`test.support.os_helper.EnvironmentVarGuard` is not thread safe

[Wulian233]

Bug report

Bug description:

I ran it multiple times with the following code and got different results

import os
import threading
import time
from test.support.os_helper import EnvironmentVarGuard

def worker1(guard):
    for i in range(1000):
        guard['MY_VAR'] = f'value1'
        time.sleep(0.0001)  # Small delay to increase chance of thread switching

def worker2(guard):
    for i in range(1000):
        guard['MY_VAR'] = f'value2'
        time.sleep(0.0001)

guard = EnvironmentVarGuard()
t1 = threading.Thread(target=worker1, args=(guard,))
t2 = threading.Thread(target=worker2, args=(guard,))

t1.start()
t2.start()
t1.join()
t2.join()

final_value = os.getenv('MY_VAR')
print(f"Final value: {final_value}")

This issue I found in #131870, adding a lock to the test_zoneinfo.py, but the original implementation didn't.

BTW, 131870 PR has not been reviewed by anyone yet (more a week)

CPython versions tested on:

3.14, 3.13

Operating systems tested on:

Windows

Linked PRs

• gh-132113: Make EnvironmentVarGuard thread safe #132128

[picnixz]

BTW, 131870 PR has not been reviewed by anyone yet (more a week)

Before reviewing this PR, we first need to make EnvironmentVarGuard thread-safe then.

[Wulian233]

Yes, but I'm not good at this. I hope someone else can make it thread-safe :)

[srinivasreddy]

I am trying to understand more about use case. Even if we make thread safe, that does not mean t2 will always finish last, no?
If we really care about ordering , we must enforce in code like this,

t1.start()
t1.join()
t2.start()
t2.join()

[serhiy-storchaka]

Context managers that change the global state cannot be thread safe.

For example, let we have two threads with two guards:

• First guard saves old value and sets a new value.
• Second guard saves old value (set by the first guard) and sets its own new value.
• First guard restores the saves original value.
• Second guard restores the value which was set by the first guard.

[picnixz]

Oh ... TIL. I didn't think of this. Closing then.

[Wulian233]

Before reviewing this PR, we first need to make EnvironmentVarGuard thread-safe then.

Now we can take a look at it #131870