sd-device,udev: tag must be a valid filename

All tags are managed under /run/udev/tags, and the directories there are
named with tags. Hence, each tag must be a valid filename.

This also makes all validity check moved to sd-device side, and
makes failure caused by setting invalid tags non-critical.

With this change, an empty string cannot be assigned to TAG=, hence the
test cases are adjusted.

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

@@ -351,6 +351,8 @@ static int device_amend(sd_device *device, const char *key, const char *value) {
                                 return r;
                         if (r == 0)
                                 break;
+                        if (isempty(word))
+                                continue;
 
                         r = device_add_tag(device, word, streq(key, "CURRENT_TAGS"));
                         if (r < 0)

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

@@ -1439,7 +1439,7 @@ _public_ int sd_device_get_diskseq(sd_device *device, uint64_t *ret) {
 static bool is_valid_tag(const char *tag) {
         assert(tag);
 
-        return !strchr(tag, ':') && !strchr(tag, ' ');
+        return in_charset(tag, ALPHANUMERICAL "-_") && filename_is_valid(tag);
 }
 
 int device_add_tag(sd_device *device, const char *tag, bool both) {

src/udev/udev-rules.c

@@ -2487,16 +2487,14 @@ static int udev_rule_apply_token_to_event(
                 if (token->op == OP_ASSIGN)
                         device_cleanup_tags(dev);
 
-                if (buf[strspn(buf, ALPHANUMERICAL "-_")] != '\0') {
-                        log_event_error(dev, token, "Invalid tag name '%s', ignoring", buf);
-                        break;
-                }
                 if (token->op == OP_REMOVE)
                         device_remove_tag(dev, buf);
                 else {
                         r = device_add_tag(dev, buf, true);
+                        if (r == -ENOMEM)
+                                return log_oom();
                         if (r < 0)
-                                return log_event_error_errno(dev, token, r, "Failed to add tag '%s': %m", buf);
+                                log_event_warning_errno(dev, token, r, "Failed to add tag '%s', ignoring: %m", buf);
                 }
                 break;
         }