cargo test doesn't capture print from threads

[kpcyrd]

I've filed this bug in rust-lang/cargo but I'm afraid rust-lang/rust is the correct repo for this.

I ran the tests for one of my crates that uses threads in its example code. I've noticed it printed some lines to stdout, even though --nocapture wasn't set, but not all of them. It appears that text printed from new threads are not correctly captured, I think this is not intended.

Reproduce

#[cfg(test)]
mod tests {
    use std::thread;
    use std::time::Duration;

    #[test]
    fn it_works() {
        thread::spawn(move || {
            println!("request: {:?}", 123);
        });

        thread::sleep(Duration::from_secs(1));
        println!("response: {:?}", 456);
    }

Output

running 2 tests
request: 123
test tests::it_works ... ok
[...]

Meta

rustc --version --verbose:

rustc 1.17.0
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.17.0
LLVM version: 4.0

[retep998]

set_print (which is what tests use to capture stdout) modifies a thread local, so it only captures output on that exact thread. If you spawn a new thread, it starts with fresh thread locals and so stdout isn't captured.

[mathstuf]

In my tests, I use log in my code and the the tests set up a simple println! logger when they run. Is there some way to have the log implementation send the output to the right place?

[dtolnay]

I would be interested in seeing a fix for this in a PR.

[mathstuf]

It'd be nice if there were some way to ask the current test "what print sink are you using?" so that it can pass them down to any spawned threads. That would at least work for "manual" threads. It wouldn't work for things like Rayon (AFAIK).

[azriel91]

The following issues appear to be duplicates (or at least, related):

• libtest - capture stderr from tests #40298
• libtest captures stdout&stderr only on first thread. #31983
• Capturing stdout from test code is inconsistent #12309

[golergka]

Although this issue is closed, some issues that were closed before as duplicates as this are still not solved: std::io::stdout().write_line("example"); still is not captured by tests.

[Khouderchah-Alex]

+1. Looks like the solution (d0d0e78, 11ce918) was for print_to (and consequently _print & macros) to redirect to OUTPUT_CAPTURE if set.

IIUC, this leaves any direct usage of stdout() uncaptured.

The title doesn't cover this specific issue, but this essentially became a meta-bug by closing all the other output capture issues as dupes. Seems like either this bug or something like #12309 should be reopened.

[jsoneaday]

+1. Looks like the solution (d0d0e78, 11ce918) was for print_to (and consequently _print & macros) to redirect to OUTPUT_CAPTURE if set.

IIUC, this leaves any direct usage of stdout() uncaptured.

The title doesn't cover this specific issue, but this essentially became a meta-bug by closing all the other output capture issues as dupes. Seems like either this bug or something like #12309 should be reopened.

12309 is closed and points to this issue and this issue is claimed as a duplicate. Is this ever going to get fixed?

[Khouderchah-Alex]

12309 is closed and points to this issue and this issue is claimed as a duplicate. Is this ever going to get fixed?

#90785 is open, but I wouldn't hold my breath unless someone's throwing out PRs