patchdiff: restrict when prefetching occurs

diff.c, diffcore-rename.c, diffcore.h, t/t4067-diff-partial-clone.sh

diff --git a/diff.c b/diff.c
index 1010d806f5..19c5d638d6 100644
--- a/diff.c
+++ b/diff.c
@@ -6507,10 +6507,24 @@ static void add_if_missing(struct repository *r,
 
 void diffcore_std(struct diff_options *options)
 {
-	if (options->repo == the_repository && has_promisor_remote()) {
-		/*
-		 * Prefetch the diff pairs that are about to be flushed.
-		 */
+	int prefetched = 0;
+	int output_formats_to_prefetch = DIFF_FORMAT_DIFFSTAT |
+		DIFF_FORMAT_NUMSTAT |
+		DIFF_FORMAT_PATCH |
+		DIFF_FORMAT_SHORTSTAT |
+		DIFF_FORMAT_DIRSTAT;
+
+	/*
+	 * Check if the user requested a blob-data-requiring diff output and/or
+	 * break-rewrite detection (which requires blob data). If yes, prefetch
+	 * the diff pairs.
+	 *
+	 * If no prefetching occurs, diffcore_rename() will prefetch if it
+	 * decides that it needs inexact rename detection.
+	 */
+	if (options->repo == the_repository && has_promisor_remote() &&
+	    (options->output_format & output_formats_to_prefetch ||
+	     (!options->found_follow && options->break_opt != -1))) {
 		int i;
 		struct diff_queue_struct *q = &diff_queued_diff;
 		struct oid_array to_fetch = OID_ARRAY_INIT;
@@ -6520,6 +6534,8 @@ void diffcore_std(struct diff_options *options)
 			add_if_missing(options->repo, &to_fetch, p->one);
 			add_if_missing(options->repo, &to_fetch, p->two);
 		}
+		prefetched = 1;
+
 		if (to_fetch.nr)
 			/*
 			 * NEEDSWORK: Consider deduplicating the OIDs sent.
@@ -6538,7 +6554,7 @@ void diffcore_std(struct diff_options *options)
 			diffcore_break(options->repo,
 				       options->break_opt);
 		if (options->detect_rename)
-			diffcore_rename(options);
+			diffcore_rename(options, prefetched);
 		if (options->break_opt != -1)
 			diffcore_merge_broken();
 	}
diff --git a/diffcore-rename.c b/diffcore-rename.c
index e189f407af..962565f066 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -7,6 +7,7 @@
 #include "object-store.h"
 #include "hashmap.h"
 #include "progress.h"
+#include "promisor-remote.h"
 
 /* Table of rename/copy destinations */
 
@@ -448,7 +449,18 @@ static int find_renames(struct diff_score *mx, int dst_cnt, int minimum_score, i
 	return count;
 }
 
-void diffcore_rename(struct diff_options *options)
+static void add_if_missing(struct repository *r,
+			   struct oid_array *to_fetch,
+			   const struct diff_filespec *filespec)
+{
+	if (filespec && filespec->oid_valid &&
+	    !S_ISGITLINK(filespec->mode) &&
+	    oid_object_info_extended(r, &filespec->oid, NULL,
+				     OBJECT_INFO_FOR_PREFETCH))
+		oid_array_append(to_fetch, &filespec->oid);
+}
+
+void diffcore_rename(struct diff_options *options, int prefetched)
 {
 	int detect_rename = options->detect_rename;
 	int minimum_score = options->rename_score;
@@ -538,6 +550,32 @@ void diffcore_rename(struct diff_options *options)
 		break;
 	}
 
+	if (!prefetched) {
+		/*
+		 * 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.
+		 *
+		 * If we haven't already prefetched, it's worth pre-fetching
+		 * them as a group now.
+		 */
+		int i;
+		struct oid_array to_fetch = OID_ARRAY_INIT;
+
+		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);
+		oid_array_clear(&to_fetch);
+	}
+
 	if (options->show_rename_progress) {
 		progress = start_delayed_progress(
 				_("Performing inexact rename detection"),
diff --git a/diffcore.h b/diffcore.h
index 7c07347e42..9f69506574 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -144,7 +144,7 @@ struct diff_filepair *diff_queue(struct diff_queue_struct *,
 void diff_q(struct diff_queue_struct *, struct diff_filepair *);
 
 void diffcore_break(struct repository *, int);
-void diffcore_rename(struct diff_options *);
+void diffcore_rename(struct diff_options *, int prefetched);
 void diffcore_merge_broken(void);
 void diffcore_pickaxe(struct diff_options *);
 void diffcore_order(const char *orderfile);
diff --git a/t/t4067-diff-partial-clone.sh b/t/t4067-diff-partial-clone.sh
index 4831ad35e6..7acb64727d 100755
--- a/t/t4067-diff-partial-clone.sh
+++ b/t/t4067-diff-partial-clone.sh
@@ -131,4 +131,53 @@ test_expect_success 'diff with rename detection batches blobs' '
 	test_line_count = 1 done_lines
 '
 
+test_expect_success 'diff does not fetch anything if inexact rename detection is not needed' '
+	test_when_finished "rm -rf server client trace" &&
+
+	test_create_repo server &&
+	echo a >server/a &&
+	printf "b\nb\nb\nb\nb\n" >server/b &&
+	git -C server add a b &&
+	git -C server commit -m x &&
+	rm server/b &&
+	printf "b\nb\nb\nb\nb\n" >server/c &&
+	git -C server add c &&
+	git -C server commit -a -m x &&
+
+	test_config -C server uploadpack.allowfilter 1 &&
+	test_config -C server uploadpack.allowanysha1inwant 1 &&
+	git clone --bare --filter=blob:limit=0 "file://$(pwd)/server" client &&
+
+	# Ensure no fetches.
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --raw -M HEAD^ HEAD &&
+	! test_path_exists trace
+'
+
+test_expect_success 'diff --break-rewrites fetches only if necessary, and batches blobs if it does' '
+	test_when_finished "rm -rf server client trace" &&
+
+	test_create_repo server &&
+	echo a >server/a &&
+	printf "b\nb\nb\nb\nb\n" >server/b &&
+	git -C server add a b &&
+	git -C server commit -m x &&
+	printf "c\nc\nc\nc\nc\n" >server/b &&
+	git -C server commit -a -m x &&
+
+	test_config -C server uploadpack.allowfilter 1 &&
+	test_config -C server uploadpack.allowanysha1inwant 1 &&
+	git clone --bare --filter=blob:limit=0 "file://$(pwd)/server" client &&
+
+	# Ensure no fetches.
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --raw -M HEAD^ HEAD &&
+	! test_path_exists trace &&
+
+	# But with --break-rewrites, ensure that there is exactly 1 negotiation
+	# by checking that there is only 1 "done" line sent. ("done" marks the
+	# end of negotiation.)
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --break-rewrites --raw -M HEAD^ HEAD &&
+	grep "git> done" trace >done_lines &&
+	test_line_count = 1 done_lines
+'
+
 test_done

builtin/index-pack.c, diff.c, promisor-remote.c, promisor-remote.h, unpack-trees.c

