Re: [PATCH v2] sha1-file: remove OBJECT_INFO_SKIP_CACHED

[Junio C Hamano <gitster@pobox.com>]

Jonathan Tan <jonathantanmy@google.com> writes:

> As a historical note, the function now known as repo_read_object_file()
> was taught the empty tree in 346245a1bb ("hard-code the empty tree
> object", 2008-02-13), and the function now known as oid_object_info()
> was taught the empty tree in c4d9986f5f ("sha1_object_info: examine
> cached_object store too", 2011-02-07). repo_has_object_file() was never
> updated, perhaps due to oversight. The flag OBJECT_INFO_SKIP_CACHED,
> introduced later in dfdd4afcf9 ("sha1_file: teach
> sha1_object_info_extended more flags", 2017-06-26) and used in
> e83e71c5e1 ("sha1_file: refactor has_sha1_file_with_flags", 2017-06-26),
> was introduced to preserve this difference in empty-tree handling, but
> now it can be removed.

I am not 100% sure if the implication of this change is safe to
allow us to say "now it can be".

The has_object_file() helper wanted to say "no" when given a
non-existing object registered via the pretend_object_file(),
presumably because we wanted to allow a use pattern like:

 - prepare an in-core representation of an object we tentatively
   expect, but not absolutely sure, to be necessary.

 - perform operations, using the object data obtained via
   read_object() API, which is capable of yielding data even for
   such "pretend" objects (perhaps we are creating a tentative merge
   parents during a recursive merge).

 - write out final set of objects by enumerating those that do not
   really exist yet (via has_object_file() API).

Teaching about the empty tree to has_object_file() is a good thing
(especially because we do not necessarily write an empty tree object
to our repositories), but as a collateral damage of doing so, we
make such use pattern impossible.  

It is not a large loss---the third bullet in the above list can just
be made to unconditionally call write_object_file() without
filtering with has_object_file() and write_object_file() will apply
the right optimization anyway, so it probably is OK.

Will queue.

Thanks.