loro_internal/utils/

subscription.rs

use either::Either;
/*
This file is modified from the original file in the following repo:
https://github.com/zed-industries/zed


Copyright 2022 - 2024 Zed Industries, Inc.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at


       http://www.apache.org/licenses/LICENSE-2.0


   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.




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*/
use crate::sync::{thread, AtomicBool, Mutex};
use smallvec::SmallVec;
use std::collections::{BTreeMap, BTreeSet};
use std::sync::atomic::Ordering;
use std::sync::{Arc, Weak};
use std::{fmt::Debug, mem};
use thread::ThreadId;

#[derive(Debug)]
pub enum SubscriptionError {
    CannotEmitEventDueToRecursiveCall,
}

impl std::fmt::Display for Subscription {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "SubscriptionError")
    }
}

pub struct SubscriberSet<EmitterKey, Callback>(
    Arc<Mutex<SubscriberSetState<EmitterKey, Callback>>>,
);

impl<EmitterKey, Callback> Clone for SubscriberSet<EmitterKey, Callback> {
    fn clone(&self) -> Self {
        SubscriberSet(self.0.clone())
    }
}

struct SubscriberSetState<EmitterKey, Callback> {
    subscribers: BTreeMap<EmitterKey, Either<BTreeMap<usize, Subscriber<Callback>>, ThreadId>>,
    /// Subscribers added while their emitter was mid-emit.
    ///
    /// `retain` swaps an emitter's map out for a `ThreadId` marker
    /// while it invokes callbacks, so during that window there is no
    /// map to insert into. The marker cannot simply be replaced: it is
    /// what makes a concurrent `retain` wait instead of emitting the
    /// same emitter twice, and what lets `is_recursive_calling` spot a
    /// re-entrant emit. Parking the new subscriber here keeps the
    /// marker intact, and `retain` folds these back in when it
    /// restores the map.
    pending_subscribers: BTreeMap<EmitterKey, BTreeMap<usize, Subscriber<Callback>>>,
    dropped_subscribers: BTreeSet<(EmitterKey, usize)>,
    next_subscriber_id: usize,
}

struct Subscriber<Callback> {
    active: Arc<AtomicBool>,
    callback: Callback,
    /// This field is used to drop the subscription when the subscriber is dropped.
    _sub: InnerSubscription,
}

impl<EmitterKey, Callback> SubscriberSet<EmitterKey, Callback>
where
    EmitterKey: 'static + Ord + Clone + Debug + Send + Sync,
    Callback: 'static + Send + Sync,
{
    pub fn new() -> Self {
        Self(Arc::new(Mutex::new(SubscriberSetState {
            subscribers: Default::default(),
            pending_subscribers: Default::default(),
            dropped_subscribers: Default::default(),
            next_subscriber_id: 0,
        })))
    }

    /// Inserts a new [`Subscription`] for the given `emitter_key`.
    ///
    /// By default, subscriptions are inert, meaning that they won't be listed when calling
    /// `[SubscriberSet::remove]` or `[SubscriberSet::retain]`. This method returns a tuple
    /// of a [`Subscription`] and an `impl FnOnce`, and you can use the latter to activate
    /// the [`Subscription`].
    pub fn insert(
        &self,
        emitter_key: EmitterKey,
        callback: Callback,
    ) -> (Subscription, impl FnOnce()) {
        let active = Arc::new(AtomicBool::new(false));
        let mut lock = self.0.lock();
        let subscriber_id = post_inc(&mut lock.next_subscriber_id);
        let this = Arc::downgrade(&self.0);
        let emitter_key_1 = emitter_key.clone();
        let inner_sub = InnerSubscription {
            unsubscribe: Arc::new(Mutex::new(Some(Box::new(move || {
                let Some(this) = this.upgrade() else {
                    return;
                };

                let mut lock = this.lock();
                let Some(subscribers) = lock.subscribers.get_mut(&emitter_key) else {
                    // remove was called with this emitter_key
                    return;
                };

                if let Either::Left(subscribers) = subscribers {
                    subscribers.remove(&subscriber_id);
                    if subscribers.is_empty() {
                        lock.subscribers.remove(&emitter_key);
                    }
                    return;
                }

