[issue39068] Base 85 encoding initialization race conditiong

[Brandon Stansbury]

New submission from Brandon Stansbury <brandonrstansbury at gmail.com>:

Under multi-threading scenarios a race condition may occur where a thread sees an initialized `_b85chars` table but an uninitialized `_b85chars2` table due to the guard only checking the first table.

This causes an exception like:

```
  File "/usr/lib/python3.6/base64.py", line 434, in b85encode
    return _85encode(b, _b85chars, _b85chars2, pad),
  File "/usr/lib/python3.6/base64.py", line 294, in _85encode
    for word in words],
  File "/usr/lib/python3.6/base64.py", line 294, in <listcomp>
    for word in words],
 "TypeError: 'NoneType' object is not subscriptable
```

----------
components: Library (Lib)
messages: 358495
nosy: drmonkeysee
priority: normal
pull_requests: 17096
severity: normal
status: open
title: Base 85 encoding initialization race conditiong
type: crash
versions: Python 3.6

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39068>
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