Repository: tokio-rs/tokio
File: tokio/src/sync/semaphore.rs (Rust)

Code before:
/// }
/// # async fn handle_connection(_socket: &mut tokio::net::TcpStream) {
/// #   // Do work
/// # }
/// ```
///
/// ## Prevent tests from running in parallel
///
/// By default, Rust parallelly runs tests in the same file. However, In some cases, running two tests in parallel may lead to some problems.
/// For example, this can happen when tests use the same database.
///
/// Consider the following scenario:
/// 1. `test_insert`: Inserts a key-value pair into the database, then retrieves the value using the same key to verify the insertion.
/// 2. `test_update`: After insertion, this test updates the key's value to a new one and verifies if the value has been accurately updated.
/// 3. `test_others`: Other tests that dosen't modify the database state, thus can run parallely with previous tests.
///
/// For this example, `test_insert` and `test_update` need to run in sequence to ensure they can consistently pass., but it's doesn't matter which one runs first.
/// Leveraging a semaphore with a single permit elegantly addresses this challenge.
///
/// ```
/// #[cfg(test)]
/// mod tests {
///    use tokio::sync::Semaphore;
///
///    // Initialize a static semaphore with only one permit, which is used to
///    // prevent test_insert and test_update from running parallely.
///    static PERMIT: Semaphore = Semaphore::const_new(1);
///
///    // Initialize the database that will be used by the subsequent tests.
///    static DB: database = database::setup();
///
///    #[tokio::test]
///    async fn test_insert() {
///
///         // Acquire permit before proceeding. Since the semaphore has only one permit,
///         // the test will wait if the permit is already acquired by other tests.
///         let permit = PERMIT.acquire().await.unwrap();
///
///         // Do the actual test stuff with database
///
///         // Insert a key-value pair to database
///         let (key, value) = ("name", 0);
///         DB.insert((key, value)).await;
///
///         // Verify if the value has been inserted correctly.
///         assert_eq!(DB.get(key), value);
///
///         // Undo the insertation, make database status unchanged.
///         DB.delete(key).await;
///         // Permit automatically dropped here  
///     }

Review comment (suggestion):
```suggestion
///    // prevent test_insert and test_update from running in parallel.
```

Code after:
/// }
/// # async fn handle_connection(_socket: &mut tokio::net::TcpStream) {
/// #   // Do work
/// # }
/// ```
///
/// ## Prevent tests from running in parallel
///
/// By default, Rust runs tests in the same file in parallel. However, in some cases, running two tests in parallel may lead to problems.
/// For example, this can happen when tests use the same database.
///
/// Consider the following scenario:
/// 1. `test_insert`: Inserts a key-value pair into the database, then retrieves the value using the same key to verify the insertion.
/// 2. `test_update`: Inserts a key, then updates the key to a new value and verifies that the value has been accurately updated.
/// 3. `test_others`: A third test that doesn't modify the database state. It can run in parallel with the other tests.
///
/// In this example, `test_insert` and `test_update` need to run in sequence to work, but it doesn't matter which test runs first.
/// We can leverage a semaphore with a single permit to address this challenge.
///
/// ```
/// use tokio::sync::Semaphore;
/// # use tokio::sync::Mutex;
/// # use std::collections::BTreeMap;
/// # struct Database {
/// #   map: Mutex<BTreeMap<String, i32>>,
/// # }
/// # impl Database {
/// #    pub const fn setup() -> Database {
/// #        Database {
/// #            map: Mutex::const_new(BTreeMap::new()),
/// #        }
/// #    }
/// #    pub async fn insert(&self, key: &str, value: i32) {
/// #        self.map.lock().await.insert(key.to_string(), value);
/// #    }
/// #    pub async fn update(&self, key: &str, value: i32) {
/// #        self.map.lock().await
/// #            .entry(key.to_string())
/// #            .and_modify(|origin| *origin = value);
/// #    }
/// #    pub async fn delete(&self, key: &str) {
/// #        self.map.lock().await.remove(key);
/// #    }
/// #    pub async fn get(&self, key: &str) -> i32 {
/// #        *self.map.lock().await.get(key).unwrap()
/// #    }
/// # }
///
/// // Initialize a static semaphore with only one permit, which is used to
/// // prevent test_insert and test_update from running in parallel.
/// static PERMIT: Semaphore = Semaphore::const_new(1);