pack-objects: add damage-control stdin packs mode

Add a storm-oriented stdin-packs mode that skips best-effort namehash traversal and pack-order reverse-index enumeration when callers only need a valid low-latency roll-up. This keeps damage-control repacks write-bound while preserving the normal stdin-packs behavior for quality repacks.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: 2 people

builtin/pack-objects.c

@@ -290,6 +290,7 @@ enum stdin_packs_mode {
 	STDIN_PACKS_MODE_NONE,
 	STDIN_PACKS_MODE_STANDARD,
 	STDIN_PACKS_MODE_FOLLOW,
+	STDIN_PACKS_MODE_DAMAGE_CONTROL,
 };
 
 /**
@@ -3917,7 +3918,8 @@ static int stdin_packs_include_check(struct commit *commit, void *data)
 }
 
 static void stdin_packs_add_pack_entries(struct strmap *packs,
-					 struct rev_info *revs)
+					 struct rev_info *revs,
+					 enum stdin_packs_mode mode)
 {
 	struct string_list keys = STRING_LIST_INIT_NODUP;
 	struct string_list_item *item;
@@ -3958,7 +3960,8 @@ static void stdin_packs_add_pack_entries(struct strmap *packs,
 			for_each_object_in_pack(info->p,
 						add_object_entry_from_pack,
 						revs,
-						ODB_FOR_EACH_OBJECT_PACK_ORDER);
+						mode == STDIN_PACKS_MODE_DAMAGE_CONTROL ?
+						0 : ODB_FOR_EACH_OBJECT_PACK_ORDER);
 	}
 
 	string_list_clear(&keys, 0);
@@ -4045,7 +4048,7 @@ static void stdin_packs_read_input(struct rev_info *revs,
 		info->p = p;
 	}
 
-	stdin_packs_add_pack_entries(&packs, revs);
+	stdin_packs_add_pack_entries(&packs, revs, mode);
 
 	strbuf_release(&buf);
 	strmap_clear(&packs, 1);
@@ -4097,12 +4100,14 @@ static void read_stdin_packs(enum stdin_packs_mode mode, int rev_list_unpacked)
 	if (rev_list_unpacked)
 		add_unreachable_loose_objects(&revs);
 
-	if (prepare_revision_walk(&revs))
-		die(_("revision walk setup failed"));
-	traverse_commit_list(&revs,
-			     show_commit_pack_hint,
-			     show_object_pack_hint,
-			     &mode);
+	if (mode != STDIN_PACKS_MODE_DAMAGE_CONTROL) {
+		if (prepare_revision_walk(&revs))
+			die(_("revision walk setup failed"));
+		traverse_commit_list(&revs,
+				     show_commit_pack_hint,
+				     show_object_pack_hint,
+				     &mode);
+	}
 
 	release_revisions(&revs);
 
@@ -4993,6 +4998,8 @@ static int parse_stdin_packs_mode(const struct option *opt, const char *arg,
 		*mode = STDIN_PACKS_MODE_STANDARD;
 	else if (!strcmp(arg, "follow"))
 		*mode = STDIN_PACKS_MODE_FOLLOW;
+	else if (!strcmp(arg, "damage-control"))
+		*mode = STDIN_PACKS_MODE_DAMAGE_CONTROL;
 	else
 		die(_("invalid value for '%s': '%s'"), opt->long_name, arg);
 

t/t5331-pack-objects-stdin.sh

@@ -60,6 +60,31 @@ test_expect_success '--stdin-packs with excluded packs' '
 	)
 '
 
+test_expect_success '--stdin-packs=damage-control includes listed packs' '
+	(
+		cd stdin-packs &&
+
+		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
+		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
+
+		git pack-objects test-dc --stdin-packs=damage-control \
+			--window=0 --depth=0 <<-EOF &&
+		$PACK_A
+		$PACK_C
+		EOF
+
+		(
+			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
+			git show-index <$(ls .git/objects/pack/pack-C-*.idx)
+		) >expect.raw &&
+		git show-index <$(ls test-dc-*.idx) >actual.raw &&
+
+		cut -d" " -f2 <expect.raw | sort >expect &&
+		cut -d" " -f2 <actual.raw | sort >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success '--stdin-packs is incompatible with --filter' '
 	(
 		cd stdin-packs &&