nexosim/asynchronix/src/simulation.rs

//! Discrete-event simulation management.
//!
//! This module contains most notably the [`Simulation`] environment, the
//! [`SimInit`] simulation builder, the [`Mailbox`] and [`Address`] types as
//! well as miscellaneous other types related to simulation management.
//!
//! # Simulation lifecycle
//!
//! The lifecycle of a simulation bench typically comprises the following
//! stages:
//!
//! 1) instantiation of models and their [`Mailbox`]es,
//! 2) connection of the models' output/requestor ports to input/replier ports
//!    using the [`Address`]es of the target models,
//! 3) instantiation of a [`SimInit`] simulation builder and migration of all
//!    models and mailboxes to the builder with [`SimInit::add_model()`],
//! 4) initialization of a [`Simulation`] instance with [`SimInit::init()`],
//! 5) discrete-time simulation, which typically involves scheduling events and
//!    incrementing simulation time while observing the models outputs.
//!
//! Most information necessary to run a simulation is available in the root
//! crate [documentation](crate) and in the [`SimInit`] and [`Simulation`]
//! documentation. The next section complement this information with a set of
//! practical recommendations that can help run and troubleshoot simulations.
//!
//! # Practical considerations
//!
//! ## Mailbox capacity
//!
//! A [`Mailbox`] is a buffer that store incoming events and queries for a
//! single model instance. Mailboxes have a bounded capacity, which defaults to
//! [`Mailbox::DEFAULT_CAPACITY`].
//!
//! The capacity is a trade-off: too large a capacity may lead to excessive
//! memory usage, whereas too small a capacity can hamper performance and
//! increase the likelihood of deadlocks (see next section). Note that, because
//! a mailbox may receive events or queries of various sizes, it is actually the
//! largest message sent that ultimately determines the amount of allocated
//! memory.
//!
//! The default capacity should prove a reasonable trade-off in most cases, but
//! for situations where it is not appropriate, it is possible to instantiate
//! mailboxes with a custom capacity by using [`Mailbox::with_capacity()`]
//! instead of [`Mailbox::new()`].
//!
//! ## Avoiding deadlocks
//!
//! While the underlying architecture of Asynchronix—the actor model—should
//! prevent most race conditions (including obviously data races which are not
//! possible in safe Rust) it is still possible in theory to generate deadlocks.
//! Though rare in practice, these may occur due to one of the below:
//!
//! 1. *query loopback*: if a model sends a query which is further forwarded by
//!    other models until it loops back to the initial model, that model would
//!    in effect wait for its own response and block,
//! 2. *mailbox saturation*: if several models concurrently send to one another
//!    a very large number of messages in succession, these models may end up
//!    saturating all mailboxes, at which point they will wait for the other's
//!    mailboxes to free space so they can send the next message, eventually
//!    preventing all of them to make further progress.
//!
//! The first scenario is usually very easy to avoid and is typically the result
//! of an improper assembly of models. Because requestor ports are only used
//! sparingly in idiomatic simulations, this situation should be relatively
//! exceptional.
//!
//! The second scenario is rare in well-behaving models and if it occurs, it is
//! most typically at the very beginning of a simulation when all models
//! simultaneously send events during the call to
//! [`Model::init()`](crate::model::Model::init). If such a large amount of
//! concurrent messages is deemed normal behavior, the issue can be readily
//! remedied by increasing the capacity of the saturated mailboxes.
//!
//! At the moment, Asynchronix is unfortunately not able to discriminate between
//! such pathological deadlocks and the "expected" deadlock that occurs when all
//! events in a given time slice have completed and all models are starved on an
//! empty mailbox. Consequently, blocking method such as [`SimInit::init()`],
//! [`Simulation::step()`], [`Simulation::send_event()`], etc., will return
//! without error after a pathological deadlock, leaving the user responsible
//! for inferring the deadlock from the behavior of the simulation in the next
//! steps. This is obviously not ideal, but is hopefully only a temporary state
//! of things until a more precise deadlock detection algorithm is implemented.
//!
//! ## Modifying connections during simulation
//!
//! Although uncommon, there is sometimes a need for connecting and/or
//! disconnecting models after they have been migrated to the simulation.
//! Likewise, one may want to connect or disconnect an [`EventSlot`] or
//! [`EventStream`] after the simulation has been instantiated.
//!
//! There is actually a very simple solution to this problem: since the
//! [`InputFn`](crate::model::InputFn) trait also matches closures of type
//! `FnOnce(&mut impl Model)`, it is enough to invoke
//! [`Simulation::send_event()`] with a closure that connects or disconnects a
//! port, such as:
//!
//! ```
//! # use asynchronix::model::{Model, Output};
//! # use asynchronix::time::{MonotonicTime, Scheduler};
//! # use asynchronix::simulation::{Mailbox, SimInit};
//! # pub struct ModelA {
//! #     pub output: Output<i32>,
//! # }
//! # impl Model for ModelA {};
//! # pub struct ModelB {}
//! # impl ModelB {
//! #     pub fn input(&mut self, value: i32) {}
//! # }
//! # impl Model for ModelB {};
//! # let modelA_addr = Mailbox::<ModelA>::new().address();
//! # let modelB_addr = Mailbox::<ModelB>::new().address();
//! # let mut simu = SimInit::new().init(MonotonicTime::EPOCH);
//! simu.send_event(
//!     |m: &mut ModelA| {
//!         m.output.connect(ModelB::input, modelB_addr);
//!     },
//!     (),
//!     &modelA_addr
//! );
//! ```
mod endpoints;
mod mailbox;
mod sim_init;

