Re: [PATCH] fast-import: Don't count delta attempts against an empty buffer

[Mike Hommey <mh@glandium.org>]

On Tue, Jul 03, 2018 at 11:41:42AM -0700, Junio C Hamano wrote:
> Mike Hommey <mh@glandium.org> 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@glandium.org>
> > ---
> >  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