What tests would you write for this code besides unit tests? How would you test multithreading?

In addition to unit tests, for Go code it is worth writing:

• Integration tests — check the interaction between components, such as sending notifications through real or mock channels.
• End-to-End tests — simulate the full scenario from receiving an event to delivering a notification.
• Performance tests — measure processing and delivery times, verify SLA compliance.
• Error resilience tests — check behavior during failures of external services, network errors.

For multithreading testing:

• Use the -race package (flag go test -race), which detects data races.
• Write stress tests that run multiple goroutines simultaneously accessing shared resources.
• Verify synchronization correctness (mutex, channels).
• Use profiling tools (pprof) to identify bottlenecks and deadlocks.

Example of running tests with race detection:

go test -race ./...

This approach helps identify concurrency errors and ensures stability in a multithreaded environment.