                // We didn't manage to remove the subscription, which means it was dropped
                // while invoking the callback. Mark it as dropped so that we can remove it
                // later.
                lock.dropped_subscribers
                    .insert((emitter_key, subscriber_id));
            })))),
        };
        let subscription = Subscription {
            unsubscribe: Arc::downgrade(&inner_sub.unsubscribe),
        };

        let subscriber = Subscriber {
            active: active.clone(),
            callback,
            _sub: inner_sub,
        };
        // Subscribing to an emitter that is mid-emit is legal and
        // happens whenever one thread adds a listener while another is
        // delivering events for the same key (or a callback subscribes
        // re-entrantly). The map is checked out by `retain` in that
        // window, so park the subscriber instead of unwrapping a
        // `Left` that is not there.
        if matches!(lock.subscribers.get(&emitter_key_1), Some(Either::Right(_))) {
            lock.pending_subscribers
                .entry(emitter_key_1)
                .or_default()
                .insert(subscriber_id, subscriber);
        } else {
            lock.subscribers
                .entry(emitter_key_1)
                .or_insert_with(|| Either::Left(BTreeMap::new()))
                .as_mut()
                .unwrap_left()
                .insert(subscriber_id, subscriber);
        }
        (subscription, move || active.store(true, Ordering::Relaxed))
    }

    #[allow(unused)]
    pub fn remove(&self, emitter: &EmitterKey) -> impl IntoIterator<Item = Callback> {
        let mut lock = self.0.lock();
        let subscribers = lock.subscribers.remove(emitter);
        // A subscriber parked mid-emit belongs to the emitter being
        // removed; it never became visible, so it goes with it.
        lock.pending_subscribers.remove(emitter);
        subscribers
            .and_then(|x| x.left().map(|s| s.into_values()))
            .into_iter()
            .flatten()
            .filter_map(|subscriber| {
                if subscriber.active.load(Ordering::Relaxed) {
                    Some(subscriber.callback)
                } else {
                    None
                }
            })
    }

    pub fn is_recursive_calling(&self, emitter: &EmitterKey) -> bool {
        if let Some(Either::Right(thread_id)) = self.0.lock().subscribers.get(emitter) {
            *thread_id == thread::current().id()
        } else {
            false
        }
    }

    /// Call the given callback for each subscriber to the given emitter.
    /// If the callback returns false, the subscriber is removed.
    pub fn retain(
        &self,
        emitter: &EmitterKey,
        f: &mut dyn FnMut(&mut Callback) -> bool,
    ) -> Result<(), SubscriptionError> {
        let mut subscribers = {
            let inner = loop {
                let mut subscriber_set_state = self.0.lock();
                let Some(set) = subscriber_set_state.subscribers.get_mut(emitter) else {
                    return Ok(());
                };
                match set {
                    Either::Left(_) => {
                        break std::mem::replace(set, Either::Right(thread::current().id()))
                            .unwrap_left();
                    }
                    Either::Right(lock_thread) => {
                        if thread::current().id() == *lock_thread {
                            return Err(SubscriptionError::CannotEmitEventDueToRecursiveCall);
                        } else {
                            // return Ok(());
                            drop(subscriber_set_state);
                            #[cfg(loom)]
                            loom::thread::yield_now();
                            #[cfg(not(loom))]
                            std::thread::sleep(std::time::Duration::from_millis(10));
                        }
                    }
                }
            };
            inner
        };

        subscribers.retain(|_, subscriber| {
            if subscriber.active.load(Ordering::Relaxed) {
                f(&mut subscriber.callback)
            } else {
                true
            }
        });

        let mut lock = self.0.lock();

        // Add any new subscribers that were added while invoking the callback.
        if let Some(Either::Left(new_subscribers)) = lock.subscribers.remove(emitter) {
            subscribers.extend(new_subscribers);
        }
        // …including those parked because this emitter was checked
        // out. Folded in BEFORE the dropped sweep below, so one that
        // was unsubscribed again mid-emit is still dropped.
        if let Some(parked) = lock.pending_subscribers.remove(emitter) {
            subscribers.extend(parked);
        }

