Fuse might call read(...) on the file class from multiple threads, causing
a race condition with the underlying seek/read operations.

Using locking here seems to make the file access slower, than just disabling
the multithreading in Fuse with fs.multithreading = False.

A thread-local storage doesn't help either, because Fuse doesn't seem to
reuse threads, but might create new threads for each read operation.