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index d967d188a3..f176dd28c8 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1368,9 +1368,8 @@ static void fix_unresolved_deltas(struct hashfile *f)
 				continue;
 			oid_array_append(&to_fetch, &d->oid);
 		}
-		if (to_fetch.nr)
-			promisor_remote_get_direct(the_repository,
-						   to_fetch.oid, to_fetch.nr);
+		promisor_remote_get_direct(the_repository,
+					   to_fetch.oid, to_fetch.nr);
 		oid_array_clear(&to_fetch);
 	}
 
diff --git a/diff.c b/diff.c
index 1010d806f5..f01b4d91b8 100644
--- a/diff.c
+++ b/diff.c
@@ -6520,12 +6520,11 @@ void diffcore_std(struct diff_options *options)
 			add_if_missing(options->repo, &to_fetch, p->one);
 			add_if_missing(options->repo, &to_fetch, p->two);
 		}
-		if (to_fetch.nr)
-			/*
-			 * NEEDSWORK: Consider deduplicating the OIDs sent.
-			 */
-			promisor_remote_get_direct(options->repo,
-						   to_fetch.oid, to_fetch.nr);
+		/*
+		 * NEEDSWORK: Consider deduplicating the OIDs sent.
+		 */
+		promisor_remote_get_direct(options->repo,
+					   to_fetch.oid, to_fetch.nr);
 		oid_array_clear(&to_fetch);
 	}
 
diff --git a/promisor-remote.c b/promisor-remote.c
index 9f338c945f..2155dfe657 100644
--- a/promisor-remote.c
+++ b/promisor-remote.c
@@ -241,6 +241,9 @@ int promisor_remote_get_direct(struct repository *repo,
 	int to_free = 0;
 	int res = -1;
 
+	if (oid_nr == 0)
+		return 0;
+
 	promisor_remote_init();
 
 	for (r = promisors; r; r = r->next) {
diff --git a/promisor-remote.h b/promisor-remote.h
index 737bac3a33..6343c47d18 100644
--- a/promisor-remote.h
+++ b/promisor-remote.h
@@ -20,6 +20,14 @@ struct promisor_remote {
 void promisor_remote_reinit(void);
 struct promisor_remote *promisor_remote_find(const char *remote_name);
 int has_promisor_remote(void);
+
+/*
+ * Fetches all requested objects from all promisor remotes, trying them one at
+ * a time until all objects are fetched. Returns 0 upon success, and non-zero
+ * otherwise.
+ *
+ * If oid_nr is 0, this function returns 0 (success) immediately.
+ */
 int promisor_remote_get_direct(struct repository *repo,
 			       const struct object_id *oids,
 			       int oid_nr);
diff --git a/unpack-trees.c b/unpack-trees.c
index f618a644ef..4c3191b947 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -423,9 +423,8 @@ static int check_updates(struct unpack_trees_options *o)
 				continue;
 			oid_array_append(&to_fetch, &ce->oid);
 		}
-		if (to_fetch.nr)
-			promisor_remote_get_direct(the_repository,
-						   to_fetch.oid, to_fetch.nr);
+		promisor_remote_get_direct(the_repository,
+					   to_fetch.oid, to_fetch.nr);
 		oid_array_clear(&to_fetch);
 	}
 	for (i = 0; i < index->cache_nr; i++) {

diff.c, diffcore-rename.c, diffcore.h, t/t4067-diff-partial-clone.sh

diff --git a/diff.c b/diff.c
index f01b4d91b8..857f02f481 100644
--- a/diff.c
+++ b/diff.c
@@ -6494,9 +6494,9 @@ void diffcore_fix_diff_index(void)
 	QSORT(q->queue, q->nr, diffnamecmp);
 }
 
-static void add_if_missing(struct repository *r,
-			   struct oid_array *to_fetch,
-			   const struct diff_filespec *filespec)
+void diff_add_if_missing(struct repository *r,
+			 struct oid_array *to_fetch,
+			 const struct diff_filespec *filespec)
 {
 	if (filespec && filespec->oid_valid &&
 	    !S_ISGITLINK(filespec->mode) &&
@@ -6507,24 +6507,42 @@ static void add_if_missing(struct repository *r,
 
 void diffcore_std(struct diff_options *options)
 {
-	if (options->repo == the_repository && has_promisor_remote()) {
-		/*
-		 * Prefetch the diff pairs that are about to be flushed.
-		 */
+	int prefetched = 0;
+	int output_formats_to_prefetch = DIFF_FORMAT_DIFFSTAT |
+		DIFF_FORMAT_NUMSTAT |
+		DIFF_FORMAT_PATCH |
+		DIFF_FORMAT_SHORTSTAT |
+		DIFF_FORMAT_DIRSTAT;
+
+	/*
+	 * Check if the user requested a blob-data-requiring diff output and/or
+	 * break-rewrite detection (which requires blob data). If yes, prefetch
+	 * the diff pairs.
+	 *
+	 * If no prefetching occurs, diffcore_rename() will prefetch if it
+	 * decides that it needs inexact rename detection.
+	 */
+	if (options->repo == the_repository && has_promisor_remote() &&
+	    (options->output_format & output_formats_to_prefetch ||
+	     (!options->found_follow && options->break_opt != -1))) {
 		int i;
 		struct diff_queue_struct *q = &diff_queued_diff;
 		struct oid_array to_fetch = OID_ARRAY_INIT;
 
 		for (i = 0; i < q->nr; i++) {
 			struct diff_filepair *p = q->queue[i];
-			add_if_missing(options->repo, &to_fetch, p->one);
-			add_if_missing(options->repo, &to_fetch, p->two);
+			diff_add_if_missing(options->repo, &to_fetch, p->one);
+			diff_add_if_missing(options->repo, &to_fetch, p->two);
 		}
+
+		prefetched = 1;
+
 		/*
 		 * NEEDSWORK: Consider deduplicating the OIDs sent.
 		 */
 		promisor_remote_get_direct(options->repo,
 					   to_fetch.oid, to_fetch.nr);
+
 		oid_array_clear(&to_fetch);
 	}
 
