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Well, on the one hand this does sound like a valid use case. On the other hand, once the urandom file descriptor is closed by third-party code, it can very well be re-opened to point to another file, and then os.urandom() will start behaving in a very bad way.
Here is a possible solution in Python:
- when opening the urandom fd for the first time, record its st_ino and st_dev
- when calling urandom() a second time, call fstat() on the fd and check the st_ino and st_dev with the known values
- if the values have changed (or if fstat() fails with EBADF), open a new fd to /dev/urandom, again