resolve: Add framework for resolution services

The framework enables the service specific implementations
to provide its own variations for the DNS installation tasks.
The selection of the address resolution service can be done
through dns_resolve_method setting.

src/resolve.c

@@ -31,28 +31,89 @@
 #include "src/iwd.h"
 #include "src/resolve.h"
 
+struct resolve_method_ops {
+	void *(*init)(void);
+	void (*exit)(void *data);
+	void (*add_dns)(uint32_t ifindex, uint8_t type, char **dns_list,
+								void *data);
+	void (*remove)(uint32_t ifindex, void *data);
+};
+
+struct resolve_method {
+	void *data;
+	const struct resolve_method_ops *ops;
+};
+
+static struct resolve_method method;
+
 void resolve_add_dns(uint32_t ifindex, uint8_t type, char **dns_list)
 {
+	if (!dns_list || !*dns_list)
+		return;
+
+	if (!method.ops || !method.ops->add_dns)
+		return;
+
+	method.ops->add_dns(ifindex, type, dns_list, method.data);
 }
 
 void resolve_remove(uint32_t ifindex)
 {
+	if (!method.ops || !method.ops->remove)
+		return;
+
+	method.ops->remove(ifindex, method.data);
 }
 
+static const struct {
+	const char *name;
+	const struct resolve_method_ops *method_ops;
+} resolve_method_ops_list[] = {
+	{ }
+};
+
 static int resolve_init(void)
 {
+	const char *method_name;
 	bool enabled;
+	uint8_t i;
 
 	if (!l_settings_get_bool(iwd_get_config(), "General",
 					"enable_network_config", &enabled) ||
 								!enabled)
 		return 0;
 
+	method_name = l_settings_get_value(iwd_get_config(), "General",
+							"dns_resolve_method");
+
+	if (!method_name)
+		/* Default to systemd-resolved service. */
+		method_name = "systemd";
+
+	for (i = 0; resolve_method_ops_list[i].name; i++) {
+		if (strcmp(resolve_method_ops_list[i].name, method_name))
+			continue;
+
+		method.ops = resolve_method_ops_list[i].method_ops;
+
+		break;
+	}
+
+	if (!method.ops)
+		return -EINVAL;
+
+	if (method.ops->init)
+		method.data = method.ops->init();
+
 	return 0;
 }
 
 static void resolve_exit(void)
 {
+	if (!method.ops->exit)
+		return;
+
+	method.ops->exit(method.data);
 }
 
 IWD_MODULE(resolve, resolve_init, resolve_exit)