        // Remove any dropped subscriptions that were dropped while invoking the callback.
        for (dropped_emitter, dropped_subscription_id) in mem::take(&mut lock.dropped_subscribers) {
            if *emitter == dropped_emitter {
                subscribers.remove(&dropped_subscription_id);
            } else {
                lock.dropped_subscribers
                    .insert((dropped_emitter, dropped_subscription_id));
            }
        }

        lock.subscribers
            .insert(emitter.clone(), Either::Left(subscribers));
        Ok(())
    }

    pub fn is_empty(&self) -> bool {
        self.0.lock().subscribers.is_empty()
    }

    pub fn may_include(&self, emitter: &EmitterKey) -> bool {
        self.0.lock().subscribers.contains_key(emitter)
    }
}

impl<EmitterKey, Callback> Default for SubscriberSet<EmitterKey, Callback>
where
    EmitterKey: 'static + Ord + Clone + Debug + Send + Sync,
    Callback: 'static + Send + Sync,
{
    fn default() -> Self {
        Self::new()
    }
}

impl<EmitterKey, Callback> std::fmt::Debug for SubscriberSet<EmitterKey, Callback>
where
    EmitterKey: 'static + Ord + Clone + Debug + Send + Sync,
    Callback: 'static + Send + Sync,
{
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let lock = self.0.lock();
        f.debug_struct("SubscriberSet")
            .field("subscriber_count", &lock.subscribers.len())
            .field("dropped_subscribers_count", &lock.dropped_subscribers.len())
            .field("next_subscriber_id", &lock.next_subscriber_id)
            .finish()
    }
}

fn post_inc(next_subscriber_id: &mut usize) -> usize {
    let ans = *next_subscriber_id;
    *next_subscriber_id += 1;
    ans
}
type Callback = Box<dyn FnOnce() + 'static + Send + Sync>;

/// A handle to a subscription created by GPUI. When dropped, the subscription
/// is cancelled and the callback will no longer be invoked.
#[must_use]
pub struct Subscription {
    unsubscribe: Weak<Mutex<Option<Callback>>>,
}

impl std::fmt::Debug for Subscription {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Subscription").finish()
    }
}

impl Subscription {
    /// Detaches the subscription from this handle. The callback will
    /// continue to be invoked until the doc has been subscribed to
    /// are dropped
    pub fn detach(self) {
        if let Some(unsubscribe) = self.unsubscribe.upgrade() {
            unsubscribe.lock().take();
        }
    }

    /// Unsubscribes the subscription.
    #[inline]
    pub fn unsubscribe(self) {
        drop(self)
    }
}

impl Drop for Subscription {
    fn drop(&mut self) {
        if let Some(unsubscribe) = self.unsubscribe.upgrade() {
            let unsubscribe = unsubscribe.lock().take();
            if let Some(unsubscribe) = unsubscribe {
                unsubscribe();
            }
        }
    }
}

struct InnerSubscription {
    unsubscribe: Arc<Mutex<Option<Callback>>>,
}

impl Drop for InnerSubscription {
    fn drop(&mut self) {
        self.unsubscribe.lock().take();
    }
}

/// A wrapper around `SubscriberSet` that automatically handles recursive event emission.
///
/// This struct differs from `SubscriberSet` in the following ways:
/// 1. It automatically handles the `CannotEmitEventDueToRecursiveCall` error that can occur in `SubscriberSet`.
/// 2. When a recursive event emission is detected, it queues the event instead of throwing an error.
/// 3. After the current event processing is complete, it automatically processes the queued events.
///
/// This behavior ensures that all events are processed in the order they were emitted, even in cases
/// where recursive event emission would normally cause an error.
#[derive(Clone)]
pub struct SubscriberSetWithQueue<EmitterKey, Callback, Payload> {
    subscriber_set: SubscriberSet<EmitterKey, Callback>,
    queue: Arc<Mutex<BTreeMap<EmitterKey, Vec<Payload>>>>,
}

