Re: [PATCHv7 8/8] clone: recursive and reference option triggers submodule alternates

+submodule.alternateErrorStrategy
+	Specifies how to treat errors with the alternates for a submodule
+	as computed via `submodule.alternateLocation`. Possible values are
+	`ignore`, `info`, `die`. Default is `die`.
+
 tag.forceSignAnnotated::
 	A boolean to specify whether annotated tags created should be GPG signed.
 	If `--annotate` is specified on the command line, it takes

diff --git a/builtin/clone.c b/builtin/clone.c
index 0182665..404c5e8 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c

@@ -947,6 +947,25 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 		else
 			fprintf(stderr, _("Cloning into '%s'...\n"), dir);
 	}
+
+	if (option_recursive) {
+		if (option_required_reference.nr &&
+		    option_optional_reference.nr)
+			die(_("clone --recursive is not compatible with "
+			      "both --reference and --reference-if-able"));
+		else if (option_required_reference.nr) {
+			string_list_append(&option_config,
+				"submodule.alternateLocation=superproject");
+			string_list_append(&option_config,
+				"submodule.alternateErrorStrategy=die");
+		} else if (option_optional_reference.nr) {
+			string_list_append(&option_config,
+				"submodule.alternateLocation=superproject");
+			string_list_append(&option_config,
+				"submodule.alternateErrorStrategy=info");
+		}
+	}
+
 	init_db(option_template, INIT_DB_QUIET);
 	write_config(&option_config);
 

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 7096848..f8f35c1 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c

@@ -472,6 +472,96 @@ static int clone_submodule(const char *path, const char *gitdir, const char *url
 	return run_command(&cp);
 }
 
+struct submodule_alternate_setup {
+	const char *submodule_name;
+	enum SUBMODULE_ALTERNATE_ERROR_MODE {
+		SUBMODULE_ALTERNATE_ERROR_DIE,
+		SUBMODULE_ALTERNATE_ERROR_INFO,
+		SUBMODULE_ALTERNATE_ERROR_IGNORE
+	} error_mode;
+	struct string_list *reference;
+};
+#define SUBMODULE_ALTERNATE_SETUP_INIT { NULL, \
+	SUBMODULE_ALTERNATE_ERROR_IGNORE, NULL }
+
+static int add_possible_reference_from_superproject(
+		struct alternate_object_database *alt, void *sas_cb)
+{
+	struct submodule_alternate_setup *sas = sas_cb;
+
+	/* directory name, minus trailing slash */
+	size_t namelen = alt->name - alt->base - 1;
+	struct strbuf name = STRBUF_INIT;
+	strbuf_add(&name, alt->base, namelen);
+
+	/*
+	 * If the alternate object store is another repository, try the
+	 * standard layout with .git/modules/<name>/objects
+	 */
+	if (ends_with(name.buf, ".git/objects")) {
+		char *sm_alternate;
+		struct strbuf sb = STRBUF_INIT;
+		struct strbuf err = STRBUF_INIT;
+		strbuf_add(&sb, name.buf, name.len - strlen("objects"));
+		/*
+		 * We need to end the new path with '/' to mark it as a dir,
+		 * otherwise a submodule name containing '/' will be broken
+		 * as the last part of a missing submodule reference would
+		 * be taken as a file name.
+		 */
+		strbuf_addf(&sb, "modules/%s/", sas->submodule_name);
+
+		sm_alternate = compute_alternate_path(sb.buf, &err);
+		if (sm_alternate) {
+			string_list_append(sas->reference, xstrdup(sb.buf));
+			free(sm_alternate);
+		} else {
+			switch (sas->error_mode) {
+			case SUBMODULE_ALTERNATE_ERROR_DIE:
+				die(_("submodule '%s' cannot add alternate: %s"),
+				    sas->submodule_name, err.buf);
+			case SUBMODULE_ALTERNATE_ERROR_INFO:
+				fprintf(stderr, _("submodule '%s' cannot add alternate: %s"),
+					sas->submodule_name, err.buf);
+			case SUBMODULE_ALTERNATE_ERROR_IGNORE:
+				; /* nothing */
+			}
+		}
+		strbuf_release(&sb);
+	}
+
+	strbuf_release(&name);
+	return 0;
+}
+
+static void prepare_possible_alternates(const char *sm_name,
+		struct string_list *reference)
+{
+	char *sm_alternate = NULL, *error_strategy = NULL;
+	struct submodule_alternate_setup sas = SUBMODULE_ALTERNATE_SETUP_INIT;
+
+	git_config_get_string("submodule.alternateLocation", &sm_alternate);
+	if (!sm_alternate)
+		return;
+
+	git_config_get_string("submodule.alternateErrorStrategy", &error_strategy);
+
+	if (!error_strategy)
+		error_strategy = xstrdup("die");
+
+	sas.submodule_name = sm_name;
+	sas.reference = reference;
+	if (!strcmp(error_strategy, "die"))
+		sas.error_mode = SUBMODULE_ALTERNATE_ERROR_DIE;
+	if (!strcmp(error_strategy, "info"))
+		sas.error_mode = SUBMODULE_ALTERNATE_ERROR_INFO;
+	if (!strcmp(sm_alternate, "superproject"))
+		foreach_alt_odb(add_possible_reference_from_superproject, &sas);
+
+	free(sm_alternate);
+	free(error_strategy);
+}
+
 static int module_clone(int argc, const char **argv, const char *prefix)
 {
 	const char *name = NULL, *url = NULL, *depth = NULL;