@@ -6537,7 +6555,7 @@ void diffcore_std(struct diff_options *options)
 			diffcore_break(options->repo,
 				       options->break_opt);
 		if (options->detect_rename)
-			diffcore_rename(options);
+			diffcore_rename(options, prefetched);
 		if (options->break_opt != -1)
 			diffcore_merge_broken();
 	}
diff --git a/diffcore-rename.c b/diffcore-rename.c
index e189f407af..79ac1b4bee 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -7,6 +7,7 @@
 #include "object-store.h"
 #include "hashmap.h"
 #include "progress.h"
+#include "promisor-remote.h"
 
 /* Table of rename/copy destinations */
 
@@ -448,7 +449,7 @@ static int find_renames(struct diff_score *mx, int dst_cnt, int minimum_score, i
 	return count;
 }
 
-void diffcore_rename(struct diff_options *options)
+void diffcore_rename(struct diff_options *options, int prefetched)
 {
 	int detect_rename = options->detect_rename;
 	int minimum_score = options->rename_score;
@@ -538,6 +539,40 @@ void diffcore_rename(struct diff_options *options)
 		break;
 	}
 
+	if (!prefetched) {
+		/*
+		 * 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.
+		 *
+		 * If we haven't already prefetched, it's worth pre-fetching
+		 * them as a group now.
+		 */
+		int i;
+		struct oid_array to_fetch = OID_ARRAY_INIT;
+
+		for (i = 0; i < rename_dst_nr; i++) {
+			if (rename_dst[i].pair)
+				continue; /* already found exact match */
+			diff_add_if_missing(options->repo, &to_fetch, rename_dst[i].two);
+		}
+		for (i = 0; i < rename_src_nr; i++) {
+			if (skip_unmodified &&
+			    diff_unmodified_pair(rename_src[i].p))
+				/*
+				 * The "for" loop below will not need these
+				 * blobs, so skip prefetching.
+				 */
+				continue;
+			diff_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);
+		oid_array_clear(&to_fetch);
+	}
+
 	if (options->show_rename_progress) {
 		progress = start_delayed_progress(
 				_("Performing inexact rename detection"),
diff --git a/diffcore.h b/diffcore.h
index 7c07347e42..d7af6ab018 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -144,7 +144,7 @@ struct diff_filepair *diff_queue(struct diff_queue_struct *,
 void diff_q(struct diff_queue_struct *, struct diff_filepair *);
 
 void diffcore_break(struct repository *, int);
-void diffcore_rename(struct diff_options *);
+void diffcore_rename(struct diff_options *, int prefetched);
 void diffcore_merge_broken(void);
 void diffcore_pickaxe(struct diff_options *);
 void diffcore_order(const char *orderfile);
@@ -182,4 +182,12 @@ int diffcore_count_changes(struct repository *r,
 			   unsigned long *src_copied,
 			   unsigned long *literal_added);
 
+/*
+ * If filespec contains an OID and if that object is missing from the given
+ * repository, add that OID to to_fetch.
+ */
+void diff_add_if_missing(struct repository *r,
+			 struct oid_array *to_fetch,
+			 const struct diff_filespec *filespec);
+
 #endif
diff --git a/t/t4067-diff-partial-clone.sh b/t/t4067-diff-partial-clone.sh
index 4831ad35e6..c1ed1c2fc4 100755
--- a/t/t4067-diff-partial-clone.sh
+++ b/t/t4067-diff-partial-clone.sh
@@ -131,4 +131,52 @@ test_expect_success 'diff with rename detection batches blobs' '
 	test_line_count = 1 done_lines
 '
 
+test_expect_success 'diff does not fetch anything if inexact rename detection is not needed' '
+	test_when_finished "rm -rf server client trace" &&
+
+	test_create_repo server &&
+	echo a >server/a &&
+	printf "b\nb\nb\nb\nb\n" >server/b &&
+	git -C server add a b &&
+	git -C server commit -m x &&
+	mv server/b server/c &&
+	git -C server add c &&
+	git -C server commit -a -m x &&
+
+	test_config -C server uploadpack.allowfilter 1 &&
+	test_config -C server uploadpack.allowanysha1inwant 1 &&
+	git clone --bare --filter=blob:limit=0 "file://$(pwd)/server" client &&
+
+	# Ensure no fetches.
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --raw -M HEAD^ HEAD &&
+	! test_path_exists trace
+'
+
+test_expect_success 'diff --break-rewrites fetches only if necessary, and batches blobs if it does' '
+	test_when_finished "rm -rf server client trace" &&
+
+	test_create_repo server &&
+	echo a >server/a &&
+	printf "b\nb\nb\nb\nb\n" >server/b &&
+	git -C server add a b &&
+	git -C server commit -m x &&
+	printf "c\nc\nc\nc\nc\n" >server/b &&
+	git -C server commit -a -m x &&
+
+	test_config -C server uploadpack.allowfilter 1 &&
+	test_config -C server uploadpack.allowanysha1inwant 1 &&
+	git clone --bare --filter=blob:limit=0 "file://$(pwd)/server" client &&
+
+	# Ensure no fetches.
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --raw -M HEAD^ HEAD &&
+	! test_path_exists trace &&
+
+	# But with --break-rewrites, ensure that there is exactly 1 negotiation
+	# by checking that there is only 1 "done" line sent. ("done" marks the
+	# end of negotiation.)
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --break-rewrites --raw -M HEAD^ HEAD &&
+	grep "git> done" trace >done_lines &&
+	test_line_count = 1 done_lines
+'
+
 test_done

builtin/index-pack.c, diff.c, promisor-remote.c, promisor-remote.h, unpack-trees.c

