Re: [PATCH v5 03/19] builtin/diff-tree: convert to struct object_id

[Jeff King <peff@peff.net>]

On Wed, Feb 22, 2017 at 10:50:21AM -0800, Junio C Hamano wrote:

> "brian m. carlson" <sandals@crustytoothpaste.net> writes:
> 
> > Convert most leaf functions to struct object_id.  Change several
> > hardcoded numbers to uses of parse_oid_hex.  In doing so, verify that we
> > when we want two trees, we have exactly two trees.
> >
> > Finally, in stdin_diff_commit, avoid accessing the byte after the NUL.
> > This will be a NUL as well, since the first NUL was a newline we
> > overwrote.  However, with parse_oid_hex, we no longer need to increment
> > the pointer directly, and can simply increment it as part of our check
> > for the space character.
> 
> After reading the pre- and post-image twice, I think I convinced
> myself that this is a faithful conersion and they do the same thing.

I think this is correct, too (but then, I think it largely comes from
the patch I wrote the other night. So I did look at it carefully, but
it's not exactly an independent review).

> What the function does appears somewhat iffy in the modern world.
> We are relying on the fact that while Git is operating in this mode
> of reading a tuple of commits per line and doing log-tree, that Git
> itself will not do the history traversal (instead, another instance
> of Git that feeds us via our standard input is walking the history)
> for this piece of code to work correctly.  
> 
> Of course, the "diff-tree --stdin" command was meant to sit on the
> downstream of "rev-list --parents", so the assumption the code makes
> (i.e. the parents field of the in-core commit objects do not have to
> be usable for history traversal) may be reasonable, but still...

I'm not sure it's that weird. "diff-tree" is about diffing, not
traversal. The only reason it touches commit->parents at all (and
doesn't just kick off a diff between the arguments it gets) is that it's
been stuck with pretty-printing the commits, which might ask to show the
parents.

-Peff