Test execution hangs if guards are used in multiple tests

[rasviitanen]

Environment

How do you use Sentry?
sentry.io

Which SDK and version?
sentry-rust (0.22.0)

Toolchain:
stable-x86_64-pc-windows-msvc (default)
rustc 1.50.0 (cb75ad5db 2021-02-10)

Steps to Reproduce

1. Define two tests that creates guards:

#[test]
fn test_a() {
    let _guard = sentry::init(sentry::ClientOptions {
        ..Default::default()
    });
}

#[test]
fn test_b() {
    let _guard = sentry::init(sentry::ClientOptions {
        ..Default::default()
    });
}

2. Run cargo test
3. Get output:

running 2 tests
test tests::test_a ... ok
test tests::test_b ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.03s 

4. Test execution freezes and doesn't return
5. Here is an example application to reproduce the problem: https://github.com/rasviitanen/sentry-bug

Running the tests individually works fine

Expected Result

The test execution should finish with exit 0.

Actual Result

running 2 tests
test observability::tests::test_a ... ok
test observability::tests::test_b ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 63 filtered out; finished in 0.03s 
<here it just freezes and you have to press ctrl+c to exit>

We didn't experience any issues with this in 0.19.0

[rasviitanen]

I think I have tracked down the issue to these lines:

sentry-rust/sentry-core/src/hub.rs

Lines 158 to 164 in f86a4db

	// note on safety: this is safe because even though we change the Arc
	// by temorary binding we guarantee that the original Arc stays alive.
	// For more information see: run
	THREAD_HUB.with(|stack| unsafe {
	let ptr = stack.get();
	f(&*ptr)
	})

I guess it's the handeling of the global statics (THREAD_HUB/PROCESS_HUB) that doesn't work correctly when initializing sentry on multiple threads. Using the PROCESS_HUB in both cases fixes the symptomps but is not a fix.

---

Here is another example of reproducing the bug:

fn main() {
    let _guard = sentry::init(sentry::ClientOptions {
        ..Default::default()
    });

    std::thread::spawn(|| {
        let _guard = sentry::init(sentry::ClientOptions {
            ..Default::default()
        });
    }).join();
}

[Swatinem]

I think I have tracked this down to the Drop impl of our SessionFlusher which was implemented in #279.
It seems that the PROCESS_HUB holds on to a reference to the Client and thus to the SessionFlusher, which will never be dropped.

[Swatinem]

Oh, I see you were also on Windows, and interestingly enough I was also tracking this down on Windows.

This seems to have been a manifestation of rust-lang/rust#74875 I guess since that matches the conclusion I came to in #326

[rasviitanen]

@Swatinem Amazing work, thank you!