Re: [PATCH 09/20] try_partial_reuse(): convert to new revindex API

On Fri, Jan 08, 2021 at 01:17:17PM -0500, Taylor Blau wrote:

Remove another instance of direct revindex manipulation by calling
'pack_pos_to_offset()' instead (the caller here does not care about the
index position of the object at position 'pos').

Somewhat confusingly, the subsequent call to unpack_object_header()
takes a pointer to &offset and then updates it with a new value. But,
try_partial_reuse() cares about the offset of both the base's header and
contents. The existing code made a copy of the offset field, and only
addresses and manipulates one of them.

Instead, store the return of pack_pos_to_offset twice: once in header
and another in offset. Header will be left untouched, but offset will be
addressed and modified by unpack_object_header().

I had to read these second two paragraphs a few times to parse them.
Really we are just replacing revidx->offset with "header", and "offset"
retains its same role within the function.

So it's definitely doing the right thing, but it makes more sense to me
as:

  Note that we cannot just use the existing "offset" variable to store
  the value we get from pack_pos_to_offset(). It is incremented by
  unpack_object_header(), but we later need the original value. Since
  we'll no longer have revindex->offset to read it from, we'll store
  that in a separate variable ("header" since it points to the entry's
  header bytes).

Another option would be to just call pack_pos_to_offset() again for the
later call. Like the code it's replacing, it's constant-time anyway. But
I think the "header" variable actually makes things more readable.

-Peff