unlockStderr

fn unlockStderr(userdata: ?*anyopaque) void {
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    if (t.stderr_writer.err == null) t.stderr_writer.interface.flush() catch {};
    if (t.stderr_writer.err) |err| {
        switch (err) {
            error.Canceled => recancelInner(),
            else => {},
        }
        t.stderr_writer.err = null;
    }
    t.stderr_writer.interface.end = 0;
    t.stderr_writer.interface.buffer = &.{};

    t.stderr_mutex_lock_count -= 1;
    if (t.stderr_mutex_lock_count == 0) {
        @atomicStore(std.Thread.Id, &t.stderr_mutex_locker, Thread.invalid_id, .unordered);
        mutexUnlock(&t.stderr_mutex);
    }
}