Improve async channels error handling and replace unbounded channels with bounded channels

Remove all unbounded channels to prevent unbounded memory usage and
potential crashes.

Use `FuturesUnordered` for sending to multiple channels simultaneously.
This prevents the sending loop from blocking if one channel is blocked,
and helps handle errors properly.

5 files changed, 104 insertions, 39 deletions

diff --git a/core/Cargo.toml b/core/Cargo.toml
index d30b956..895ddf6 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -10,9 +10,9 @@ authors.workspace = true
 
 [features]
 default = ["smol"]
-crypto = ["dep:ed25519-dalek"]
+crypto = ["ed25519-dalek"]
 tokio = ["dep:tokio"]
-smol = ["dep:smol", "dep:async-process"]
+smol = ["dep:smol", "async-process"]
 
 [dependencies]
 log = "0.4.21"
@@ -29,6 +29,9 @@ pin-project-lite = "0.2.14"
 async-process = { version = "2.2.3", optional = true }
 smol = { version = "2.0.0", optional = true }
 tokio = { version = "1.38.0", features = ["full"], optional = true }
+futures-util = { version = "0.3.5", features = [
+  "alloc",
+], default-features = false }
 
 # encode
 bincode = "2.0.0-rc.3"
diff --git a/core/src/async_runtime/executor.rs b/core/src/async_runtime/executor.rs
index 9335f12..88f6370 100644
--- a/core/src/async_runtime/executor.rs
+++ b/core/src/async_runtime/executor.rs
@@ -25,6 +25,11 @@ impl Executor {
     ) -> Task<T> {
         self.inner.spawn(future).into()
     }
+
+    #[cfg(feature = "tokio")]
+    pub fn handle(&self) -> &tokio::runtime::Handle {
+        return self.inner.handle();
+    }
 }
 
 static GLOBAL_EXECUTOR: OnceCell<Executor> = OnceCell::new();
diff --git a/core/src/async_util/task_group.rs b/core/src/async_util/task_group.rs
index 63c1541..c55b9a1 100644
--- a/core/src/async_util/task_group.rs
+++ b/core/src/async_util/task_group.rs
@@ -87,6 +87,7 @@ impl TaskGroup {
         self.stop_signal.broadcast().await;
 
         loop {
+            // XXX BE CAREFUL HERE, it hold synchronous mutex across .await point.
             let task = self.tasks.lock().pop();
             if let Some(t) = task {
                 t.cancel().await
diff --git a/core/src/event.rs b/core/src/event.rs
index 771d661..1632df3 100644
--- a/core/src/event.rs
+++ b/core/src/event.rs
@@ -7,17 +7,19 @@ use std::{
 
 use async_channel::{Receiver, Sender};
 use chrono::{DateTime, Utc};
+use futures_util::stream::{FuturesUnordered, StreamExt};
 use log::{debug, error};
 
-use crate::{async_runtime::lock::Mutex, util::random_16, Result};
+use crate::{async_runtime::lock::Mutex, util::random_32, Result};
+
+const CHANNEL_BUFFER_SIZE: usize = 1000;
 
 pub type ArcEventSys<T> = Arc<EventSys<T>>;
-pub type WeakEventSys<T> = Weak<EventSys<T>>;
-pub type EventListenerID = u16;
+pub type EventListenerID = u32;
 
 type Listeners<T> = HashMap<T, HashMap<String, HashMap<EventListenerID, Sender<Event>>>>;
 
-/// EventSys supports event emission to registered listeners based on topics.
+/// EventSys emits events to registered listeners based on topics.
 /// # Example
 ///
 /// ```
@@ -74,22 +76,41 @@ type Listeners<T> = HashMap<T, HashMap<String, HashMap<EventListenerID, Sender<E
 ///
 pub struct EventSys<T> {
     listeners: Mutex<Listeners<T>>,
+    listener_buffer_size: usize,
 }
 
 impl<T> EventSys<T>
 where
     T: std::hash::Hash + Eq + std::fmt::Debug + Clone,
 {
-    /// Creates a new `EventSys`
+    /// Creates a new [`EventSys`]
     pub fn new() -> ArcEventSys<T> {
         Arc::new(Self {
             listeners: Mutex::new(HashMap::new()),
+            listener_buffer_size: CHANNEL_BUFFER_SIZE,
+        })
+    }
+
+    /// Creates a new [`EventSys`] with the provided buffer size for the
+    /// [`EventListener`] channel.
+    ///
+    /// This is important to control the memory used by the listener channel.
+    /// If the consumer for the event listener can't keep up with the new events
+    /// coming, then the channel buffer will fill with new events, and if the
+    /// buffer is full, the emit function will block until the listener
+    /// starts to consume the buffered events.
+    ///
+    /// If `size` is zero, this function will panic.
+    pub fn with_buffer_size(size: usize) -> ArcEventSys<T> {
+        Arc::new(Self {
+            listeners: Mutex::new(HashMap::new()),
+            listener_buffer_size: size,
         })
     }
 
