Commits on Apr 25, 2024

1. Update CONTRIBUTING.md with proper LOOM test invocation …

   Loom tests currently fail when `LOOM_MAX_PREEMPTION` is set to 1. Specifically, `yield_calls_park_before_scheduling_again` across all schedulers (current thread, multi thread, multi thread alt).
   
   For example: 
   
   ```
   RUSTFLAGS="-Dwarnings --cfg loom  -C debug_assertions" LOOM_MAX_PREEMPTIONS=1 LOOM_MAX_BRANCHES=10000 cargo test --lib --release --features full -- --nocapture loom_current_thread
       Finished release [optimized] target(s) in 0.05s
        Running unittests src/lib.rs (/Users/ant/Developer/oss/tokio/target/release/deps/tokio-32b7cb1c593730d4)
   
   running 3 tests
   test runtime::tests::loom_current_thread::assert_no_unnecessary_polls ... ok
   thread 'runtime::tests::loom_current_thread::yield_now::yield_calls_park_before_scheduling_again' panicked at tokio/src/runtime/tests/loom_current_thread/yield_now.rs:22:17:
   assertion `left == right` failed
     left: 1
    right: 0
   note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
   thread 'runtime::tests::loom_current_thread::yield_now::yield_calls_park_before_scheduling_again' panicked at /Users/ant/.cargo/registry/src/index.crates.io-6f17d22bba15001f/loom-0.7.1/src/rt/execution.rs:216:13:
   deadlock; threads = [(Id(0), Blocked(Location(None))), (Id(1), Blocked(Location(None))), (Id(2), Blocked(Location(None)))]
   thread 'runtime::tests::loom_current_thread::yield_now::yield_calls_park_before_scheduling_again' panicked at /Users/ant/.cargo/registry/src/index.crates.io-6f17d22bba15001f/loom-0.7.1/src/rt/thread.rs:276:39:
   called `Option::unwrap()` on a `None` value
   stack backtrace:
      0:        0x104dfbfe8 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h3be43c6214e69043
      1:        0x104e19a70 - core::fmt::write::h12f9049830901a82
      2:        0x104df9bbc - std::io::Write::write_fmt::h2ff514dfe799c882
      3:        0x104dfbe1c - std::sys_common::backtrace::print::h722d6ff0889d39c2
      4:        0x104dfd958 - std::panicking::default_hook::{{closure}}::hd7f9985c1b9ae0aa
      5:        0x104dfd6a0 - std::panicking::default_hook::h29fb35819c2a32ca
      6:        0x104dfdecc - std::panicking::rust_panic_with_hook::h401df0236a0e7dae
      7:        0x104dfdc5c - std::panicking::begin_panic_handler::{{closure}}::h67f4ee7322151c3c
      8:        0x104dfc46c - std::sys_common::backtrace::__rust_end_short_backtrace::hbce128a2ff88406e
      9:        0x104dfda10 - _rust_begin_unwind
     10:        0x104e3192c - core::panicking::panic_fmt::hecc1dfe94cb15bc7
     11:        0x104e319b4 - core::panicking::panic::h73b079eecd9000fe
     12:        0x104e318b0 - core::option::unwrap_failed::h2a1d2a89129dbc81
     13:        0x104cd4880 - loom::rt::object::Ref<T>::set_action::h606daf31b2ba97a6
     14:        0x104cadf58 - scoped_tls::ScopedKey<T>::with::h634eac6aaf7668ce
     15:        0x104cb6d64 - loom::rt::atomic::Atomic<T>::store::he9584501adecba00
   error: test failed, to rerun pass `--lib`
   
   Caused by:
     process didn't exit successfully: `/Users/ant/Developer/oss/tokio/target/release/deps/tokio-32b7cb1c593730d4 --nocapture loom_current_thread` (signal: 10, SIGBUS: access to undefined memory)
   ```
   
   Although the test consistently fails with this invocation (tried on my Mac and Linux using the repo's main branch), it's not failing on CI.
   
   When looking into how we invoke loom in CI, I noticed we pass LOOM_MAX_PREEMPTIONS=2 (e.g. https://github.com/tokio-rs/tokio/actions/runs/8814945156/job/24195889256). This env var change makes the test pass.
   
   And so this diff updates the invocation to match what we do in CI.

   

   adelavegaf committed Apr 25, 2024
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