Test end-of-space errors

[giovannipizzi]

One of the most common errors that users will face is a full disk. We need to assess that the implementation is at least robust against these errors.

We should check if using pyfakefs can help in testing these cases, and add tests for those.

[giovannipizzi]

An example pytest with pyfakefs:

import errno
import os
import pytest

## Note: you need to install pyfakefs==4.0.2 to make the 'fs' fixture available

def test_fakefs(fs):  # pytest: disable=invalid-name
    # "fs" is the reference to the fake file system
    fs.set_disk_usage(100) # bytes
    fname = '/var/data/xx1.txt'
    fs.create_file(fname)
    assert os.path.exists(fname)
    with open(fname, 'w') as fhandle:
        fhandle.write("a" * 80)

    with pytest.raises(OSError) as excinfo:
        with open(fname, 'w') as fhandle:
            fhandle.write("a" * 120)

    print(dir(excinfo))
    assert excinfo.value.errno == errno.ENOSPC