pub use endpoints::{EventSlot, EventStream};
pub use mailbox::{Address, Mailbox};
pub use sim_init::SimInit;

use std::error::Error;
use std::fmt;
use std::future::Future;
use std::sync::{Arc, Mutex, MutexGuard};
use std::time::Duration;

use recycle_box::{coerce_box, RecycleBox};

use crate::executor::Executor;
use crate::model::{InputFn, Model, ReplierFn};
use crate::time::{
    self, Deadline, EventKey, MonotonicTime, ScheduledEvent, SchedulerQueue, SchedulingError,
    TearableAtomicTime,
};
use crate::util::futures::SeqFuture;
use crate::util::slot;
use crate::util::sync_cell::SyncCell;

/// Simulation environment.
///
/// A `Simulation` is created by calling the
/// [`SimInit::init()`](crate::simulation::SimInit::init) method on a simulation
/// initializer. It contains an asynchronous executor that runs all simulation
/// models added beforehand to [`SimInit`](crate::simulation::SimInit).
///
/// A [`Simulation`] object also manages an event scheduling queue and
/// simulation time. The scheduling queue can be accessed from the simulation
/// itself, but also from models via the optional
/// [`&Scheduler`][time::Scheduler] argument of input and replier port methods.
/// Likewise, simulation time can be accessed with the [`Simulation::time()`]
/// method, or from models with the [`Scheduler::time()`](time::Scheduler::time)
/// method.
///
/// Events and queries can be scheduled immediately, *i.e.* for the current
/// simulation time, using [`send_event()`](Simulation::send_event) and
/// [`send_query()`](Simulation::send_query). Calling these methods will block
/// until all computations triggered by such event or query have completed. In
/// the case of queries, the response is returned.
///
/// Events can also be scheduled at a future simulation time using one of the
/// [`schedule_*()`](Simulation::schedule_event) method. These methods queue an
/// event without blocking.
///
/// Finally, the [`Simulation`] instance manages simulation time. Calling
/// [`step()`](Simulation::step) will increment simulation time until that of
/// the next scheduled event in chronological order, whereas
/// [`step_by()`](Simulation::step_by) and
/// [`step_until()`](Simulation::step_until) can increment time by an arbitrary
/// duration, running the computations for all intermediate time slices
/// sequentially. These methods will block until all computations for the
/// relevant time slice(s) have completed.
pub struct Simulation {
    executor: Executor,
    scheduler_queue: Arc<Mutex<SchedulerQueue>>,
    time: SyncCell<TearableAtomicTime>,
}

impl Simulation {
    /// Creates a new `Simulation`.
    pub(crate) fn new(
        executor: Executor,
        scheduler_queue: Arc<Mutex<SchedulerQueue>>,
        time: SyncCell<TearableAtomicTime>,
    ) -> Self {
        Self {
            executor,
            scheduler_queue,
            time,
        }
    }

