From ca571025d86b55933d493e38d6e72824bcf5a80a Mon Sep 17 00:00:00 2001
From: Patrick Steinhardt <ps@pks.im>
Date: Fri, 7 Aug 2026 05:34:25 +0200
Subject: [PATCH] loose: load loose object map for the correct source

When loading the loose object map via `load_one_loose_object_map()` we
pass in both a repository and the corresponding source. We ultimately
don't really respect the passed-in source though as we instead always
load the map via the common directory. This doesn't make any sense
though, as the function is called in a loop through all sources, and as
such the expectation is that we'll load the map that belongs to the
given source. The consequence is that we'll ignore loose object maps of
any configured alternates.

Fix this bug by instead loading the map via the loose source's path.

Helped-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 loose.c                       | 18 ++++++++++--------
 t/t1016-compatObjectFormat.sh | 18 ++++++++++++++++++
 2 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/loose.c b/loose.c
index bf01d3e42d..9dad75373b 100644
--- a/loose.c
+++ b/loose.c
@@ -61,9 +61,11 @@ static int insert_loose_map(struct odb_source_loose *loose,
 	return inserted;
 }
 
-static int load_one_loose_object_map(struct repository *repo, struct odb_source_loose *loose)
+static int load_one_loose_object_map(struct odb_source_loose *loose)
 {
-	struct strbuf buf = STRBUF_INIT, path = STRBUF_INIT;
+	struct repository *repo = loose->base.odb->repo;
+	struct strbuf buf = STRBUF_INIT;
+	char *path;
 	FILE *fp;
 	int ret = -1;
 
@@ -78,10 +80,10 @@ static int load_one_loose_object_map(struct repository *repo, struct odb_source_
 	insert_loose_map(loose, repo->hash_algo->empty_blob, repo->compat_hash_algo->empty_blob);
 	insert_loose_map(loose, repo->hash_algo->null_oid, repo->compat_hash_algo->null_oid);
 
-	repo_common_path_replace(repo, &path, "objects/loose-object-idx");
-	fp = fopen(path.buf, "rb");
+	path = xstrfmt("%s/loose-object-idx", loose->base.path);
+	fp = fopen(path, "rb");
 	if (!fp) {
-		strbuf_release(&path);
+		free(path);
 		return 0;
 	}
 
@@ -102,7 +104,7 @@ static int load_one_loose_object_map(struct repository *repo, struct odb_source_
 err:
 	fclose(fp);
 	strbuf_release(&buf);
-	strbuf_release(&path);
+	free(path);
 	return ret;
 }
 
@@ -117,10 +119,10 @@ int repo_read_loose_object_map(struct repository *repo)
 
 	for (source = repo->objects->sources; source; source = source->next) {
 		struct odb_source_files *files = odb_source_files_downcast(source);
-		if (load_one_loose_object_map(repo, files->loose) < 0) {
+		if (load_one_loose_object_map(files->loose) < 0)
 			return -1;
-		}
 	}
+
 	return 0;
 }
 
diff --git a/t/t1016-compatObjectFormat.sh b/t/t1016-compatObjectFormat.sh
index 92d48b96a1..9cafcee509 100755
--- a/t/t1016-compatObjectFormat.sh
+++ b/t/t1016-compatObjectFormat.sh
@@ -187,6 +187,24 @@ do
 		eval signedtag3_${hash}_oid=$(git hash-object -t tag -w ../${hash}_signedtag3) &&
 		eval signedtag4_${hash}_oid=$(git hash-object -t tag -w ../${hash}_signedtag4)
 	'
+
+	test_expect_success 'rev-parse maps oid of object borrowed from alternate' '
+		for repo in alt borrow
+		do
+			test_when_finished "rm -rf $repo" &&
+			git init --object-format=$hash $repo &&
+			git -C $repo config set core.repositoryformatversion 1 &&
+			git -C $repo config set extensions.compatObjectFormat $(compat_hash $hash) || exit 1
+		done &&
+
+		git -C alt commit --allow-empty --message A &&
+		echo "$(pwd)/alt/.git/objects" >borrow/.git/objects/info/alternates &&
+
+		oid=$(git -C alt rev-parse HEAD) &&
+		git -C alt    rev-parse --output-object-format=$(compat_hash $hash) "$oid" >expect &&
+		git -C borrow rev-parse --output-object-format=$(compat_hash $hash) "$oid" >actual &&
+		test_cmp expect actual
+	'
 done
 cd "$base"