CVE-2020-36446: Data races in signal-simple

Affected versions of this crate unconditionally implement Send/Sync for SyncChannel. SyncChannel doesn’t provide access to &T but merely serves as a channel that consumes and returns owned T.

Users can create UB in safe Rust by sending T: !Send to other threads with SyncChannel::send/recv APIs. Using T = Arc<Cell<_> allows to create data races (which can lead to memory corruption), and using T = MutexGuard allows to unlock a mutex from a thread that didn’t lock the mutex.