Bug 3.11.x behavioral, open file buffers not flushed til file closed.

[Eryk Sun]

On 3/5/23, aapost <aapost at idontexist.club> wrote:
>
> If a file is still open, even if all the operations on the file have
> ceased for a time, the tail of the written operation data does not get
> flushed to the file until close is issued and the file closes cleanly.

This is normal behavior for buffered file I/O. There's no timer set to
flush the buffer after operations have "ceased for a time". It
automatically flushes only when the buffer is full or, for line
buffering, when a newline is written.

The default buffer size is based on the raw file object's _blksize
attribute. If st_blksize can't be determined via fstat(), the default
_blksize is 8 KiB.

Here's an example on Linux. In this example, the buffer size is 4 KiB.

    >>> f = open('abc', 'w')
    >>> os.fstat(f.fileno()).st_blksize
    4096
    >>> f.buffer.raw._blksize
    4096
    >>> f.writelines(f'{i}\n' for i in range(50000))
    >>> with open('abc') as g: g.readlines()[-1]
    ...
    '49626\n'

    >>> pre_flush_size = os.path.getsize('abc')
    >>> f.flush()
    >>> post_flush_size = os.path.getsize('abc')
    >>> post_flush_size - pre_flush_size
    2238

Verify that this makes sense, based on what was left in the buffer
prior to flushing:

    >>> remaining_lines = 50000 - 49626 - 1
    >>> bytes_per_line = 6
    >>> remaining_lines * bytes_per_line
    2238