unit-name: generate a clear error code when converting an overly long fs path to a unit name

src/basic/unit-name.c

@@ -528,6 +528,9 @@ int unit_name_from_path(const char *path, const char *suffix, char **ret) {
         if (!s)
                 return -ENOMEM;
 
+        if (strlen(s) >= UNIT_NAME_MAX) /* Return a slightly more descriptive error for this specific condition */
+                return -ENAMETOOLONG;
+
         /* Refuse this if this got too long or for some other reason didn't result in a valid name */
         if (!unit_name_is_valid(s, UNIT_NAME_PLAIN))
                 return -EINVAL;
@@ -559,6 +562,9 @@ int unit_name_from_path_instance(const char *prefix, const char *path, const cha
         if (!s)
                 return -ENOMEM;
 
+        if (strlen(s) >= UNIT_NAME_MAX) /* Return a slightly more descriptive error for this specific condition */
+                return -ENAMETOOLONG;
+
         /* Refuse this if this got too long or for some other reason didn't result in a valid name */
         if (!unit_name_is_valid(s, UNIT_NAME_INSTANCE))
                 return -EINVAL;