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index d967d188a3..f176dd28c8 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1368,9 +1368,8 @@ static void fix_unresolved_deltas(struct hashfile *f)
 				continue;
 			oid_array_append(&to_fetch, &d->oid);
 		}
-		if (to_fetch.nr)
-			promisor_remote_get_direct(the_repository,
-						   to_fetch.oid, to_fetch.nr);
+		promisor_remote_get_direct(the_repository,
+					   to_fetch.oid, to_fetch.nr);
 		oid_array_clear(&to_fetch);
 	}
 
diff --git a/diff.c b/diff.c
index 1010d806f5..f01b4d91b8 100644
--- a/diff.c
+++ b/diff.c
@@ -6520,12 +6520,11 @@ void diffcore_std(struct diff_options *options)
 			add_if_missing(options->repo, &to_fetch, p->one);
 			add_if_missing(options->repo, &to_fetch, p->two);
 		}
-		if (to_fetch.nr)
-			/*
-			 * NEEDSWORK: Consider deduplicating the OIDs sent.
-			 */
-			promisor_remote_get_direct(options->repo,
-						   to_fetch.oid, to_fetch.nr);
+		/*
+		 * NEEDSWORK: Consider deduplicating the OIDs sent.
+		 */
+		promisor_remote_get_direct(options->repo,
+					   to_fetch.oid, to_fetch.nr);
 		oid_array_clear(&to_fetch);
 	}
 
diff --git a/promisor-remote.c b/promisor-remote.c
index 9f338c945f..2155dfe657 100644
--- a/promisor-remote.c
+++ b/promisor-remote.c
@@ -241,6 +241,9 @@ int promisor_remote_get_direct(struct repository *repo,
 	int to_free = 0;
 	int res = -1;
 
+	if (oid_nr == 0)
+		return 0;
+
 	promisor_remote_init();
 
 	for (r = promisors; r; r = r->next) {
diff --git a/promisor-remote.h b/promisor-remote.h
index 737bac3a33..6343c47d18 100644
--- a/promisor-remote.h
+++ b/promisor-remote.h
@@ -20,6 +20,14 @@ struct promisor_remote {
 void promisor_remote_reinit(void);
 struct promisor_remote *promisor_remote_find(const char *remote_name);
 int has_promisor_remote(void);
+
+/*
+ * Fetches all requested objects from all promisor remotes, trying them one at
+ * a time until all objects are fetched. Returns 0 upon success, and non-zero
+ * otherwise.
+ *
+ * If oid_nr is 0, this function returns 0 (success) immediately.
+ */
 int promisor_remote_get_direct(struct repository *repo,
 			       const struct object_id *oids,
 			       int oid_nr);
diff --git a/unpack-trees.c b/unpack-trees.c
index f618a644ef..4c3191b947 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -423,9 +423,8 @@ static int check_updates(struct unpack_trees_options *o)
 				continue;
 			oid_array_append(&to_fetch, &ce->oid);
 		}
-		if (to_fetch.nr)
-			promisor_remote_get_direct(the_repository,
-						   to_fetch.oid, to_fetch.nr);
+		promisor_remote_get_direct(the_repository,
+					   to_fetch.oid, to_fetch.nr);
 		oid_array_clear(&to_fetch);
 	}
 	for (i = 0; i < index->cache_nr; i++) {

diff.c, diffcore-break.c, diffcore-rename.c, diffcore.h, line-log.c

diff --git a/diff.c b/diff.c
index f01b4d91b8..f337d837ac 100644
--- a/diff.c
+++ b/diff.c
@@ -573,7 +573,7 @@ static int fill_mmfile(struct repository *r, mmfile_t *mf,
 		mf->size = 0;
 		return 0;
 	}
-	else if (diff_populate_filespec(r, one, 0))
+	else if (diff_populate_filespec(r, one, NULL))
 		return -1;
 
 	mf->ptr = one->data;
@@ -585,9 +585,13 @@ static int fill_mmfile(struct repository *r, mmfile_t *mf,
 static unsigned long diff_filespec_size(struct repository *r,
 					struct diff_filespec *one)
 {
+	struct diff_populate_filespec_options dpf_options = {
+		.check_size_only = 1,
+	};
+
 	if (!DIFF_FILE_VALID(one))
 		return 0;
-	diff_populate_filespec(r, one, CHECK_SIZE_ONLY);
+	diff_populate_filespec(r, one, &dpf_options);
 	return one->size;
 }
 
@@ -3020,6 +3024,9 @@ static void show_dirstat(struct diff_options *options)
 		struct diff_filepair *p = q->queue[i];
 		const char *name;
 		unsigned long copied, added, damage;
+		struct diff_populate_filespec_options dpf_options = {
+			.check_size_only = 1,
+		};
 
 		name = p->two->path ? p->two->path : p->one->path;
 
@@ -3047,19 +3054,19 @@ static void show_dirstat(struct diff_options *options)
 		}
 
 		if (DIFF_FILE_VALID(p->one) && DIFF_FILE_VALID(p->two)) {
-			diff_populate_filespec(options->repo, p->one, 0);
-			diff_populate_filespec(options->repo, p->two, 0);
+			diff_populate_filespec(options->repo, p->one, NULL);
+			diff_populate_filespec(options->repo, p->two, NULL);
 			diffcore_count_changes(options->repo,
 					       p->one, p->two, NULL, NULL,
 					       &copied, &added);
 			diff_free_filespec_data(p->one);
 			diff_free_filespec_data(p->two);
 		} else if (DIFF_FILE_VALID(p->one)) {
-			diff_populate_filespec(options->repo, p->one, CHECK_SIZE_ONLY);
+			diff_populate_filespec(options->repo, p->one, &dpf_options);
 			copied = added = 0;
 			diff_free_filespec_data(p->one);
 		} else if (DIFF_FILE_VALID(p->two)) {
-			diff_populate_filespec(options->repo, p->two, CHECK_SIZE_ONLY);
+			diff_populate_filespec(options->repo, p->two, &dpf_options);
 			copied = 0;
 			added = p->two->size;
 			diff_free_filespec_data(p->two);
@@ -3339,13 +3346,17 @@ static void emit_binary_diff(struct diff_options *o,
 int diff_filespec_is_binary(struct repository *r,
 			    struct diff_filespec *one)
 {
+	struct diff_populate_filespec_options dpf_options = {
+		.check_binary = 1,
+	};
+
 	if (one->is_binary == -1) {
 		diff_filespec_load_driver(one, r->index);
 		if (one->driver->binary != -1)
 			one->is_binary = one->driver->binary;
 		else {
 			if (!one->data && DIFF_FILE_VALID(one))
-				diff_populate_filespec(r, one, CHECK_BINARY);
+				diff_populate_filespec(r, one, &dpf_options);
 			if (one->is_binary == -1 && one->data)
 				one->is_binary = buffer_is_binary(one->data,
 						one->size);
@@ -3677,8 +3688,8 @@ static void builtin_diffstat(const char *name_a, const char *name_b,
 	}
 