impl<EmitterKey, Callback, Payload> Debug
    for SubscriberSetWithQueue<EmitterKey, Callback, Payload>
{
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("SubscriberSetWithQueue").finish()
    }
}

impl<EmitterKey, Callback, Payload> Default
    for SubscriberSetWithQueue<EmitterKey, Callback, Payload>
where
    EmitterKey: 'static + Ord + Clone + Debug + Send + Sync,
    Callback: 'static + Send + Sync + for<'a> FnMut(&'a Payload) -> bool,
    Payload: Send + Sync + Debug,
{
    fn default() -> Self {
        Self::new()
    }
}

pub struct WeakSubscriberSetWithQueue<EmitterKey, Callback, Payload> {
    subscriber_set: Weak<Mutex<SubscriberSetState<EmitterKey, Callback>>>,
    queue: Weak<Mutex<BTreeMap<EmitterKey, Vec<Payload>>>>,
}

impl<EmitterKey, Callback, Payload> Debug
    for WeakSubscriberSetWithQueue<EmitterKey, Callback, Payload>
{
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("WeakSubscriberSetWithQueue").finish()
    }
}

impl<EmitterKey, Callback, Payload> Clone
    for WeakSubscriberSetWithQueue<EmitterKey, Callback, Payload>
{
    fn clone(&self) -> Self {
        Self {
            subscriber_set: self.subscriber_set.clone(),
            queue: self.queue.clone(),
        }
    }
}

impl<EmitterKey, Callback, Payload> WeakSubscriberSetWithQueue<EmitterKey, Callback, Payload> {
    pub fn upgrade(self) -> Option<SubscriberSetWithQueue<EmitterKey, Callback, Payload>> {
        Some(SubscriberSetWithQueue {
            subscriber_set: SubscriberSet(self.subscriber_set.upgrade()?),
            queue: self.queue.upgrade()?,
        })
    }
}

impl<EmitterKey, Callback, Payload> SubscriberSetWithQueue<EmitterKey, Callback, Payload>
where
    EmitterKey: 'static + Ord + Clone + Debug + Send + Sync,
    Callback: 'static + Send + Sync + for<'a> FnMut(&'a Payload) -> bool,
    Payload: Send + Sync + Debug,
{
    pub fn new() -> Self {
        Self {
            subscriber_set: SubscriberSet::new(),
            queue: Arc::new(Mutex::new(Default::default())),
        }
    }

    pub fn downgrade(&self) -> WeakSubscriberSetWithQueue<EmitterKey, Callback, Payload> {
        WeakSubscriberSetWithQueue {
            subscriber_set: Arc::downgrade(&self.subscriber_set.0),
            queue: Arc::downgrade(&self.queue),
        }
    }

    pub fn inner(&self) -> &SubscriberSet<EmitterKey, Callback> {
        &self.subscriber_set
    }

    pub fn emit(&self, key: &EmitterKey, payload: Payload) {
        let mut pending_events: SmallVec<[Payload; 1]> = SmallVec::new();
        pending_events.push(payload);
        while let Some(payload) = pending_events.pop() {
            let result = self
                .subscriber_set
                .retain(key, &mut |callback| (callback)(&payload));
            match result {
                Ok(_) => {
                    let mut queue = self.queue.lock();
                    if let Some(new_pending_events) = queue.remove(key) {
                        pending_events.extend(new_pending_events);
                    }
                }
                Err(SubscriptionError::CannotEmitEventDueToRecursiveCall) => {
                    let mut queue = self.queue.lock();
                    queue.entry(key.clone()).or_default().push(payload);
                }
            }
        }
    }
}

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

    #[test]
    fn test_inner_subscription_drop() {
        let subscriber_set = SubscriberSet::<i32, Box<dyn Fn(&i32) -> bool + Send + Sync>>::new();
        let (subscription, activate) = subscriber_set.insert(1, Box::new(move |_: &i32| true));
        activate();
        drop(subscriber_set);
        assert!(subscription.unsubscribe.upgrade().is_none());
    }

