It was leaky and required a substantial number of `loc := #caller_location` additions to parts of the core library to make it easier to track down how and where it leaked.
The tests now run fine multi-threaded.
Returns the actual error if one is set, instead of swallowing it for the
less descriptive negative error.
Also fixes a out-of-bounds slice error in `bufio.writer_write` because
it wasn't checking the returned `m`.