 	else if (complete_rewrite) {
-		diff_populate_filespec(o->repo, one, 0);
-		diff_populate_filespec(o->repo, two, 0);
+		diff_populate_filespec(o->repo, one, NULL);
+		diff_populate_filespec(o->repo, two, NULL);
 		data->deleted = count_lines(one->data, one->size);
 		data->added = count_lines(two->data, two->size);
 	}
@@ -3914,9 +3925,10 @@ static int diff_populate_gitlink(struct diff_filespec *s, int size_only)
  */
 int diff_populate_filespec(struct repository *r,
 			   struct diff_filespec *s,
-			   unsigned int flags)
+			   const struct diff_populate_filespec_options *options)
 {
-	int size_only = flags & CHECK_SIZE_ONLY;
+	int size_only = options ? options->check_size_only : 0;
+	int check_binary = options ? options->check_binary : 0;
 	int err = 0;
 	int conv_flags = global_conv_flags_eol;
 	/*
@@ -3986,7 +3998,7 @@ int diff_populate_filespec(struct repository *r,
 		 * opening the file and inspecting the contents, this
 		 * is probably fine.
 		 */
-		if ((flags & CHECK_BINARY) &&
+		if (check_binary &&
 		    s->size > big_file_threshold && s->is_binary == -1) {
 			s->is_binary = 1;
 			return 0;
@@ -4012,7 +4024,7 @@ int diff_populate_filespec(struct repository *r,
 	}
 	else {
 		enum object_type type;
-		if (size_only || (flags & CHECK_BINARY)) {
+		if (size_only || check_binary) {
 			type = oid_object_info(r, &s->oid, &s->size);
 			if (type < 0)
 				die("unable to read %s",
@@ -4144,7 +4156,7 @@ static struct diff_tempfile *prepare_temp_file(struct repository *r,
 		return temp;
 	}
 	else {
-		if (diff_populate_filespec(r, one, 0))
+		if (diff_populate_filespec(r, one, NULL))
 			die("cannot read data blob for %s", one->path);
 		prep_temp_blob(r->index, name, temp,
 			       one->data, one->size,
@@ -6410,9 +6422,9 @@ static int diff_filespec_is_identical(struct repository *r,
 {
 	if (S_ISGITLINK(one->mode))
 		return 0;
-	if (diff_populate_filespec(r, one, 0))
+	if (diff_populate_filespec(r, one, NULL))
 		return 0;
-	if (diff_populate_filespec(r, two, 0))
+	if (diff_populate_filespec(r, two, NULL))
 		return 0;
 	return !memcmp(one->data, two->data, one->size);
 }
@@ -6420,6 +6432,10 @@ static int diff_filespec_is_identical(struct repository *r,
 static int diff_filespec_check_stat_unmatch(struct repository *r,
 					    struct diff_filepair *p)
 {
+	struct diff_populate_filespec_options dpf_options = {
+		.check_size_only = 1,
+	};
+
 	if (p->done_skip_stat_unmatch)
 		return p->skip_stat_unmatch_result;
 
@@ -6442,8 +6458,8 @@ static int diff_filespec_check_stat_unmatch(struct repository *r,
 	    !DIFF_FILE_VALID(p->two) ||
 	    (p->one->oid_valid && p->two->oid_valid) ||
 	    (p->one->mode != p->two->mode) ||
-	    diff_populate_filespec(r, p->one, CHECK_SIZE_ONLY) ||
-	    diff_populate_filespec(r, p->two, CHECK_SIZE_ONLY) ||
+	    diff_populate_filespec(r, p->one, &dpf_options) ||
+	    diff_populate_filespec(r, p->two, &dpf_options) ||
 	    (p->one->size != p->two->size) ||
 	    !diff_filespec_is_identical(r, p->one, p->two)) /* (2) */
 		p->skip_stat_unmatch_result = 1;
@@ -6773,7 +6789,7 @@ size_t fill_textconv(struct repository *r,
 			*outbuf = "";
 			return 0;
 		}
-		if (diff_populate_filespec(r, df, 0))
+		if (diff_populate_filespec(r, df, NULL))
 			die("unable to read files to diff");
 		*outbuf = df->data;
 		return df->size;
diff --git a/diffcore-break.c b/diffcore-break.c
index 9d20a6a6fc..e8f6322c6a 100644
--- a/diffcore-break.c
+++ b/diffcore-break.c
@@ -62,8 +62,8 @@ static int should_break(struct repository *r,
 	    oideq(&src->oid, &dst->oid))
 		return 0; /* they are the same */
 
-	if (diff_populate_filespec(r, src, 0) ||
-	    diff_populate_filespec(r, dst, 0))
+	if (diff_populate_filespec(r, src, NULL) ||
+	    diff_populate_filespec(r, dst, NULL))
 		return 0; /* error but caught downstream */
 
 	max_size = ((src->size > dst->size) ? src->size : dst->size);
diff --git a/diffcore-rename.c b/diffcore-rename.c
index e189f407af..bf4c0b8740 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -148,6 +148,9 @@ static int estimate_similarity(struct repository *r,
 	 */
 	unsigned long max_size, delta_size, base_size, src_copied, literal_added;
 	int score;
+	struct diff_populate_filespec_options dpf_options = {
+		.check_size_only = 1
+	};
 
 	/* We deal only with regular files.  Symlink renames are handled
 	 * only when they are exact matches --- in other words, no edits
@@ -166,10 +169,10 @@ static int estimate_similarity(struct repository *r,
 	 * say whether the size is valid or not!)
 	 */
 	if (!src->cnt_data &&
-	    diff_populate_filespec(r, src, CHECK_SIZE_ONLY))
+	    diff_populate_filespec(r, src, &dpf_options))
 		return 0;
 	if (!dst->cnt_data &&
-	    diff_populate_filespec(r, dst, CHECK_SIZE_ONLY))
+	    diff_populate_filespec(r, dst, &dpf_options))
 		return 0;
 
 	max_size = ((src->size > dst->size) ? src->size : dst->size);
@@ -187,9 +190,9 @@ static int estimate_similarity(struct repository *r,
 	if (max_size * (MAX_SCORE-minimum_score) < delta_size * MAX_SCORE)
 		return 0;
 
