Deal with parallel execution of test cases

[Genki-S]

Currently, when test cases are executed in parallel (like with parallelize_me! of MiniTest), mapping file is read and written at the same time and the file becomes corrupt.

I noticed this when I was trying TTNT in actionpack directory of rails/rails. The parallelization was specified here and when I comment it out, rake ttnt:test:anchor finished successfully (without commenting out, mapping file became corrupt).

I have 2 options for now:

1. Disable parallelize_me! somehow
2. Lock mapping file and hope parallelized executions stop when trying to access locked file

I think users can make tests run in parallel even without parallelize_me!, so second approach seems better.

[Genki-S]

Just a mental note:

at_exit is inherited to forked processes, so at_exit block in lib/anchor might get called multiple times. (within actionpack which is using ForkingExecutor, it's called 4 times (by default) due to these forks).

It's nice if TTNT can work without any problem even if tests run in parallel.

[robin850]

I'm saying that out of the blue, I didn't test but to avoid such problems, did you try to gather coverage as explained in Tenderlove's article (i.e. directly hooking inside Minitest's run_one_method) ? You can import Coverage#peek_result if you want. Feel free to ask if you are struggling against any problem.

[Genki-S]

That sounds worth trying! Thank you for the idea 😄
I will try that first since it would bring additional benefit of making it possible to select tests by test methods, not by test files.