JRuby thread safety test nondeterministically fails

[JacobEvelyn]

In this CI run, running against 403a4e7 with this JRuby version:

jruby 10.0.0.0 (3.4.2) 2025-04-13 6ed59bc847 OpenJDK 64-Bit Server VM 21.0.7+6-LTS on 21.0.7+6-LTS +indy +jit [x86_64-linux]

we got the following test failure:

Failures:

  1) thread safety when the method takes one positional argument when the method has already been called with another argument behaves like provides thread safety guarantees when checking condition: are different values returned? returns different values to each thread, and memoizes one of them
     Failure/Error: expect(thread_return_values.uniq.size).to be > 1

       expected: > 1
            got:   1
     Shared Example Group: "provides thread safety guarantees" called from ./spec/thread_safety_spec.rb:102
     # ./spec/thread_safety_spec.rb:74:in 'block in <main>'

Finished in 3 minutes 48.4 seconds (files took 1.67 seconds to load)
616 examples, 1 failure

Failed examples:

rspec './spec/thread_safety_spec.rb[1:2:1:1:2:1]' # thread safety when the method takes one positional argument when the method has already been called with another argument behaves like provides thread safety guarantees when checking condition: are different values returned? returns different values to each thread, and memoizes one of them

Randomized with seed 5952

[JacobEvelyn]

This failed again in this CI run, running against 5720bf6 on the same JRuby version as above:

jruby 10.0.0.0 (3.4.2) 2025-04-13 6ed59bc847 OpenJDK 64-Bit Server VM 21.0.7+6-LTS on 21.0.7+6-LTS +indy +jit [x86_64-linux]

with the following test failure:

Failures:

  1) thread safety when the method takes required and optional positional arguments when the method has already been called with other arguments behaves like provides thread safety guarantees when checking condition: are different values returned? returns different values to each thread, and memoizes one of them
     Failure/Error: expect(thread_return_values.uniq.size).to be > 1

       expected: > 1
            got:   1
     Shared Example Group: "provides thread safety guarantees" called from ./spec/thread_safety_spec.rb:136
     # ./spec/thread_safety_spec.rb:74:in 'block in <main>'

Finished in 3 minutes 36.3 seconds (files took 1.61 seconds to load)
616 examples, 1 failure

Failed examples:

rspec './spec/thread_safety_spec.rb[1:4:1:1:2:1]' # thread safety when the method takes required and optional positional arguments when the method has already been called with other arguments behaves like provides thread safety guarantees when checking condition: are different values returned? returns different values to each thread, and memoizes one of them

Randomized with seed 49988

[ms-ati]

Ah. A possible path is to conditionally include concurrent-ruby on JRuby, or even just use it if its already included, so that we use thread-safe Array and Hash?

[JacobEvelyn]

I'm hesitant to introduce a dependency, which would be a departure from how this library has existed thus far. @ms-ati do you have a clear understanding of what's causing the bug? I haven't investigated it.

[JacobEvelyn]

Note that this may be caused by the same underlying issue as #264