Decouple number of concurrent tests from RUST_THREADS env var

[brson]

Currently the test runner will adjust the number of tests run concurrently based on RUST_THREADS. I don't think this coupling really makes sense and we should instead use RUST_TEST_TASKS.

[vijaykramesh]

Where were you imagining the env would load RUST_TEST_TASKS instead of RUST_THREADS? It'd be simple enough to also check getenv(RUST_TEST_TASKS) and use that if it exists (at https://github.com/mozilla/rust/blob/master/src/rt/rust_env.cpp#L87) but it seems a bit messy to include test logic in the main env loader.

Unless I'm mistaken, the result of that environmental setting gets loaded for both tests and non-tests when creating the scheduler at https://github.com/mozilla/rust/blob/master/src/rt/rust_kernel.cpp#L49 so not sure if there is a cleaner place to add in the test-specific env stuff, or if it belongs in src/rt/rust_env.cpp?

[vijaykramesh]

I don't think that is quite right though, as if you specify RUST_TEST_TASKS=X RUST_THREADS=Y it will start the scheduler with X even if you aren't running tests. Hmm.

[brson]

@vijaykramesh sorry for the delayed response. I would put a call to os::getenv("RUST_TEST_TASKS") directly in extra::test::get_concurrency, and if that's not set fallback to the current behavior of using the same number of tasks as RUST_THREADS * some constant, minus the special case when RUST_THREADS=1 (when RUST_THREADS=1 we don't multiply times the constant).

[huonw]

Triage: will be fixed when #8823 lands.