nutex 0.1.0

#![allow(unreachable_pub, dead_code)]
use std::{
    ops::{Deref, DerefMut},
    sync::{
        Arc,
        atomic::{AtomicBool, Ordering},
    },
};
use tokio::sync::{Mutex, MutexGuard};

#[derive(Debug)]
/// A nullable `Mutex`-like type based on [`tokio::sync::Mutex`] implementation.
///
/// [`Nutex`] stands for **NU**llable mu**TEX** and it gives you
/// convenience to work with [`Option`]al values, such as connection that
/// cannot be set at the time when they was created:
/// you no more need to unwrap the value all the time in functions
/// that cannot be accessed without `Mutex<Option<T>>` value defining.
///
/// Note that locking [`Nutex`] requires to be certain that value is not `None`
/// and if you are not sure that function cannot be accessed without value defining
/// use [`Nutex::safe_lock`] or [`Nutex::safe_blocking_lock`] instead.
///
/// Otherwise if value is [`None`], [`Nutex::lock`]ing will lead you to panic.
///
/// ---
///
/// # Usage Examples
///
/// For example, you created an app and you want to access and process
/// the user client data stored in mutex in the some function
/// that may be called only if user is logged in.
///
/// With using the standard Tokio mutex you should unwrap the value
/// all the time:
///
/// ```rust,no_run
/// pub struct AppState {
///     pub client: Mutex<Option<Client>>,
///     // ...
/// }
///
/// #[tauri::command]
/// pub async fn send_message(
///     state: State<'_, AppState>,
///     message: String,
///     to: RecId
/// ) -> Result<(), SendError> {
///     // ? You need to do THIS:
///     state
///         .client
///         .lock()
///         .await
///         .expect("User must be logged in, blah, blah, blah...")
///         .user_id()
///         .expect("User must be logged in, blah blah blah...")
///         .process()
///         .yet_another_process()
///         .et_cetera();
///     // ...
///     Ok(())
/// }
///
/// #[tauri::command]
/// pub async fn register(
///     state: State<'_, AppState>,
///     auth_data: AuthData,
/// ) -> Result<(), ErrorResponse> {
///     // ? And THIS...
///     state.client.lock().await = Some(
///         Client::builder()
///             .register(auth_data)
///             .build()
///             .await?
///             .map_err(/* ... */)?;
///     )
///     // ...
///     Ok(())
/// }
/// ```
///
/// Double checking. Double headache.
/// But `Nutex` gives you this:
///
/// ```rust,no_run
/// pub struct AppState {
///     pub client: Nutex<Client>,
///     // ...
/// }
///
/// #[tauri::command]
/// pub async fn send_message(
///     state: State<'_, AppState>,
///     message: String,
///     to: RecId
/// ) -> Result<(), SendError> {
///     state
///         .client
///         .lock()
///         .await
///         .user_id()
///         .expect("User must be logged in, blah blah blah...")
///         .process();
///     // ...
///     Ok(())
/// }
///
/// #[tauri::command]
/// pub async fn register(
///     state: State<'_, AppState>,
///     auth_data: AuthData,
/// ) -> Result<(), ErrorResponse> {
///     state.client.set(
///         Client::builder()
///             .register(auth_data)
///             .build()
///             .await?
///             .map_err(/* ... */)?;
///             // ...
///     ).await;
///
///     Ok(())
/// }
/// ```
///
/// It simplifies the code a lot because `Mutex`es frequently
/// used with values that cannot be known at the time when
/// they are created.
///
/// # Examples
///
/// If you certainly know that value is not [`None`], then you
/// can just lock the [`Nutex`] and access the value:
/// ```rust,no_run
/// let nutex = Nutex::from(String::from("foo"));
/// *nutex.lock().await = String::from("bar");
/// assert_eq!(String::from("bar"), *nutex.lock().await);
/// ```
///
/// But if you're not sure that it's not [`None`], prefer using [`Nutex::safe_lock`]:
/// ```rust,no_run
/// if let Some(mut guard) = nutex.safe_lock().await {
///     *guard = String::from("foo");
/// }
/// ```
///
/// ---
///
/// Other documentation about [`Nutex`] may be found in the [`tokio::sync::Mutex`] docs:
/// [`Nutex`] is just a wrapper for this anyway.
/// ```
pub struct Nutex<T: Sized> {
    mutex: Mutex<Option<T>>,
    is_some: Arc<AtomicBool>,
}

