Re: [RFC PATCH] diff: only prefetch for certain output formats

[Jeff King <peff@peff.net>]

On Wed, Jan 29, 2020 at 05:39:00PM -0800, Jonathan Tan wrote:

> > You could get pretty specific by putting logic inside diffcore_rename(),
> > which would know if anything is left over after exact rename detection,
> > but I suspect just checking:
> > 
> >   (options->break_opt != -1 || options->detect_rename)
> > 
> > in diffcore_std() would be OK in practice.
> 
> Thanks for taking a look at this patch and for the pointers, especially
> to rename detection. I investigated and found that in practice, with
> respect to rename detection, options->detect_rename is insufficient to
> determine exactly when we need to fetch; we need to fetch when
> (for example) a file is deleted and another added, but not when a file
> is merely changed, and these rules are not reflected in
> options->detect_rename. These rules indeed are in diffcore_rename(), as
> you mentioned, but putting logic inside diffcore_rename() (or copying
> the same logic over to diffcore_std()) complicates things for too little
> benefit, I think.
> 
> To add to this, rename detection is turned on by default, so it wouldn't
> even fix the original issue with "status".
> 
> So I'll abandon this patch, at least until someone finds a use case for
> diffing with no rename detection on a partial clone and would rather not
> have a prefetch.

Ah, true, "options->detect_rename" would be overly broad.

I actually don't think it would be that bad to put the logic in
diffcore_rename(). If we wait until the right moment (after inexact
renames have been resolved, and when we see if there are any candidates
left), it should just be a matter of walking over the candidate lists.

Something like this (it would need the add_if_missing() helper from
diffcore_std()):

diff --git a/diffcore-rename.c b/diffcore-rename.c
index 531d7adeaf..d519ffcc45 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -458,6 +458,7 @@ void diffcore_rename(struct diff_options *options)
 	int i, j, rename_count, skip_unmodified = 0;
 	int num_create, dst_cnt;
 	struct progress *progress = NULL;
+	struct oid_array to_fetch = OID_ARRAY_INIT;
 
 	if (!minimum_score)
 		minimum_score = DEFAULT_RENAME_SCORE;
@@ -538,6 +539,25 @@ void diffcore_rename(struct diff_options *options)
 		break;
 	}
 
+	/*
+	 * At this point we know there's actual work to do: we have rename
+	 * destinations that didn't find an exact match, and we have potential
+	 * sources. So we'll have to do inexact rename detection, which
+	 * requires looking at the blobs. It's worth pre-fetching them as a
+	 * group now.
+	 */
+	for (i = 0; i < rename_dst_nr; i++) {
+		if (rename_dst[i].pair)
+			continue; /* already found exact match */
+		add_if_missing(options->repo, &to_fetch, rename_dst[i].two);
+	}
+	for (i = 0; i < rename_src_nr; i++) {
+		add_if_missing(options->repo, &to_fetch, rename_src[i].p->one);
+	}
+	if (to_fetch.nr)
+		promisor_remote_get_direct(options->repo,
+					   to_fetch.oid, to_fetch.nr);
+
 	if (options->show_rename_progress) {
 		progress = start_delayed_progress(
 				_("Performing inexact rename detection"),