    /// Returns the current simulation time.
    pub fn time(&self) -> MonotonicTime {
        self.time.read()
    }

    /// Advances simulation time to that of the next scheduled event, processing
    /// that event as well as all other event scheduled for the same time.
    ///
    /// This method may block. Once it returns, it is guaranteed that all newly
    /// processed event (if any) have completed.
    pub fn step(&mut self) {
        self.step_to_next_bounded(MonotonicTime::MAX);
    }

    /// Iteratively advances the simulation time by the specified duration and
    /// processes all events scheduled up to the target time.
    ///
    /// This method may block. Once it returns, it is guaranteed that (i) all
    /// events scheduled up to the specified target time have completed and (ii)
    /// the final simulation time has been incremented by the specified
    /// duration.
    pub fn step_by(&mut self, duration: Duration) {
        let target_time = self.time.read() + duration;

        self.step_until_unchecked(target_time);
    }

    /// Iteratively advances the simulation time and processes all events
    /// scheduled up to the specified target time.
    ///
    /// This method may block. Once it returns, it is guaranteed that (i) all
    /// events scheduled up to the specified target time have completed and (ii)
    /// the final simulation time matches the target time.
    pub fn step_until(&mut self, target_time: MonotonicTime) -> Result<(), SchedulingError> {
        if self.time.read() >= target_time {
            return Err(SchedulingError::InvalidScheduledTime);
        }
        self.step_until_unchecked(target_time);

        Ok(())
    }

    /// Schedules an event at a future time.
    ///
    /// An error is returned if the specified time is not in the future of the
    /// current simulation time.
    ///
    /// Events scheduled for the same time and targeting the same model are
    /// guaranteed to be processed according to the scheduling order.
    ///
    /// See also: [`time::Scheduler::schedule_event`].
    pub fn schedule_event<M, F, T, S>(
        &mut self,
        deadline: impl Deadline,
        func: F,
        arg: T,
        address: impl Into<Address<M>>,
    ) -> Result<(), SchedulingError>
    where
        M: Model,
        F: for<'a> InputFn<'a, M, T, S>,
        T: Send + Clone + 'static,
        S: Send + 'static,
    {
        let now = self.time();
        let time = deadline.into_time(now);
        if now >= time {
            return Err(SchedulingError::InvalidScheduledTime);
        }
        time::schedule_event_at_unchecked(time, func, arg, address.into().0, &self.scheduler_queue);

        Ok(())
    }

    /// Schedules a cancellable event at a future time and returns an event key.
    ///
    /// An error is returned if the specified time is not in the future of the
    /// current simulation time.
    ///
    /// Events scheduled for the same time and targeting the same model are
    /// guaranteed to be processed according to the scheduling order.
    ///
    /// See also: [`time::Scheduler::schedule_keyed_event`].
    pub fn schedule_keyed_event<M, F, T, S>(
        &mut self,
        deadline: impl Deadline,
        func: F,
        arg: T,
        address: impl Into<Address<M>>,
    ) -> Result<EventKey, SchedulingError>
    where
        M: Model,
        F: for<'a> InputFn<'a, M, T, S>,
        T: Send + Clone + 'static,
        S: Send + 'static,
    {
        let now = self.time();
        let time = deadline.into_time(now);
        if now >= time {
            return Err(SchedulingError::InvalidScheduledTime);
        }
        let event_key = time::schedule_keyed_event_at_unchecked(
            time,
            func,
            arg,
            address.into().0,
            &self.scheduler_queue,
        );

