Re: [PATCH] Don't dereference NULL upon lookup_tree failure.

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

Jim Meyering <jim@meyering.net> writes:

>> While this change may be a prudent safeguard, there is something
>> else going on.  Can you provide the callchain that led to the
>> parse_object_buffer() that gave SHA1 of a commit object with
>> type set to OBJ_TREE?  Which caller does that bogus combination?
>
> Sure.
> Here's valgrind output from running this (from my reproducer):
>
>     valgrind --trace-children=yes git clone . k
> ---------------
>   error: Object 0d57588da39d10795486bd5451bc2660832228e6 is a commit, not a tree
>   ==9483== Invalid read of size 1
>   ==9483==    at 0x405C27: parse_object_buffer (object.c:146)
>   ==9483==    by 0x405CE4: parse_object (object.c:187)
>   ==9483==    by 0x403185: send_ref (upload-pack.c:561)
>   ==9483==    by 0x408EEF: do_for_each_ref (refs.c:546)
>   ==9483==    by 0x4036EC: main (upload-pack.c:587)

Sorry, I asked for a wrong thing.  parse_object() reads and
finds out the type, so type there is presumably the right type
of the object (which is OBJ_TREE).  Then parse_object_buffer()
checks if it has already seen the object of the same SHA-1 and
finds that somebody had earlier told that SHA-1 name is of a
commit object.  Either you found a SHA-1 collision (highly
unlikely) or the earlier caller had lied.  And I think what
really needs to be fixed is that lying caller.  That is not in
the above call chain.