MemoryError: Cannot allocate write+execute memory for ffi.callback()

For more information, see https://cffi.readthedocs.io/en/latest/using.html#callbacks

Found originally here: https://github.com/scrapy/scrapy/issues/4117
Seems like pyOpenSSL issue, that's why creating it here.

Any advice or workaround is appreciated.

Traceback:

2019-10-31 20:24:51 [scrapy.downloadermiddlewares.robotstxt] ERROR: Error downloading <GET https://xxx.yyy/robots.txt>: Cannot allocate write+execute memory for ffi.callback(). You might be running on a system that prevents this. For more information, see https://cffi.readthedocs.io/en/latest/using.html#callbacks
Traceback (most recent call last):
.... skipped as non-relevant ...
 File "/home/scrapy/env/local/lib/python2.7/site-packages/twisted/internet/_sslverify.py", line 1709, in _makeContext
    ctx.set_verify(verifyFlags, _verifyCallback)
  File "/home/scrapy/env/local/lib/python2.7/site-packages/OpenSSL/SSL.py", line 1103, in set_verify
    self._verify_helper = _VerifyHelper(callback)

OS:

$ cat /etc/os-release
NAME="Ubuntu"
VERSION="18.04.2 LTS (Bionic Beaver)"
PRETTY_NAME="Ubuntu 18.04.2 LTS"
VERSION_ID="18.04"

Start with the traceback at Twisted's _sslverify.py and pyOpenSSL's OpenSSL.py set_verify(), including _VerifyHelper(callback). Read the linked cffi callback documentation and compare the Ubuntu 18.04 environment details with the callback failure. Done means the cause or a supported workaround is established and covered by an appropriate regression test.