        Ok(event_key)
    }

    /// Schedules a periodically recurring event at a future time.
    ///
    /// An error is returned if the specified time is not in the future of the
    /// current simulation time or if the specified period is null.
    ///
    /// Events scheduled for the same time and targeting the same model are
    /// guaranteed to be processed according to the scheduling order.
    ///
    /// See also: [`time::Scheduler::schedule_periodic_event`].
    pub fn schedule_periodic_event<M, F, T, S>(
        &mut self,
        deadline: impl Deadline,
        period: Duration,
        func: F,
        arg: T,
        address: impl Into<Address<M>>,
    ) -> Result<(), SchedulingError>
    where
        M: Model,
        F: for<'a> InputFn<'a, M, T, S> + Clone,
        T: Send + Clone + 'static,
        S: Send + 'static,
    {
        let now = self.time();
        let time = deadline.into_time(now);
        if now >= time {
            return Err(SchedulingError::InvalidScheduledTime);
        }
        if period.is_zero() {
            return Err(SchedulingError::NullRepetitionPeriod);
        }
        time::schedule_periodic_event_at_unchecked(
            time,
            period,
            func,
            arg,
            address.into().0,
            &self.scheduler_queue,
        );

        Ok(())
    }

    /// Schedules a cancellable, periodically recurring event at a future time
    /// and returns an event key.
    ///
    /// An error is returned if the specified time is not in the future of the
    /// current simulation time or if the specified period is null.
    ///
    /// Events scheduled for the same time and targeting the same model are
    /// guaranteed to be processed according to the scheduling order.
    ///
    /// See also: [`time::Scheduler::schedule_keyed_periodic_event`].
    pub fn schedule_keyed_periodic_event<M, F, T, S>(
        &mut self,
        deadline: impl Deadline,
        period: Duration,
        func: F,
        arg: T,
        address: impl Into<Address<M>>,
    ) -> Result<EventKey, SchedulingError>
    where
        M: Model,
        F: for<'a> InputFn<'a, M, T, S> + Clone,
        T: Send + Clone + 'static,
        S: Send + 'static,
    {
        let now = self.time();
        let time = deadline.into_time(now);
        if now >= time {
            return Err(SchedulingError::InvalidScheduledTime);
        }
        if period.is_zero() {
            return Err(SchedulingError::NullRepetitionPeriod);
        }
        let event_key = time::schedule_periodic_keyed_event_at_unchecked(
            time,
            period,
            func,
            arg,
            address.into().0,
            &self.scheduler_queue,
        );

        Ok(event_key)
    }

    /// Sends and processes an event, blocking until completion.
    ///
    /// Simulation time remains unchanged.
    pub fn send_event<M, F, T, S>(&mut self, func: F, arg: T, address: impl Into<Address<M>>)
    where
        M: Model,
        F: for<'a> InputFn<'a, M, T, S>,
        T: Send + Clone + 'static,
    {
        let sender = address.into().0;
        let fut = async move {
            // Ignore send errors.
            let _ = sender
                .send(
                    move |model: &mut M,
                          scheduler,
                          recycle_box: RecycleBox<()>|
                          -> RecycleBox<dyn Future<Output = ()> + Send + '_> {
                        let fut = func.call(model, arg, scheduler);

                        coerce_box!(RecycleBox::recycle(recycle_box, fut))
                    },
                )
                .await;
        };

        self.executor.spawn_and_forget(fut);
        self.executor.run();
    }

    /// Sends and processes a query, blocking until completion.
    ///
    /// Simulation time remains unchanged.
    pub fn send_query<M, F, T, R, S>(
        &mut self,
        func: F,
        arg: T,
        address: impl Into<Address<M>>,
    ) -> Result<R, QueryError>
    where
        M: Model,
        F: for<'a> ReplierFn<'a, M, T, R, S>,
        T: Send + Clone + 'static,
        R: Send + 'static,
    {
        let (reply_writer, mut reply_reader) = slot::slot();
        let sender = address.into().0;

        let fut = async move {
            // Ignore send errors.
            let _ = sender
                .send(
                    move |model: &mut M,
                          scheduler,
                          recycle_box: RecycleBox<()>|
                          -> RecycleBox<dyn Future<Output = ()> + Send + '_> {
                        let fut = async move {
                            let reply = func.call(model, arg, scheduler).await;
                            let _ = reply_writer.write(reply);
                        };