@@ -532,6 +622,9 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 	if (!file_exists(sm_gitdir)) {
 		if (safe_create_leading_directories_const(sm_gitdir) < 0)
 			die(_("could not create directory '%s'"), sm_gitdir);
+
+		prepare_possible_alternates(name, &reference);
+
 		if (clone_submodule(path, sm_gitdir, url, depth, &reference, quiet))
 			die(_("clone of '%s' into submodule path '%s' failed"),
 			    url, path);

diff --git a/t/t7408-submodule-reference.sh b/t/t7408-submodule-reference.sh
index dff47af..1c1e289 100755
--- a/t/t7408-submodule-reference.sh
+++ b/t/t7408-submodule-reference.sh

@@ -82,4 +82,47 @@ test_expect_success 'updating superproject keeps alternates' '
 	test_alternate_is_used super-clone/.git/modules/sub/objects/info/alternates super-clone/sub
 '
 
+test_expect_success 'submodules use alternates when cloning a superproject' '
+	test_when_finished "rm -rf super-clone" &&
+	git clone --reference super --recursive super super-clone &&
+	(
+		cd super-clone &&
+		# test superproject has alternates setup correctly
+		test_alternate_is_used .git/objects/info/alternates . &&
+		# test submodule has correct setup
+		test_alternate_is_used .git/modules/sub/objects/info/alternates sub
+	)
+'
+
+test_expect_success 'missing submodule alternate fails clone and submodule update' '
+	test_when_finished "rm -rf super-clone" &&
+	git clone super super2 &&
+	test_must_fail git clone --recursive --reference super2 super2 super-clone &&
+	(
+		cd super-clone &&
+		# test superproject has alternates setup correctly
+		test_alternate_is_used .git/objects/info/alternates . &&
+		# update of the submodule succeeds
+		test_must_fail git submodule update --init &&
+		# and we have no alternates:
+		test_must_fail test_alternate_is_used .git/modules/sub/objects/info/alternates sub &&
+		test_must_fail test_path_is_file sub/file1
+	)
+'
+
+test_expect_success 'ignoring missing submodule alternates passes clone and submodule update' '
+	test_when_finished "rm -rf super-clone" &&
+	git clone --reference-if-able super2 --recursive super2 super-clone &&
+	(
+		cd super-clone &&
+		# test superproject has alternates setup correctly
+		test_alternate_is_used .git/objects/info/alternates . &&
+		# update of the submodule succeeds
+		git submodule update --init &&
+		# and we have no alternates:
+		test_must_fail test_alternate_is_used .git/modules/sub/objects/info/alternates sub &&
+		test_path_is_file sub/file1
+	)
+'
+
 test_done