tests: config: verify functionality with read-only backends

tests/config/readonly.c

+#include "clar_libgit2.h"
+#include "config_file.h"
+#include "config.h"
+
+static git_config *cfg;
+
+void test_config_readonly__initialize(void)
+{
+	cl_git_pass(git_config_new(&cfg));
+}
+
+void test_config_readonly__cleanup(void)
+{
+	git_config_free(cfg);
+	cfg = NULL;
+}
+
+void test_config_readonly__writing_to_readonly_fails(void)
+{
+	git_config_backend *backend;
+
+	cl_git_pass(git_config_file__ondisk(&backend, "global"));
+	backend->readonly = 1;
+	cl_git_pass(git_config_add_backend(cfg, backend, GIT_CONFIG_LEVEL_GLOBAL, 0));
+
+	cl_git_fail_with(GIT_ENOTFOUND, git_config_set_string(cfg, "foo.bar", "baz"));
+	cl_assert(!git_path_exists("global"));
+}
+
+void test_config_readonly__writing_to_cfg_with_rw_precedence_succeeds(void)
+{
+	git_config_backend *backend;
+
+	cl_git_pass(git_config_file__ondisk(&backend, "global"));
+	backend->readonly = 1;
+	cl_git_pass(git_config_add_backend(cfg, backend, GIT_CONFIG_LEVEL_GLOBAL, 0));
+
+	cl_git_pass(git_config_file__ondisk(&backend, "local"));
+	cl_git_pass(git_config_add_backend(cfg, backend, GIT_CONFIG_LEVEL_LOCAL, 0));
+
+	cl_git_pass(git_config_set_string(cfg, "foo.bar", "baz"));
+
+	cl_assert(git_path_exists("local"));
+	cl_assert(!git_path_exists("global"));
+	cl_git_pass(p_unlink("local"));
+}
+
+void test_config_readonly__writing_to_cfg_with_ro_precedence_succeeds(void)
+{
+	git_config_backend *backend;
+
+	cl_git_pass(git_config_file__ondisk(&backend, "local"));
+	backend->readonly = 1;
+	cl_git_pass(git_config_add_backend(cfg, backend, GIT_CONFIG_LEVEL_LOCAL, 0));
+
+	cl_git_pass(git_config_file__ondisk(&backend, "global"));
+	cl_git_pass(git_config_add_backend(cfg, backend, GIT_CONFIG_LEVEL_GLOBAL, 0));
+
+	cl_git_pass(git_config_set_string(cfg, "foo.bar", "baz"));
+
+	cl_assert(!git_path_exists("local"));
+	cl_assert(git_path_exists("global"));
+	cl_git_pass(p_unlink("global"));
+}