device-util: introduce device_get_seat() helper function

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

@@ -191,6 +191,21 @@ int device_sysname_startswith_strv(sd_device *device, char * const *prefixes, co
         return !!suffix;
 }
 
+int device_get_seat(sd_device *device, const char **ret) {
+        const char *seat = NULL;
+        int r;
+
+        assert(device);
+        assert(ret);
+
+        r = sd_device_get_property_value(device, "ID_SEAT", &seat);
+        if (r < 0 && r != -ENOENT)
+                return r;
+
+        *ret = isempty(seat) ? "seat0" : seat;
+        return 0;
+}
+
 bool device_property_can_set(const char *property) {
         return property &&
                 !STR_IN_SET(property,

src/libsystemd/sd-device/device-util.h

@@ -111,6 +111,8 @@ int device_is_devtype(sd_device *device, const char *devtype);
 
 int device_is_subsystem_devtype(sd_device *device, const char *subsystem, const char *devtype);
 
+int device_get_seat(sd_device *device, const char **ret);
+
 int device_sysname_startswith_strv(sd_device *device, char * const *prefixes, const char **ret_suffix);
 #define device_sysname_startswith(device, ...) \
         device_sysname_startswith_strv(device, STRV_MAKE(__VA_ARGS__), NULL)