Size limits (OSError Errno 28) causing content do disappear

[couling]

Size limits (OSError Errno 28) causing content do disappear. Content that has been flushed to the file disappears after a further failed write.

The following test should pass:

def test_fake_fs(fs):
    fs.set_disk_usage(100)  # sets the file system size in bytes
    os.makedirs("/foo")
    with open('/foo/bar.txt', 'wb') as f:
        f.write(b'a' * 50)
        f.flush()
        # By this point 50 bytes must be "committed" into the file whatever happens next.
        with open('/foo/bar.txt', "rb") as r:
            x = r.read()
            # Yes, it's there!
            assert x.startswith(b'a' * 50)
        with pytest.raises(OSError):
            f.write(b'b' * 200)
            f.flush()
        with open('/foo/bar.txt', "rb") as r:
            x = r.read()
            # Wait, where did it go
            assert x.startswith(b'a' * 50)
        # Because of the above assertion failure, the truncate never happens
        f.truncate(50)
    # The implicit close causes another OSError because of the missing truncate().

My enviroment

(.venv) $ python -c "import platform; print(platform.platform())"
Linux-5.4.0-42-generic-x86_64-with-glibc2.29
(.venv) $ python -c "import sys; print('Python', sys.version)"
Python 3.8.2 (default, Jul 16 2020, 14:00:26) 
(.venv) $ python -c "from pyfakefs.fake_filesystem import __version__; print('pyfakefs', __version__)"
pyfakefs 4.1.0
(.venv) $ 

[couling]

Apologies, I've just spotted the above test should fail but for different reasons. With buffered file access, the truncate() would also fail. But the above assertion failure is still wrong. There's no way that the file should randomly blank like that.

[mrbean-bremen]

Please check with master - should be fixed now.