-	if (!src->cnt_data && diff_populate_filespec(r, src, 0))
+	if (!src->cnt_data && diff_populate_filespec(r, src, NULL))
 		return 0;
-	if (!dst->cnt_data && diff_populate_filespec(r, dst, 0))
+	if (!dst->cnt_data && diff_populate_filespec(r, dst, NULL))
 		return 0;
 
 	if (diffcore_count_changes(r, src, dst,
@@ -261,7 +264,7 @@ static unsigned int hash_filespec(struct repository *r,
 				  struct diff_filespec *filespec)
 {
 	if (!filespec->oid_valid) {
-		if (diff_populate_filespec(r, filespec, 0))
+		if (diff_populate_filespec(r, filespec, NULL))
 			return 0;
 		hash_object_file(r->hash_algo, filespec->data, filespec->size,
 				 "blob", &filespec->oid);
diff --git a/diffcore.h b/diffcore.h
index 7c07347e42..3b2020ce93 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -65,9 +65,12 @@ void free_filespec(struct diff_filespec *);
 void fill_filespec(struct diff_filespec *, const struct object_id *,
 		   int, unsigned short);
 
-#define CHECK_SIZE_ONLY 1
-#define CHECK_BINARY    2
-int diff_populate_filespec(struct repository *, struct diff_filespec *, unsigned int);
+struct diff_populate_filespec_options {
+	unsigned check_size_only : 1;
+	unsigned check_binary : 1;
+};
+int diff_populate_filespec(struct repository *, struct diff_filespec *,
+			   const struct diff_populate_filespec_options *);
 void diff_free_filespec_data(struct diff_filespec *);
 void diff_free_filespec_blob(struct diff_filespec *);
 int diff_filespec_is_binary(struct repository *, struct diff_filespec *);
diff --git a/line-log.c b/line-log.c
index 9010e00950..40e1738dbb 100644
--- a/line-log.c
+++ b/line-log.c
@@ -519,7 +519,7 @@ static void fill_line_ends(struct repository *r,
 	unsigned long *ends = NULL;
 	char *data = NULL;
 
-	if (diff_populate_filespec(r, spec, 0))
+	if (diff_populate_filespec(r, spec, NULL))
 		die("Cannot read blob %s", oid_to_hex(&spec->oid));
 
 	ALLOC_ARRAY(ends, size);
@@ -1045,12 +1045,12 @@ static int process_diff_filepair(struct rev_info *rev,
 		return 0;
 
 	assert(pair->two->oid_valid);
-	diff_populate_filespec(rev->diffopt.repo, pair->two, 0);
+	diff_populate_filespec(rev->diffopt.repo, pair->two, NULL);
 	file_target.ptr = pair->two->data;
 	file_target.size = pair->two->size;
 
 	if (pair->one->oid_valid) {
-		diff_populate_filespec(rev->diffopt.repo, pair->one, 0);
+		diff_populate_filespec(rev->diffopt.repo, pair->one, NULL);
 		file_parent.ptr = pair->one->data;
 		file_parent.size = pair->one->size;
 	} else {

diff --git a/diff.c b/diff.c
index f337d837ac..8db981b906 100644
--- a/diff.c
+++ b/diff.c
@@ -4023,12 +4023,22 @@ int diff_populate_filespec(struct repository *r,
 		}
 	}
 	else {
-		enum object_type type;
+		struct object_info info = {
+			.sizep = &s->size
+		};
+
+		if (!(size_only || check_binary))
+			/*
+			 * Set contentp, since there is no chance that merely
+			 * the size is sufficient.
+			 */
+			info.contentp = &s->data;
+
+		if (oid_object_info_extended(r, &s->oid, &info,
+					     OBJECT_INFO_LOOKUP_REPLACE))
+			die("unable to read %s", oid_to_hex(&s->oid));
+
 		if (size_only || check_binary) {
-			type = oid_object_info(r, &s->oid, &s->size);
-			if (type < 0)
-				die("unable to read %s",
-				    oid_to_hex(&s->oid));
 			if (size_only)
 				return 0;
 			if (s->size > big_file_threshold && s->is_binary == -1) {
@@ -4036,9 +4046,12 @@ int diff_populate_filespec(struct repository *r,
 				return 0;
 			}
 		}
-		s->data = repo_read_object_file(r, &s->oid, &type, &s->size);
-		if (!s->data)
-			die("unable to read %s", oid_to_hex(&s->oid));
+		if (!info.contentp) {
+			info.contentp = &s->data;
+			if (oid_object_info_extended(r, &s->oid, &info,
+						     OBJECT_INFO_LOOKUP_REPLACE))
+				die("unable to read %s", oid_to_hex(&s->oid));
+		}
 		s->should_free = 1;
 	}
 	return 0;

diff.c, diffcore-break.c, diffcore-rename.c, diffcore.h, t/t4067-diff-partial-clone.sh

diff --git a/diff.c b/diff.c
index 8db981b906..d1ad6a3c4a 100644
--- a/diff.c
+++ b/diff.c
@@ -4034,10 +4034,18 @@ int diff_populate_filespec(struct repository *r,
 			 */
 			info.contentp = &s->data;
 
+		if (options && options->missing_object_cb) {
+			if (!oid_object_info_extended(r, &s->oid, &info,
+						      OBJECT_INFO_LOOKUP_REPLACE |
+						      OBJECT_INFO_SKIP_FETCH_OBJECT))
+				goto object_read;
+			options->missing_object_cb(options->missing_object_data);
+		}
 		if (oid_object_info_extended(r, &s->oid, &info,
 					     OBJECT_INFO_LOOKUP_REPLACE))
 			die("unable to read %s", oid_to_hex(&s->oid));
 
