"Cargo test" uses a thread pool in 1.33.0

[asomers]

Previous versions of Cargo would create and destroy a new thread for each test. Cargo 1.33.0 instead creates a thread pool and reuses the same thread for multiple tests. This can cause test failures for crates that implicitly rely on Cargo's old behavior for isolation. I'm not saying that the new behavior is wrong, but it deserves a mention in the release notes.

nix-rust/nix#1033

[RalfJung]

There is indeed a change in behavior, but not the way you suggest: behavior only changed for the case of a machine with exactly 1 core. Previously, it would spawn a thread per test and then basically immediately wait on that thread to finish; now instead it runs the tests in the main thread directly. There is no thread pool.

[asomers]

Perhaps there are two relevant PRs, because I absolutely do see thread-pool-like behavior. Just run strace -e clone cargo test on an n-core system, on almost any crate, for n > 1. With Rust and Cargo 1.33.0 there will be n clones. With older Rust there will be one clone per test.

Actually I just tried again and I'm not seeing that behavior anymore. Perhaps I did something stupid the first time. It may be that the only behavior change is for --test-threads=1.

[RalfJung]

Ah yes, indeed it's not about the number of cores but the concurrency level -- which defaults to the number of cores.

[Mark-Simulacrum]

Nominating for libs team -- I expect we'll probably want to not change anything here since the new behavior seems strictly better. However, as a regression, we should discuss.

[asomers]

I agree that the new behavior is better; Nix shouldn't have been relying on the old behavior. I just think that the new behavior should've been mentioned in the release notes.

[Mark-Simulacrum]

Yep, we missed it when compiling the release notes. If you'd like to propose a PR to master with this added to 1.33.0's notes that'd be appreciated!

[alexcrichton]

This was discussed during libs triage the other day and the conclusion was that adding this to the release notes is fine, so denominating.