#[derive(Debug)]
/// A custom handle that holds `Nutex` and contains `MutexGuard` of original Tokio `Mutex`.
/// The guard can be held across any `.await` point as it is [`Send`] thanks to the
/// original Tokio `MutexGuard` and `Mutex`.
///
/// As long as you have this guard, you have exclusive access to the underlying
/// `T`. The guard internally borrows the `Nutex`, so it guarantees that
/// [`Nutex`] reference won't be dropped early.
///
/// [`Nutex`] will unlock automatically when guard drop.
#[clippy::has_significant_drop]
#[must_use = "if unused the Mutex will immediately unlock"]
pub struct NutexGuard<'a, T: Sized> {
    lock: &'a Nutex<T>,
    guard: MutexGuard<'a, Option<T>>,
}

impl<'a, T: Sized> Nutex<T> {
    /// Creates a new `Nutex` that contains `None`.
    /// Value may be set later by [`Nutex::set`] or
    /// dereferencing [`Nutex::safe_lock`].
    pub fn new() -> Self {
        Self::default()
    }

    /// Asynchronously gets the value snapshot.
    /// Because of result value is clone and not the immutable reference,
    /// it's not reactive and cannot be observed later.
    ///
    /// ---
    ///
    /// # Panics
    ///
    /// * Panics if [`Nutex`] value is not set.
    /// If you are not sure that calling function cannot be called without
    /// setting the value, use [`Nutex::safe_get`] instead.
    pub async fn get(&self) -> T
    where
        T: Clone + Sized,
    {
        self.lock().await.clone()
    }

    /// Same as [`Nutex::get`], but blocks the code execution until value is get.
    pub fn blocking_get(&self) -> T
    where
        T: Clone + Sized,
    {
        self.blocking_lock().clone()
    }

    /// "Safe" edition of the original [`Nutex::get`] method.
    ///
    /// Note that "safe" doesn't mean that original
    /// method usage is discouraged: this method may be used
    /// if you are not sure that value exist.
    ///
    /// Although if more than half of your code contains this method,
    /// think about the original [`tokio::sync::Mutex`] instead of [`Nutex`].
    pub async fn safe_get(&self) -> Option<T>
    where
        T: Clone + Sized,
    {
        self.safe_lock().await.map(|g| g.clone())
    }

    /// Same as [`Nutex::safe_get`], but blocks the code execution until value is get.
    pub fn safe_blocking_get(&self) -> Option<T>
    where
        T: Clone + Sized,
    {
        self.safe_blocking_lock().map(|g| g.clone())
    }

    /// Sets the value in the [`Nutex`], replacing inner [`None`]
    /// to the `Some(value)`.
    ///
    /// Note that this method does nothing if value is already set:
    /// if you need to mutate the value that `Nutex` contains,
    /// use [`Nutex::lock`] or [`Nutex::blocking_lock`] instead.
    pub async fn set(&self, val: T) {
        if self.is_some.load(Ordering::Relaxed) {
            *self.mutex.lock().await = Some(val);
            self.is_some.swap(true, Ordering::Relaxed);
        }
    }

    /// Same as [`Nutex::set`], but blocks the code execution until value is set.
    pub fn blocking_set(&self, val: T) {
        if self.is_some.load(Ordering::Relaxed) {
            *self.mutex.blocking_lock() = Some(val);
            self.is_some.swap(true, Ordering::Relaxed);
        }
    }

    /// Sets inner value of the `Nutex` as `None`.
    pub async fn clear(&self) {
        *self.mutex.lock().await = None;
    }

    /// Same as [`Nutex::clear`], but blocks the code execution until value is cleared.
    pub fn blocking_clear(&self) {
        *self.mutex.blocking_lock() = None;
    }

    fn guard(&'a self, guard: MutexGuard<'a, Option<T>>) -> NutexGuard<'a, T> {
        if self.is_some.load(Ordering::Relaxed) {
            NutexGuard {
                lock: self,
                guard: guard,
            }
        } else {
            panic!(
                "Accessed inner Nutex value that's `None`.
                If you're not sure that value is certainly set,
                prefer using `.safe_lock()` or `.safe_blocking_lock()` methods instead."
            )
        }
    }

    fn guard_expect(&'a self, guard: MutexGuard<'a, Option<T>>, msg: &str) -> NutexGuard<'a, T> {
        if self.is_some.load(Ordering::Relaxed) {
            NutexGuard {
                lock: self,
                guard: guard,
            }
        } else {
            panic!("{}", msg)
        }
    }

    fn safe_guard(&'a self, guard: MutexGuard<'a, Option<T>>) -> Option<NutexGuard<'a, T>> {
        if self.is_some.load(Ordering::Relaxed) {
            Some(NutexGuard {
                lock: self,
                guard: guard,
            })
        } else {
            None
        }
    }

    /// Locks this [`Nutex`], causing the current task to yield until the lock has
    /// been acquired.  When the lock has been acquired, function returns a
    /// [`NutexGuard`].
    ///
    /// If the `Nutex` is available to be acquired immediately, then this call
    /// will typically not yield to the runtime. However, this is not guaranteed
    /// under all circumstances.
    ///
    /// ---
    ///
    /// # Panics
    /// * Panics if [`Nutex`] value is not set.
    /// If you are not sure that calling function cannot be called without
    /// setting the value, use [`Nutex::safe_lock`] instead.
    pub async fn lock(&'a self) -> NutexGuard<'a, T> {
        self.guard(self.mutex.lock().await)
    }

