Description of problem: The documentation for mount-local claims: "Note you must not make concurrent libguestfs calls on the same handle from another thread, with the exception of guestfs_umount_local." implying that guestfs_umount_local is thread-safe. It certainly is not safe to call guestfs_umount_local from another thread. guestfs_umount_local could generate events, call the error handler, or access other fields in the handle, with no locking at all. Version-Release number of selected component (if applicable): libguestfs 1.21.16 Additional info: I have added a commit to fix the documentation. However we ought to make guestfs_umount_local thread-safe in future.