sysconfig.get_config_var is not thread-safe

[gareth-rees]

Reproducer

This program starts many threads racing to call sysconfig.get_config_var:

import sysconfig
from multiprocessing.pool import ThreadPool

def thread(i):
    assert sysconfig.get_config_var("srcdir") is not None

with ThreadPool() as pool:
    pool.map(thread, range(100))

When run with Python 3.11.0, this asserts as follows:

Traceback (most recent call last):
  File "/home/grees/core/junk.py", line 8, in <module>
    pool.map(thread, range(100))
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/grees/github.com/cpython/Lib/multiprocessing/pool.py", line 364, in map
    return self._map_async(func, iterable, mapstar, chunksize).get()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/grees/github.com/cpython/Lib/multiprocessing/pool.py", line 771, in get
    raise self._value
    ^^^^^^^^^^^^^^^^^
  File "/home/grees/github.com/cpython/Lib/multiprocessing/pool.py", line 125, in worker
    result = (True, func(*args, **kwds))
                    ^^^^^^^^^^^^^^^^^^^
  File "/home/grees/github.com/cpython/Lib/multiprocessing/pool.py", line 48, in mapstar
    return list(map(*args))
           ^^^^^^^^^^^^^^^^
  File "/home/grees/core/junk.py", line 5, in thread
    assert sysconfig.get_config_var("srcdir") is not None
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

Analysis

In the sysconfig module, get_config_var calls get_config_vars which initializes the global variable _CONFIG_VARS in a thread-unsafe manner.

Use case

The reproducer above is simplified from a multi-threaded build system. Each build thread needs to get some information from sysconfig, so that the threads are racing to be the first to call get_config_vars.

Workaround

Adding an initial call to get_config_vars before starting any thread ensures that _CONFIG_VARS is reliably initialized.