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

[Junio C Hamano <gitster@pobox.com>]

Mike Hommey <mh@glandium.org> writes:

> 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?

For a single-patch topic like this one, if you like what was in the
e-mail verbatim, saying so is sufficient, as I can just use the
material to run "git commit --amend".  For anything more involved
(e.g. "oh, then insert a code to do this before that function"
and/or a fix to an earlier patch in a multi-patch series), I'd
prefer a re-submission, which can be processed just the same way as
any other new topic.

Thanks.