Tests do not capture stdout of foreign threads

[jstarks]

If a test writes to stdout on a thread created outside of Rust (e.g. directly via pthread_create on Linux or CreateThread on Windows), then that output will not be captured but will go straight to stdout when run with cargo test. This is problematic for tests that need to run code on threads created by non-Rust components.

Repro:

#![cfg(test)]

#[test]
fn foo() {
    unsafe {
        let mut n = std::mem::zeroed();
        assert_eq!(
            0,
            libc::pthread_create(&mut n, std::ptr::null(), print_thread, std::ptr::null_mut())
        );
        assert_eq!(0, libc::pthread_join(n, std::ptr::null_mut()));
    }
}

extern "C" fn print_thread(_: *mut std::ffi::c_void) -> *mut std::ffi::c_void {
    println!("should be captured");
    std::ptr::null_mut()
}

Expected (output is captured):

    Finished test [unoptimized + debuginfo] target(s) in 0.11s
     Running unittests (target/debug/deps/repro-4d64f2a51a952b52)

running 1 test
test foo ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Actual (output is not captured):

    Finished test [unoptimized + debuginfo] target(s) in 0.11s
     Running unittests (target/debug/deps/repro-4d64f2a51a952b52)

running 1 test
should be captured
test foo ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Meta

rustc --version --verbose:

rustc 1.56.0 (09c42c458 2021-10-18)
binary: rustc
commit-hash: 09c42c45858d5f3aedfa670698275303a3d19afa
commit-date: 2021-10-18
host: x86_64-unknown-linux-musl
release: 1.56.0
LLVM version: 13.0.0

[sfackler]

How would this actually be implemented? We cannot override the behavior of pthread_create AFAIK.