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

Mike Hommey [off-list ref] writes:

On Tue, Jul 03, 2018 at 11:41:42AM -0700, Junio C Hamano wrote:

quoted

Mike Hommey [off-list ref] writes:

quoted

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 ...

quoted

Signed-off-by: Mike Hommey <redacted>
---
 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.