+object_read:
 		if (size_only || check_binary) {
 			if (size_only)
 				return 0;
@@ -6447,6 +6455,8 @@ static int diff_filespec_check_stat_unmatch(struct repository *r,
 {
 	struct diff_populate_filespec_options dpf_options = {
 		.check_size_only = 1,
+		.missing_object_cb = diff_queued_diff_prefetch,
+		.missing_object_data = r,
 	};
 
 	if (p->done_skip_stat_unmatch)
@@ -6523,9 +6533,9 @@ void diffcore_fix_diff_index(void)
 	QSORT(q->queue, q->nr, diffnamecmp);
 }
 
-static void add_if_missing(struct repository *r,
-			   struct oid_array *to_fetch,
-			   const struct diff_filespec *filespec)
+void diff_add_if_missing(struct repository *r,
+			 struct oid_array *to_fetch,
+			 const struct diff_filespec *filespec)
 {
 	if (filespec && filespec->oid_valid &&
 	    !S_ISGITLINK(filespec->mode) &&
@@ -6534,29 +6544,48 @@ static void add_if_missing(struct repository *r,
 		oid_array_append(to_fetch, &filespec->oid);
 }
 
-void diffcore_std(struct diff_options *options)
+void diff_queued_diff_prefetch(void *repository)
 {
-	if (options->repo == the_repository && has_promisor_remote()) {
-		/*
-		 * Prefetch the diff pairs that are about to be flushed.
-		 */
-		int i;
-		struct diff_queue_struct *q = &diff_queued_diff;
-		struct oid_array to_fetch = OID_ARRAY_INIT;
+	struct repository *repo = repository;
+	int i;
+	struct diff_queue_struct *q = &diff_queued_diff;
+	struct oid_array to_fetch = OID_ARRAY_INIT;
 
-		for (i = 0; i < q->nr; i++) {
-			struct diff_filepair *p = q->queue[i];
-			add_if_missing(options->repo, &to_fetch, p->one);
-			add_if_missing(options->repo, &to_fetch, p->two);
-		}
-		/*
-		 * NEEDSWORK: Consider deduplicating the OIDs sent.
-		 */
-		promisor_remote_get_direct(options->repo,
-					   to_fetch.oid, to_fetch.nr);
-		oid_array_clear(&to_fetch);
+	for (i = 0; i < q->nr; i++) {
+		struct diff_filepair *p = q->queue[i];
+		diff_add_if_missing(repo, &to_fetch, p->one);
+		diff_add_if_missing(repo, &to_fetch, p->two);
 	}
 
+	/*
+	 * NEEDSWORK: Consider deduplicating the OIDs sent.
+	 */
+	promisor_remote_get_direct(repo, to_fetch.oid, to_fetch.nr);
+
+	oid_array_clear(&to_fetch);
+}
+
+void diffcore_std(struct diff_options *options)
+{
+	int output_formats_to_prefetch = DIFF_FORMAT_DIFFSTAT |
+		DIFF_FORMAT_NUMSTAT |
+		DIFF_FORMAT_PATCH |
+		DIFF_FORMAT_SHORTSTAT |
+		DIFF_FORMAT_DIRSTAT;
+
+	/*
+	 * Check if the user requested a blob-data-requiring diff output and/or
+	 * break-rewrite detection (which requires blob data). If yes, prefetch
+	 * the diff pairs.
+	 *
+	 * If no prefetching occurs, diffcore_rename() will prefetch if it
+	 * decides that it needs inexact rename detection.
+	 */
+	if (options->repo == the_repository && has_promisor_remote() &&
+	    (options->output_format & output_formats_to_prefetch ||
+	     options->pickaxe_opts & DIFF_PICKAXE_KINDS_MASK))
+		diff_queued_diff_prefetch(options->repo);
+
 	/* NOTE please keep the following in sync with diff_tree_combined() */
 	if (options->skip_stat_unmatch)
 		diffcore_skip_stat_unmatch(options);
diff --git a/diffcore-break.c b/diffcore-break.c
index e8f6322c6a..0d4a14964d 100644
--- a/diffcore-break.c
+++ b/diffcore-break.c
@@ -4,6 +4,7 @@
 #include "cache.h"
 #include "diff.h"
 #include "diffcore.h"
+#include "promisor-remote.h"
 
 static int should_break(struct repository *r,
 			struct diff_filespec *src,
@@ -49,6 +50,8 @@ static int should_break(struct repository *r,
 	unsigned long delta_size, max_size;
 	unsigned long src_copied, literal_added, src_removed;
 
+	struct diff_populate_filespec_options options = { 0 };
+
 	*merge_score_p = 0; /* assume no deletion --- "do not break"
 			     * is the default.
 			     */
@@ -62,8 +65,13 @@ static int should_break(struct repository *r,
 	    oideq(&src->oid, &dst->oid))
 		return 0; /* they are the same */
 
-	if (diff_populate_filespec(r, src, NULL) ||
-	    diff_populate_filespec(r, dst, NULL))
+	if (r == the_repository && has_promisor_remote()) {
+		options.missing_object_cb = diff_queued_diff_prefetch;
+		options.missing_object_data = r;
+	}
+
+	if (diff_populate_filespec(r, src, &options) ||
+	    diff_populate_filespec(r, dst, &options))
 		return 0; /* error but caught downstream */
 
 	max_size = ((src->size > dst->size) ? src->size : dst->size);
diff --git a/diffcore-rename.c b/diffcore-rename.c
index bf4c0b8740..99e63e90f8 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -1,4 +1,5 @@
 /*
+ *
  * Copyright (C) 2005 Junio C Hamano
  */
 #include "cache.h"
@@ -7,6 +8,7 @@
 #include "object-store.h"
 #include "hashmap.h"
 #include "progress.h"
+#include "promisor-remote.h"
 
 /* Table of rename/copy destinations */
 
@@ -128,10 +130,46 @@ struct diff_score {
 	short name_score;
 };
 
