[PATCH 0/7] ensure index is set for all OBJ_COMMIT objects variable

[Jeff King <peff@peff.net>]

Here's a series to address the bug I mentioned earlier by catching the
conversion of OBJ_NONE to OBJ_COMMIT in a central location and setting
the index there.

I've included your patch 1/2 unchanged in the beginning, as I build on
top of it (and your patch 2/2 is no longer applicable).  The rest is
refactoring leading up to patch 6 to fix the bug. Patch 7 is a bonus
cleanup.

I'd hoped to cap off the series by converting the "type" field of
"struct commit" to a "const unsigned type : 3", which would avoid any
new callers being added that would touch it without going through the
proper procedure.  However, it's a bitfield, which makes it hard to cast
the constness away in the actual setter function. My best attempt was to
use a union with matching const and non-const members, but that would
mean changing all of the sites which read the field (and there are many)
to use "object->type.read".

There may be a clever solution hiding in a dark corner of C, but I
suspect we are entering a realm of portability problems with older
compilers (I even saw one compiler's documentation claim that "const"
was forbidden on bitfields, even though C99 has an example which does
it).

  [1/7]: alloc.c: remove the alloc_raw_commit_node() function
  [2/7]: move setting of object->type to alloc_* functions
  [3/7]: parse_object_buffer: do not set object type
  [4/7]: add object_as_type helper for casting objects
  [5/7]: alloc: factor out commit index
  [6/7]: object_as_type: set commit index
  [7/7]: diff-tree: avoid lookup_unknown_object

-Peff