    /// Subscribing while the same emitter is mid-emit must not panic.
    /// `retain` checks the emitter's map out and leaves a `ThreadId`
    /// marker in its place, so an `insert` landing in that window used
    /// to `unwrap_left()` the marker and take the process down with
    /// `called Either::unwrap_left() on a Right value`. The new
    /// subscriber is parked and folded in when `retain` restores the
    /// map — so it is registered, but does not fire for the event
    /// already being delivered.
    #[test]
    fn insert_during_emit_does_not_panic() {
        let set = SubscriberSet::<i32, Box<dyn Fn(&i32) -> bool + Send + Sync>>::new();
        let late_fired = Arc::new(AtomicBool::new(false));

        // The first subscriber subscribes AGAIN from inside its own
        // callback: the emitter is checked out at that moment, which
        // is exactly the state that used to panic.
        let set_inner = set.clone();
        let flag = late_fired.clone();
        let (_sub, activate) = set.insert(
            1,
            Box::new(move |_: &i32| {
                let flag = flag.clone();
                let (sub, activate) = set_inner.insert(
                    1,
                    Box::new(move |_: &i32| {
                        flag.store(true, Ordering::Relaxed);
                        true
                    }),
                );
                activate();
                std::mem::forget(sub);
                true
            }),
        );
        activate();

        set.retain(&1, &mut |callback| callback(&1)).unwrap();
        assert!(
            !late_fired.load(Ordering::Relaxed),
            "a subscriber added mid-emit must not receive the event being delivered"
        );

        // The parked subscriber was folded back in, so the NEXT emit
        // reaches it.
        set.retain(&1, &mut |callback| callback(&1)).unwrap();
        assert!(
            late_fired.load(Ordering::Relaxed),
            "a subscriber added mid-emit must be registered for later events"
        );
    }

    /// The cross-thread flavour: one thread emits while another
    /// subscribes to the same emitter.
    #[test]
    fn concurrent_insert_and_emit_do_not_panic() {
        use std::sync::{atomic::AtomicUsize, mpsc};
        let set = SubscriberSet::<i32, Box<dyn Fn(&i32) -> bool + Send + Sync>>::new();

        // The first emit parks inside the callback until released, which
        // keeps the emitter checked out for as long as the test needs.
        let (entered_tx, entered_rx) = mpsc::channel::<()>();
        let (release_tx, release_rx) = mpsc::channel::<()>();
        let entered_tx = std::sync::Mutex::new(Some(entered_tx));
        let release_rx = std::sync::Mutex::new(release_rx);
        let (_sub, activate) = set.insert(
            1,
            Box::new(move |_: &i32| {
                if let Some(tx) = entered_tx.lock().unwrap().take() {
                    tx.send(()).unwrap();
                    release_rx.lock().unwrap().recv().unwrap();
                }
                true
            }),
        );
        activate();

        let emitter = set.clone();
        let t = std::thread::spawn(move || {
            emitter.retain(&1, &mut |callback| callback(&1)).unwrap();
        });

        // Only subscribe once the emitting thread is inside the callback,
        // so this insert is guaranteed to land while the emitter is
        // checked out.
        entered_rx.recv().unwrap();
        let late_calls = Arc::new(AtomicUsize::new(0));
        let late_calls_clone = late_calls.clone();
        let (sub, activate) = set.insert(
            1,
            Box::new(move |_: &i32| {
                late_calls_clone.fetch_add(1, Ordering::SeqCst);
                true
            }),
        );
        activate();
        release_tx.send(()).unwrap();
        t.join().unwrap();
        assert_eq!(late_calls.load(Ordering::SeqCst), 0);

        set.retain(&1, &mut |callback| callback(&1)).unwrap();
        assert_eq!(late_calls.load(Ordering::SeqCst), 1);
        drop(sub);
    }

    #[test]
    fn test_inner_subscription_drop_2() {
        let subscriber_set = SubscriberSet::<i32, Box<dyn Fn(&i32) -> bool + Send + Sync>>::new();
        let (subscription, activate) = subscriber_set.insert(1, Box::new(move |_: &i32| false));
        activate();
        subscriber_set
            .retain(&1, &mut |callback| callback(&1))
            .unwrap();
        assert!(subscription.unsubscribe.upgrade().is_none());
    }
}