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

From: Jeff King <peff@peff.net>
Date: Jan 12, 2021, 09:06 UTC
Message-ID: <X/1mpl/ZmL4NPIEm@coredump.intra.peff.net>
In-Reply-To: <54f4ad329f56808432549aa885f2847d5c9a8ac6.1610129796.git.me@ttaylorr.com>

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

Show 13 quoted lines

> 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