Drop support for ruby 2.7 and add 3.3 to test matrix

[pmor]

• Bump minimum ruby version to 3.0

[swiknaba]

A wild guess: https://github.com/countries/countries/blob/v5.7.1/spec/thread_safety_spec.rb#L14

100 Threads might not be enough to consistently get a thread safety issue considering Ruby's improved M:N Thread scheduler: https://github.com/ruby/ruby/blob/ruby_3_3/NEWS.md#mn-thread-scheduler

I tried to run it with 250 threads, but still don't get a race-condition. After 251 threads, my systems fails with Errno::EMFILE: Too many open files for the test that skips using a mutex.

I've injected the current thread ID into the cache object in each loop, which resulted in a unique ID for each thread, so the cache object is not thread-local I guess (to be expected?).

Other stuff I tried that didn't help:

• directly call load_data! in each loop in each thread
• adding sleep Kernel.rand(0.0001..0.001) in the synchronized block to have some artificial jitter with accessing the cache
• adding random time delays in the file opening method
• run for 250 threads and loop over 10k language codes

I ran tests on a M1 Max with 10 cores.

[pmor]

@swiknaba Thanks for digging into that. I haven't had any success either.

That code has been around for about 4 years, since before I was maintainer of the gem, and I don't think anyone reported thread safety issues since then. The #use_mutex? method is only there to force tests to run without the mutex anyway, so I think I'm just going to remove it and update the specs.