On Tue, Jul 03, 2018 at 11:41:42AM -0700, Junio C Hamano wrote:
> Mike Hommey <mh@xxxxxxxxxxxx> writes:
>
> > When the reference buffer is empty, diff_delta returns NULL without
> > really attempting anything, yet fast-import counts that as a delta
> > attempt.
>
> But that is an attempt nevertheless, no? Attempted and failed to
> find anything useful, that is. What problem are you trying to solve
> and what issue are you trying to address, exactly?
>
> ... goes and reads the patch ...
>
> > Signed-off-by: Mike Hommey <mh@xxxxxxxxxxxx>
> > ---
> > fast-import.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fast-import.c b/fast-import.c
> > index 4d55910ab9..12195d54d7 100644
> > --- a/fast-import.c
> > +++ b/fast-import.c
> > @@ -1076,7 +1076,7 @@ static int store_object(
> > return 1;
> > }
> >
> > - if (last && last->data.buf && last->depth < max_depth
> > + if (last && last->data.len && last->data.buf && last->depth < max_depth
> > && dat->len > the_hash_algo->rawsz) {
> >
> > delta_count_attempts_by_type[type]++;
>
> This is a misleading patch as the title and the proposed log message
> both talk about "attempts are counted but we didn't actually do
> anything", implying that the only problem is that the counter is not
> aligned with reality. The fact that the post-context we see here
> only shows the "counting" part does not help us, either.
>
> But the next line in the post-context is actually code that does
> something important, which is ...
>
> delta = diff_delta(last->data.buf, last->data.len,
> dat->buf, dat->len,
> &deltalen, dat->len - the_hash_algo->rawsz);
> } else
> delta = NULL;
>
>
> ... and makes the reader realize that the change itself is much
> better than the patch with 3-line context, the title, and the
> proposed log message advertises it as ;-)
>
> How about selling it this way instead?
>
> fast-import: do not call diff_delta() with empty buffer
>
> We know diff_delta() returns NULL, saying "no good delta
> exists for it", when fed an empty data. Check the length of
> the data in the caller to avoid such a call.
>
> This incidentally reduces the number of attempted deltification
> we see in the final statistics.
>
> or something like that?
Fair enough. Do you want me to send a v2 with this description?
Mike
