>> a new copy, we switch to ask has_sha1_file_locally() and even if an
>> alternate object database we borrow from has it, we keep our own
>> copy in our repository.

This is not a tangent, but if you want to go this "forbid making our
repository depend on objects we do not have but we know about after
we peek submodule odb" route [*1*], write_sha1_file() needs to be
told about has_sha1_file_proper().  We may "git add" a new blob in
the superproject, the blob may not yet exist in *our* repository,
but may happen to already exist in the submodue odb.  In such a
case, write_sha1_file() has to write that blob in our repository,
without the existing has_sha1_file() check bypassing it.  Otherwise
our attempt to create a tree that contains that blob will fail,
saying that the blob only seems to exist to us via submodule odb but
not in our repository.


*1* which I do not necessarily agree with---I am in favor of getting
rid of add_submodule_odb() to fix these issues at the root cause of
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