    /// Same as [`Nutex::lock`], but if value is not set, it panics with a custom message.
    ///
    /// Useful if panic impossibility reason is opaque and you should explain
    /// why panic is impossible.
    pub async fn lock_expect(&'a self, msg: &str) -> NutexGuard<'a, T> {
        self.guard_expect(self.mutex.lock().await, msg)
    }

    /// Same as [`Nutex::lock`], but blocks the code execution until `Nutex` is locked.
    pub fn blocking_lock(&'a self) -> NutexGuard<'a, T> {
        self.guard(self.mutex.blocking_lock())
    }

    /// Same as [`Nutex::lock_expect`], but blocks the code execution until `Nutex` is locked.
    pub fn blocking_lock_expect(&'a self, msg: &str) -> NutexGuard<'a, T> {
        self.guard_expect(self.mutex.blocking_lock(), msg)
    }

    /// "Safe" edition of the original [`Nutex::lock`] method.
    ///
    /// Note that "safe" doesn't mean that original
    /// method usage is discouraged: this method may be used
    /// if you are not sure that value exist.
    ///
    /// Although if more than half of your code contains this method,
    /// think about the original [`tokio::sync::Mutex`] instead of `Nutex`.
    pub async fn safe_lock(&'a self) -> Option<NutexGuard<'a, T>> {
        self.safe_guard(self.mutex.lock().await)
    }

    /// Same as [`Nutex::safe_lock`], but blocks the code execution until `Nutex` is locked.
    pub fn safe_blocking_lock(&'a self) -> Option<NutexGuard<'a, T>> {
        self.safe_guard(self.mutex.blocking_lock())
    }

    /// Atomic checker that inner value is not [`None`].
    pub fn is_some(&self) -> bool {
        self.is_some.load(Ordering::Relaxed)
    }

    /// Atomic checker that inner value is [`None`].
    pub fn is_none(&self) -> bool {
        !self.is_some.load(Ordering::Relaxed)
    }

    /// Consumes the [`Nutex`], returning the underlying data.
    pub fn into_inner(self) -> Option<T> {
        self.mutex.into_inner()
    }
}

impl<T: Sized> Default for Nutex<T> {
    /// Creates default value of [`Nutex`].
    /// Shorthand of this expression is [`Nutex::new`].
    ///
    /// Inner value of [`Nutex`] is [`None`] by default.
    fn default() -> Self {
        Self {
            mutex: Mutex::new(None),
            is_some: Arc::new(AtomicBool::new(false)),
        }
    }
}

impl<T: Sized> From<T> for Nutex<T> {
    /// Creates [`Nutex`] with the inner value from the argument.
    fn from(value: T) -> Self {
        Self {
            mutex: Mutex::new(Some(value)),
            is_some: Arc::new(AtomicBool::new(true)),
        }
    }
}

impl<'a, T: Sized> NutexGuard<'a, T> {
    /// Returns a reference to the inner [`Nutex`] that lives
    /// the same time as the guard.
    pub fn nutex(&self) -> &'a Nutex<T> {
        self.lock
    }
}

impl<'a, T: Sized> Deref for NutexGuard<'a, T> {
    type Target = T;

    fn deref(&self) -> &'a Self::Target {
        match &*self.guard {
            // SAFETY: Man what the fuck it's just a lifetime annotation
            Some(val) => unsafe { std::mem::transmute::<&T, &'a T>(val) },
            None => unreachable!("MutexGuard can be accessed only if value exist"),
        }
    }
}

impl<'a, T: Sized> DerefMut for NutexGuard<'a, T> {
    fn deref_mut(&mut self) -> &'a mut Self::Target {
        match &mut *self.guard {
            // SAFETY: Man what the fuck it's just a lifetime annotation
            Some(val) => unsafe { std::mem::transmute::<&mut T, &'a mut T>(val) },
            None => unreachable!("MutexGuard can be accessed only if value exist"),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn access_test() {
        let nutex = Nutex::from(String::from("foo"));
        assert_eq!(String::from("foo"), *nutex.blocking_lock())
    }

    #[test]
    fn mutation_test() {
        let nutex: Nutex<String> = Nutex::new();
        nutex.blocking_set(String::from("bar"));
        assert_eq!(String::from("bar"), *nutex.blocking_lock())
    }

    #[test]
    fn clear_test() {
        let nutex: Nutex<String> = Nutex::new();
        nutex.blocking_set(String::from("bar"));
        assert_eq!(String::from("bar"), *nutex.blocking_lock());
        nutex.blocking_clear();
        assert!(nutex.safe_blocking_lock().is_none());
    }
}