Junio C Hamano <gits...@pobox.com> writes:

> ...
> Is there even a single caller that is prepared to react to NULL?
>     Answer. There is a single hit inside fsck.c that wants to report
>     an error without killing ourselves in fsck_commit_buffer().  I
>     however doubt its use of get_commit_tree() is correct in the
>     first place.  The function is about validating the commit object
>     payload manually, without trusting the result of parse_commit(),
>     and it does read the object name of the tree object; the call to
>     get_commit_tree() used for reporting the error there should
>     probably become has_object() on the tree_oid.

At least we need to ensure, not just has_object(), but the object
indeed claims to be a tree object.  It is OK if it is a corrupt
tree object---we'll catch its brokenness separately anyway.

    Hmm, the should we also tolerate the pointed object to be broken
    in a way that it is not even able to claim to be a tree object?
    That would mean that has_object() is sufficient to check here.

OK, so... 

> So, after fixing the above, we may safely be able to die inside
> get_commit_tree() instead of returning NULL.
> By the way, I think get_commit_tree() and parse_commit() in fsck
> should always use the value obtained from the underlying object and
> bypass any caches like commit graph---if they pay attention to the
> caches, they should be fixed.  Secondary caches like commit graph
> should of course be validated against what are recorded in the
> underlying object, but that should be done separately.

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