sd-device: introduce sd_device_get_driver_subsystem()

To create the sd_device object of a driver, the function
sd_device_new_from_subsystem_sysname() requires "drivers" for subsystem
and e.g. "pci:iwlwifi" for sysname. Similarly, sd_device_new_from_device_id()
also requires driver subsystem. However, we have never provided a
way to get the driver subsystem ("pci" for the previous example) from
an existing sd_device object.

Let's introduce a way to get driver subsystem.

src/libsystemd/libsystemd.sym

@@ -1046,6 +1046,7 @@ global:
         sd_varlink_take_fd;
         sd_varlink_unref;
         sd_varlink_wait;
+        sd_device_get_driver_subsystem;
         sd_device_monitor_is_running;
         sd_device_monitor_get_fd;
         sd_device_monitor_receive;

src/libsystemd/sd-device/sd-device.c

@@ -1198,6 +1198,20 @@ _public_ int sd_device_get_subsystem(sd_device *device, const char **ret) {
         return 0;
 }
 
+_public_ int sd_device_get_driver_subsystem(sd_device *device, const char **ret) {
+        assert_return(device, -EINVAL);
+
+        if (!device_in_subsystem(device, "drivers"))
+                return -ENOENT;
+
+        assert(device->driver_subsystem);
+
+        if (ret)
+                *ret = device->driver_subsystem;
+
+        return 0;
+}
+
 _public_ int sd_device_get_devtype(sd_device *device, const char **devtype) {
         int r;
 

src/libsystemd/sd-device/test-sd-device.c

@@ -79,9 +79,11 @@ static void test_sd_device_one(sd_device *d) {
                                                * sd_device_new_from_device_id() may not work as expected. */
                 const char *name, *id;
 
-                if (streq(subsystem, "drivers"))
-                        name = strjoina(d->driver_subsystem, ":", sysname);
-                else
+                if (streq(subsystem, "drivers")) {
+                        const char *driver_subsystem;
+                        ASSERT_OK(sd_device_get_driver_subsystem(d, &driver_subsystem));
+                        name = strjoina(driver_subsystem, ":", sysname);
+                } else
                         name = sysname;
                 assert_se(sd_device_new_from_subsystem_sysname(&dev, subsystem, name) >= 0);
                 assert_se(sd_device_get_syspath(dev, &val) >= 0);