setup: introduce GIT_DEFAULT_REF_FORMAT envvar

Introduce a new GIT_DEFAULT_REF_FORMAT environment variable that lets
users control the default ref format used by both git-init(1) and
git-clone(1). This is modeled after GIT_DEFAULT_OBJECT_FORMAT, which
does the same thing for the repository's object format.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

Documentation/git.txt

@@ -556,6 +556,11 @@ double-quotes and respecting backslash escapes. E.g., the value
 	is always used. The default is "sha1".
 	See `--object-format` in linkgit:git-init[1].
 
+`GIT_DEFAULT_REF_FORMAT`::
+	If this variable is set, the default reference backend format for new
+	repositories will be set to this value. The default is "files".
+	See `--ref-format` in linkgit:git-init[1].
+
 Git Commits
 ~~~~~~~~~~~
 `GIT_AUTHOR_NAME`::

setup.c

@@ -2164,12 +2164,19 @@ static void validate_hash_algorithm(struct repository_format *repo_fmt, int hash
 static void validate_ref_storage_format(struct repository_format *repo_fmt,
 					unsigned int format)
 {
+	const char *name = getenv("GIT_DEFAULT_REF_FORMAT");
+
 	if (repo_fmt->version >= 0 &&
 	    format != REF_STORAGE_FORMAT_UNKNOWN &&
 	    format != repo_fmt->ref_storage_format) {
 		die(_("attempt to reinitialize repository with different reference storage format"));
 	} else if (format != REF_STORAGE_FORMAT_UNKNOWN) {
 		repo_fmt->ref_storage_format = format;
+	} else if (name) {
+		format = ref_storage_format_by_name(name);
+		if (format == REF_STORAGE_FORMAT_UNKNOWN)
+			die(_("unknown ref storage format '%s'"), name);
+		repo_fmt->ref_storage_format = format;
 	}
 }
 

t/t0001-init.sh

@@ -558,6 +558,24 @@ test_expect_success DEFAULT_REPO_FORMAT 'extensions.refStorage with unknown back
 	grep "invalid value for ${SQ}extensions.refstorage${SQ}: ${SQ}garbage${SQ}" err
 '
 
+test_expect_success DEFAULT_REPO_FORMAT 'init with GIT_DEFAULT_REF_FORMAT=files' '
+	test_when_finished "rm -rf refformat" &&
+	GIT_DEFAULT_REF_FORMAT=files git init refformat &&
+	echo 0 >expect &&
+	git -C refformat config core.repositoryformatversion >actual &&
+	test_cmp expect actual &&
+	test_must_fail git -C refformat config extensions.refstorage
+'
+
+test_expect_success 'init with GIT_DEFAULT_REF_FORMAT=garbage' '
+	test_when_finished "rm -rf refformat" &&
+	cat >expect <<-EOF &&
+	fatal: unknown ref storage format ${SQ}garbage${SQ}
+	EOF
+	test_must_fail env GIT_DEFAULT_REF_FORMAT=garbage git init refformat 2>err &&
+	test_cmp expect err
+'
+
 test_expect_success MINGW 'core.hidedotfiles = false' '
 	git config --global core.hidedotfiles false &&
 	rm -rf newdir &&