Fix race condition in init config tests

[JeremieRodon]

Problem

In the init::config::tests module, the usage of std::env::set_var and std::env::remove_var can cause a race condition which can occasionally fail the cargo test invocation.

This happens when 2 tests using the EnvManager type are running in parallel in different threads of the test binary.
You can verify that on any Linux system by running (e.g.):

cargo test test_get_multi_exporter_config_datadog_ -- --test-threads 3

By selecting those 3 datadog tests that use the EnvManager and ensuring parallelism on 3 threads, you should be able to reproduce the race condition 100% of the time. Note that without the --test-threads 3 argument, you should also run multi-threaded unless you have a single core. Also note that even when simply running cargo test to run all tests, the race condition will happen occasionally out of bad luck (that's how I found the problem).

Solution

I introduced a static Mutex<()> in the EnvManager. When calling EnvManager::new(), the Mutex guard becomes part of the EnvManager struct, ensuring only one EnvManager can exist at a time among the threads of the test binary. Essentially, it guarantees that tests using the EnvManager are run sequentially and do not interfere with each other.

[mheffner]

Nice! We've been mostly using nextest which will run these as separate processes, but it's good to keep this safe for cargo test runs too.

[JeremieRodon]

I followed DEVELOPING.md like a good boy 😄

If tests should be performed with another tool, it could be desirable to update it at some point 🙂

[mheffner]

If tests should be performed with another tool, it could be desirable to update it at some point 🙂

Ahh! Good point, I'll update those.