RacyCell<T>: Data race allowed on T

[kuzeyardabulut]

Hi,
I found a memory-safety/soundness issue in this crate while scanning Rust code for potential vulnerabilities. This PR contains a fix for the issue.

Issue Description

RacyCell<T> unconditionally implements Sync. This allows users to create data races on T: !Sync. Such data races can lead to undefined behavior.

bootloader/bios/common/src/racy_cell.rs

Line 21 in c5e5ed2

unsafe impl<T> Sync for RacyCell<T> {}

[bjorn3]

RacyCell doesn't expose any safe method that allows access to the internal value through a shared reference, so the internal value doesn't need to be Sync for RacyCell to be sync.

The fact that this PR doesn't break CI suggests that RacyCell isn't needed however. I think all uses can be replaced with the unstable core::cell::SyncOnceCell which does have the T: Sync bound on it's Sync impl.

[kuzeyardabulut]

I agree with what you said. This may not cause a direct data race. But making the above changes can be useful.

[phil-opp]

The fact that this PR doesn't break CI suggests that RacyCell isn't needed however. I think all uses can be replaced with the unstable core::cell::SyncOnceCell which does have the T: Sync bound on it's Sync impl.

I guess you mean the SyncUnsafeCell? Yes, we could use that as well, but I would prefer to wait until it's stabilized.

[phil-opp]

Looks reasonable to me, thanks for submitting!