Isolating Side Effects in Tests Using Thread-Local Mocking

use std::cell::RefCell;
 
thread_local! {
    static MOCK_DB_RESPONSE: RefCell<Option<Result<String, &'static str>>> = const { RefCell::new(None) };
}
 
fn fetch_data_from_db() -> Result<String, &'static str> {
    MOCK_DB_RESPONSE.with(|mock| {
        if let Some(ref response) = *mock.borrow() {
            response.clone()
        } else {
            Ok("real_production_data".to_string())
        }
    })
}
 
#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_database_error_handling() {
        MOCK_DB_RESPONSE.with(|mock| {
            *mock.borrow_mut() = Some(Err("Timeout"));
        });
        assert_eq!(fetch_data_from_db(), Err("Timeout"));
        MOCK_DB_RESPONSE.with(|mock| *mock.borrow_mut() = None);
    }
}