+struct prefetch_options {
+	struct repository *repo;
+	int skip_unmodified;
+};
+static void prefetch(void *prefetch_options)
+{
+	struct prefetch_options *options = prefetch_options;
+	int i;
+	struct oid_array to_fetch = OID_ARRAY_INIT;
+
+	for (i = 0; i < rename_dst_nr; i++) {
+		if (rename_dst[i].pair)
+			/*
+			 * The loop in diffcore_rename() will not need these
+			 * blobs, so skip prefetching.
+			 */
+			continue; /* already found exact match */
+		diff_add_if_missing(options->repo, &to_fetch,
+				    rename_dst[i].two);
+	}
+	for (i = 0; i < rename_src_nr; i++) {
+		if (options->skip_unmodified &&
+		    diff_unmodified_pair(rename_src[i].p))
+			/*
+			 * The loop in diffcore_rename() will not need these
+			 * blobs, so skip prefetching.
+			 */
+			continue;
+		diff_add_if_missing(options->repo, &to_fetch,
+				    rename_src[i].p->one);
+	}
+	promisor_remote_get_direct(options->repo, to_fetch.oid, to_fetch.nr);
+	oid_array_clear(&to_fetch);
+}
+
 static int estimate_similarity(struct repository *r,
 			       struct diff_filespec *src,
 			       struct diff_filespec *dst,
-			       int minimum_score)
+			       int minimum_score,
+			       int skip_unmodified)
 {
 	/* src points at a file that existed in the original tree (or
 	 * optionally a file in the destination tree) and dst points
@@ -151,6 +189,12 @@ static int estimate_similarity(struct repository *r,
 	struct diff_populate_filespec_options dpf_options = {
 		.check_size_only = 1
 	};
+	struct prefetch_options prefetch_options = {r, skip_unmodified};
+
+	if (r == the_repository && has_promisor_remote()) {
+		dpf_options.missing_object_cb = prefetch;
+		dpf_options.missing_object_data = &prefetch_options;
+	}
 
 	/* We deal only with regular files.  Symlink renames are handled
 	 * only when they are exact matches --- in other words, no edits
@@ -190,9 +234,11 @@ static int estimate_similarity(struct repository *r,
 	if (max_size * (MAX_SCORE-minimum_score) < delta_size * MAX_SCORE)
 		return 0;
 
-	if (!src->cnt_data && diff_populate_filespec(r, src, NULL))
+	dpf_options.check_size_only = 0;
+
+	if (!src->cnt_data && diff_populate_filespec(r, src, &dpf_options))
 		return 0;
-	if (!dst->cnt_data && diff_populate_filespec(r, dst, NULL))
+	if (!dst->cnt_data && diff_populate_filespec(r, dst, &dpf_options))
 		return 0;
 
 	if (diffcore_count_changes(r, src, dst,
@@ -569,7 +615,8 @@ void diffcore_rename(struct diff_options *options)
 
 			this_src.score = estimate_similarity(options->repo,
 							     one, two,
-							     minimum_score);
+							     minimum_score,
+							     skip_unmodified);
 			this_src.name_score = basename_same(one, two);
 			this_src.dst = i;
 			this_src.src = j;
diff --git a/diffcore.h b/diffcore.h
index 3b2020ce93..d2a63c5c71 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -65,9 +65,22 @@ void free_filespec(struct diff_filespec *);
 void fill_filespec(struct diff_filespec *, const struct object_id *,
 		   int, unsigned short);
 
+/*
+ * Prefetch the entries in diff_queued_diff. The parameter is a pointer to a
+ * struct repository.
+ */
+void diff_queued_diff_prefetch(void *repository);
+
 struct diff_populate_filespec_options {
 	unsigned check_size_only : 1;
 	unsigned check_binary : 1;
+
+	/*
+	 * If an object is missing, diff_populate_filespec() will invoke this
+	 * callback before attempting to read that object again.
+	 */
+	void (*missing_object_cb)(void *);
+	void *missing_object_data;
 };
 int diff_populate_filespec(struct repository *, struct diff_filespec *,
 			   const struct diff_populate_filespec_options *);
@@ -185,4 +198,12 @@ int diffcore_count_changes(struct repository *r,
 			   unsigned long *src_copied,
 			   unsigned long *literal_added);
 
+/*
+ * If filespec contains an OID and if that object is missing from the given
+ * repository, add that OID to to_fetch.
+ */
+void diff_add_if_missing(struct repository *r,
+			 struct oid_array *to_fetch,
+			 const struct diff_filespec *filespec);
+
 #endif
diff --git a/t/t4067-diff-partial-clone.sh b/t/t4067-diff-partial-clone.sh
index 4831ad35e6..c1ed1c2fc4 100755
--- a/t/t4067-diff-partial-clone.sh
+++ b/t/t4067-diff-partial-clone.sh
@@ -131,4 +131,52 @@ test_expect_success 'diff with rename detection batches blobs' '
 	test_line_count = 1 done_lines
 '
 
+test_expect_success 'diff does not fetch anything if inexact rename detection is not needed' '
+	test_when_finished "rm -rf server client trace" &&
+
+	test_create_repo server &&
+	echo a >server/a &&
+	printf "b\nb\nb\nb\nb\n" >server/b &&
+	git -C server add a b &&
+	git -C server commit -m x &&
+	mv server/b server/c &&
+	git -C server add c &&
+	git -C server commit -a -m x &&
+
+	test_config -C server uploadpack.allowfilter 1 &&
+	test_config -C server uploadpack.allowanysha1inwant 1 &&
+	git clone --bare --filter=blob:limit=0 "file://$(pwd)/server" client &&
+
+	# Ensure no fetches.
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --raw -M HEAD^ HEAD &&
+	! test_path_exists trace
+'
+
+test_expect_success 'diff --break-rewrites fetches only if necessary, and batches blobs if it does' '
+	test_when_finished "rm -rf server client trace" &&
+
+	test_create_repo server &&
+	echo a >server/a &&
+	printf "b\nb\nb\nb\nb\n" >server/b &&
+	git -C server add a b &&
+	git -C server commit -m x &&
+	printf "c\nc\nc\nc\nc\n" >server/b &&
+	git -C server commit -a -m x &&
+
+	test_config -C server uploadpack.allowfilter 1 &&
+	test_config -C server uploadpack.allowanysha1inwant 1 &&
+	git clone --bare --filter=blob:limit=0 "file://$(pwd)/server" client &&
+
+	# Ensure no fetches.
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --raw -M HEAD^ HEAD &&
+	! test_path_exists trace &&
+
+	# But with --break-rewrites, ensure that there is exactly 1 negotiation
+	# by checking that there is only 1 "done" line sent. ("done" marks the
+	# end of negotiation.)
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff --break-rewrites --raw -M HEAD^ HEAD &&
+	grep "git> done" trace >done_lines &&
+	test_line_count = 1 done_lines
+'
+
 test_done