This is one step towards supporting partial clone submodules.

Even after this patch, we will still lack partial clone submodules
support, primarily because a lot of Git code that accesses submodule
objects does so by adding their object stores as alternates, meaning
that any lazy fetches that would occur in the submodule would be done
based on the config of the superproject, not of the submodule. This also
prevents testing of the functionality in this patch by user-facing
commands. So for now, test this mechanism using a test helper.

Signed-off-by: Jonathan Tan <redacted>
---
 Makefile                      |  1 +
 object-file.c                 |  7 ++-----
 promisor-remote.c             | 14 +++++++++-----
 t/helper/test-partial-clone.c | 34 ++++++++++++++++++++++++++++++++++
 t/helper/test-tool.c          |  1 +
 t/helper/test-tool.h          |  1 +
 t/t0410-partial-clone.sh      | 24 ++++++++++++++++++++++++
 7 files changed, 72 insertions(+), 10 deletions(-)
 create mode 100644 t/helper/test-partial-clone.c

diff --git a/Makefile b/Makefile
index c3565fc0f8..f6653bcd5e 100644
--- a/Makefile
+++ b/Makefile

@@ -725,6 +725,7 @@ TEST_BUILTINS_OBJS += test-oidmap.o
 TEST_BUILTINS_OBJS += test-online-cpus.o
 TEST_BUILTINS_OBJS += test-parse-options.o
 TEST_BUILTINS_OBJS += test-parse-pathspec-file.o
+TEST_BUILTINS_OBJS += test-partial-clone.o
 TEST_BUILTINS_OBJS += test-path-utils.o
 TEST_BUILTINS_OBJS += test-pcre2-config.o
 TEST_BUILTINS_OBJS += test-pkt-line.o

diff --git a/object-file.c b/object-file.c
index f233b440b2..ebf273e9e7 100644
--- a/object-file.c
+++ b/object-file.c

@@ -1570,15 +1570,12 @@ static int do_oid_object_info_extended(struct repository *r,
 		}
 
 		/* Check if it is a missing object */
-		if (fetch_if_missing && has_promisor_remote() &&
-		    !already_retried && r == the_repository &&
+		if (fetch_if_missing && repo_has_promisor_remote(r) &&
+		    !already_retried &&

@@ -150,7 +156,7 @@ static void promisor_remote_init(struct repository *r)
 		xcalloc(sizeof(*r->promisor_remote_config), 1);
 	config->promisors_tail = &config->promisors;
 
-	git_config(promisor_remote_config, config);
+	repo_config(r, promisor_remote_config, config);

diff --git a/t/helper/test-partial-clone.c b/t/helper/test-partial-clone.c
new file mode 100644
index 0000000000..e7bc7eb21f
--- /dev/null
+++ b/t/helper/test-partial-clone.c

@@ -0,0 +1,34 @@
+#include "cache.h"
+#include "test-tool.h"
+#include "repository.h"
+#include "object-store.h"
+
+static void object_info(const char *gitdir, const char *oid_hex)
+{
+	struct repository r;
+	struct object_id oid;
+	unsigned long size;
+	struct object_info oi = {.sizep = &size};
+	const char *p;
+
+	if (repo_init(&r, gitdir, NULL))
+		die("could not init repo");
+	if (parse_oid_hex(oid_hex, &oid, &p))
+		die("could not parse oid");
+	if (oid_object_info_extended(&r, &oid, &oi, 0))
+		die("could not obtain object info");
+	printf("%d\n", (int) size);
+}
+
+int cmd__partial_clone(int argc, const char **argv)
+{
+	if (argc < 4)
+		die("too few arguments");
+
+	if (!strcmp(argv[1], "object-info"))
+		object_info(argv[2], argv[3]);
+	else
+		die("invalid argument '%s'", argv[1]);
+
+	return 0;
+}

diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index c5bd0c6d4c..b21e8f1519 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c

@@ -46,6 +46,7 @@ static struct test_cmd cmds[] = {
 	{ "online-cpus", cmd__online_cpus },
 	{ "parse-options", cmd__parse_options },
 	{ "parse-pathspec-file", cmd__parse_pathspec_file },
+	{ "partial-clone", cmd__partial_clone },
 	{ "path-utils", cmd__path_utils },
 	{ "pcre2-config", cmd__pcre2_config },
 	{ "pkt-line", cmd__pkt_line },

diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index e8069a3b22..f845ced4b3 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h

@@ -35,6 +35,7 @@ int cmd__oidmap(int argc, const char **argv);
 int cmd__online_cpus(int argc, const char **argv);
 int cmd__parse_options(int argc, const char **argv);
 int cmd__parse_pathspec_file(int argc, const char** argv);
+int cmd__partial_clone(int argc, const char **argv);
 int cmd__path_utils(int argc, const char **argv);
 int cmd__pcre2_config(int argc, const char **argv);
 int cmd__pkt_line(int argc, const char **argv);

diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
index 584a039b85..e804d267e6 100755
--- a/t/t0410-partial-clone.sh
+++ b/t/t0410-partial-clone.sh

@@ -604,6 +604,30 @@ test_expect_success 'do not fetch when checking existence of tree we construct o
 	git -C repo cherry-pick side1
 '
 
+test_expect_success 'lazy-fetch when accessing object not in the_repository' '
+	rm -rf full partial.git &&
+	test_create_repo full &&
+	printf 12345 >full/file.txt &&
+	git -C full add file.txt &&
+	git -C full commit -m "first commit" &&