`S3.read(-1)` for a large file (2^31+α bytes) fails due to an SSL `OverflowError`

[belltailjp]

I found out that when reading the entire content of a file of 2+αGiB from S3 fails by OverflowError: signed integer is greater than maximum exception raised from Python SSL library.

Here is the minimum reproduction.

import pfio
import os

path = 's3://<bucket>/foo.dat'
# size = 2**31 + 7 * 1024   # No error
size = 2**31 + 8 * 1024   # Get error

# Create test data of _size_ bytes
bs = 128 * 1024 * 1024
with pfio.v2.open_url(path, 'wb') as f:
    while 0 < size:
        s = min(bs, size)
        print('ss={}, s={}'.format(size, s))
        f.write(bytearray(os.urandom(s)))
        size -= s

# Read the entire content
with pfio.v2.open_url(path, 'rb') as f:
    assert len(f.read(-1))

Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "/usr/local/lib/python3.8/site-packages/pfio/v2/s3.py", line 149, in readall
    return self.read(-1)
  File "/usr/local/lib/python3.8/site-packages/pfio/v2/s3.py", line 82, in read
    data = body.read()
  File "/usr/local/lib/python3.8/site-packages/botocore/response.py", line 95, in read
    chunk = self._raw_stream.read(amt)
  File "/usr/local/lib/python3.8/site-packages/urllib3/response.py", line 515, in read
    data = self._fp.read() if not fp_closed else b""
  File "/usr/local/lib/python3.8/http/client.py", line 468, in read
    s = self._safe_read(self.length)
  File "/usr/local/lib/python3.8/http/client.py", line 609, in _safe_read
    data = self.fp.read(amt)
  File "/usr/local/lib/python3.8/socket.py", line 669, in readinto
    return self._sock.recv_into(b)
  File "/usr/local/lib/python3.8/ssl.py", line 1241, in recv_into
    return self.read(nbytes, buffer)
  File "/usr/local/lib/python3.8/ssl.py", line 1099, in read
    return self._sslobj.read(len, buffer)
OverflowError: signed integer is greater than maximum

Reading the error message, I thought reading a file of 2^31 bytes is fine and 2^31+1 bytes is NG, but it seems to be slightly different; the threshold is somewhere between 2147490816 (2^31+7k) ~ 2147491840 (2^31+8k).

I think the S3 API itself should support reading such a large file, but the issue is in Python SSL library layer (if so, maybe it'd be better trying Python 3.9 and 3.10).

Here is my environment:

% python --version
Python 3.8.10
% python -c "import pfio; print(pfio.__version__)"
2.2.0

[belltailjp]

It seems that this Python bug is deeply related, which is apparently dealt in Python 3.10~. https://bugs.python.org/issue42853 https://stackoverflow.com/questions/70905872

For pfio, since we cannot drop support for Python 3.8~ right now, I guess we need some workarounds to prevent attempting to read the whole content at once even when _ObjectReader.read(-1) or _ObjectReader.readall() is called.

The naive approach would be to modify _ObjectReader.read to split get_object API call when necessary, though it sounds like re-implementing kind of a buffering which is duplication with BufferedReader (#247).

I wonder if there is a way to somehow force BufferedReader to do buffering when read(-1) is called, although currently it directly calls _ObjectReader.readall. c.f., https://github.com/python/cpython/blob/v3.11.0a5/Lib/_pyio.py#L1096 In that case, we also need to consider "rt" mode which uses TextIOWrapper instead of BufferedReader. In addition, I guess it would also be preferred to prevent this issue without buffering wrapper (buffering=0). # Note: The reported issue reproduces regardless of with/without buffering option and "rb"/"rt" mode.

[kuenishi]

Strictly speaking, 42853 was fixed in Python 3.9.7 (release note). I knew this issue in January, but I didn't report here, sorry! I thought at that time reading a fairly large file (>2GB) at once was a bit rare use case so that it doesn't pay implementing a workaround for the issue. Regarding the fact you reported here, did you find this issue in your application?

Python 3.8 EoL is scheduled in 2024-10. It'll be more than two years from today, and 3.8 maintenance state is security update only. 42853 isn't a vulnerability, so it' won't be fixed in 3.8 branch. Hmmm....

[kuenishi]

We just had observed an internal use case of loading large pickle file failure like this:

import pickle
from pfio.v2 import open_url
with open_url("s3://very/large/file.pickle", "rb") as fp:
  pickle.load(fp)  # Gets the exception

[kuenishi]

Update: even after 3.9.7, this issue reproduced against loading large ndarray pickled files, possibly because the binary protocol forces to load a large array more than 2G to read at once from SSL. This is fixed in Python 3.10, as it uses SSL_read_ex().

Python 3.10 will use SSL_write_ex() and SSL_read_ex(), which support > 2 GB data.

So the complete resolution for this issue is to use Python 3.10. ¯\_(ツ)_/¯