                        coerce_box!(RecycleBox::recycle(recycle_box, fut))
                    },
                )
                .await;
        };

        self.executor.spawn_and_forget(fut);
        self.executor.run();

        reply_reader.try_read().map_err(|_| QueryError {})
    }

    /// Advances simulation time to that of the next scheduled event if its
    /// scheduling time does not exceed the specified bound, processing that
    /// event as well as all other events scheduled for the same time.
    ///
    /// If at least one event was found that satisfied the time bound, the
    /// corresponding new simulation time is returned.
    fn step_to_next_bounded(&mut self, upper_time_bound: MonotonicTime) -> Option<MonotonicTime> {
        // Function pulling the next event. If the event is periodic, it is
        // immediately cloned and re-scheduled.
        fn pull_next_event(
            scheduler_queue: &mut MutexGuard<SchedulerQueue>,
        ) -> Box<dyn ScheduledEvent> {
            let ((time, channel_id), event) = scheduler_queue.pull().unwrap();
            if let Some((event_clone, period)) = event.next() {
                scheduler_queue.insert((time + period, channel_id), event_clone);
            }

            event
        }

        // Closure returning the next key which time stamp is no older than the
        // upper bound, if any. Cancelled events are pulled and discarded.
        let peek_next_key = |scheduler_queue: &mut MutexGuard<SchedulerQueue>| {
            loop {
                match scheduler_queue.peek() {
                    Some((&k, t)) if k.0 <= upper_time_bound => {
                        if !t.is_cancelled() {
                            break Some(k);
                        }
                        // Discard cancelled events.
                        scheduler_queue.pull();
                    }
                    _ => break None,
                }
            }
        };

        // Move to the next scheduled time.
        let mut scheduler_queue = self.scheduler_queue.lock().unwrap();
        let mut current_key = peek_next_key(&mut scheduler_queue)?;
        self.time.write(current_key.0);

        loop {
            let event = pull_next_event(&mut scheduler_queue);
            let mut next_key = peek_next_key(&mut scheduler_queue);
            if next_key != Some(current_key) {
                // Since there are no other events targeting the same mailbox
                // and the same time, the event is spawned immediately.
                event.spawn_and_forget(&self.executor);
            } else {
                // To ensure that their relative order of execution is
                // preserved, all event targeting the same mailbox are executed
                // sequentially within a single compound future.
                let mut event_sequence = SeqFuture::new();
                event_sequence.push(event.into_future());
                loop {
                    let event = pull_next_event(&mut scheduler_queue);
                    event_sequence.push(event.into_future());
                    next_key = peek_next_key(&mut scheduler_queue);
                    if next_key != Some(current_key) {
                        break;
                    }
                }

                // Spawn a compound future that sequentially polls all events
                // targeting the same mailbox.
                self.executor.spawn_and_forget(event_sequence);
            }

            current_key = match next_key {
                // If the next event is scheduled at the same time, update the
                // key and continue.
                Some(k) if k.0 == current_key.0 => k,
                // Otherwise wait until all events have completed and return.
                _ => {
                    drop(scheduler_queue); // make sure the queue's mutex is released.
                    self.executor.run();

                    return Some(current_key.0);
                }
            };
        }
    }

    /// Iteratively advances simulation time and processes all events scheduled
    /// up to the specified target time.
    ///
    /// Once the method returns it is guaranteed that (i) all events scheduled
    /// up to the specified target time have completed and (ii) the final
    /// simulation time matches the target time.
    ///
    /// This method does not check whether the specified time lies in the future
    /// of the current simulation time.
    fn step_until_unchecked(&mut self, target_time: MonotonicTime) {
        loop {
            match self.step_to_next_bounded(target_time) {
                // The target time was reached exactly.
                Some(t) if t == target_time => return,
                // No events are scheduled before or at the target time.
                None => {
                    // Update the simulation time.
                    self.time.write(target_time);
                    return;
                }
                // The target time was not reached yet.
                _ => {}
            }
        }
    }
}

impl fmt::Debug for Simulation {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Simulation")
            .field("time", &self.time.read())
            .finish_non_exhaustive()
    }
}

/// Error returned when a query did not obtain a response.
///
/// This can happen either because the model targeted by the address was not
/// added to the simulation or due to a simulation deadlock.
#[derive(Debug, PartialEq, Eq, Clone, Copy)]
pub struct QueryError {}

impl fmt::Display for QueryError {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(fmt, "the query did not receive a response")
    }
}

impl Error for QueryError {}