     /// Emits an event to the listeners.
     ///
-    /// The event must implement the `EventValueTopic` trait to indicate the
+    /// The event must implement the [`EventValueTopic`] trait to indicate the
     /// topic of the event. Otherwise, you can use `emit_by_topic()`.
     pub async fn emit<E: EventValueTopic<Topic = T> + Clone>(&self, value: &E) {
         let topic = E::topic();
@@ -115,22 +136,26 @@ where
         let event_id = E::id().to_string();
 
         if !event_ids.contains_key(&event_id) {
-            debug!(
-                "Failed to emit an event to a non-existent event id: {:?}",
-                event_id
-            );
+            debug!("Failed to emit an event: unknown event id {:?}", event_id);
             return;
         }
 
-        let mut failed_listeners = vec![];
+        let mut results = FuturesUnordered::new();
 
         let listeners = event_ids.get_mut(&event_id).unwrap();
         for (listener_id, listener) in listeners.iter() {
-            if let Err(err) = listener.send(event.clone()).await {
+            let result = async { (*listener_id, listener.send(event.clone()).await) };
+            results.push(result);
+        }
+
+        let mut failed_listeners = vec![];
+        while let Some((id, fut_err)) = results.next().await {
+            if let Err(err) = fut_err {
                 debug!("Failed to emit event for topic {:?}: {}", topic, err);
-                failed_listeners.push(*listener_id);
+                failed_listeners.push(id);
             }
         }
+        drop(results);
 
         for listener_id in failed_listeners.iter() {
             listeners.remove(listener_id);
@@ -142,7 +167,7 @@ where
         self: &Arc<Self>,
         topic: &T,
     ) -> EventListener<T, E> {
-        let chan = async_channel::unbounded();
+        let chan = async_channel::bounded(self.listener_buffer_size);
 
         let topics = &mut self.listeners.lock().await;
 
@@ -159,9 +184,10 @@ where
 
         let listeners = event_ids.get_mut(&event_id).unwrap();
 
-        let mut listener_id = random_16();
+        let mut listener_id = random_32();
+        // Generate a new one if listener_id already exists
         while listeners.contains_key(&listener_id) {
-            listener_id = random_16();
+            listener_id = random_32();
         }
 
         let listener =
@@ -197,7 +223,7 @@ where
 pub struct EventListener<T, E> {
     id: EventListenerID,
     recv_chan: Receiver<Event>,
-    event_sys: WeakEventSys<T>,
+    event_sys: Weak<EventSys<T>>,
     event_id: String,
     topic: T,
     phantom: PhantomData<E>,
@@ -208,10 +234,10 @@ where
     T: std::hash::Hash + Eq + Clone + std::fmt::Debug,
     E: EventValueAny + Clone + EventValue,
 {
-    /// Create a new event listener.
+    /// Creates a new [`EventListener`].
     fn new(
         id: EventListenerID,
-        event_sys: WeakEventSys<T>,
+        event_sys: Weak<EventSys<T>>,
         recv_chan: Receiver<Event>,
         event_id: &str,
         topic: &T,
@@ -226,12 +252,12 @@ where
         }
     }
 
-    /// Receive the next event.
+    /// Receives the next event.
     pub async fn recv(&self) -> Result<E> {
         match self.recv_chan.recv().await {
             Ok(event) => match ((*event.value).value_as_any()).downcast_ref::<E>() {
                 Some(v) => Ok(v.clone()),
-                None => unreachable!("Error when attempting to downcast the event value."),
+                None => unreachable!("Failed to downcast the event value."),
             },
             Err(err) => {
                 error!("Failed to receive new event: {err}");
@@ -241,7 +267,7 @@ where
         }
     }
 
-    /// Cancels the listener and removes it from the `EventSys`.
+    /// Cancels the event listener and removes it from the [`EventSys`].
     pub async fn cancel(&self) {
         if let Some(es) = self.event_sys.upgrade() {
             es.remove(&self.topic, &self.event_id, &self.id).await;
@@ -249,12 +275,12 @@ where
     }
 
     /// Returns the topic for this event listener.
-    pub async fn topic(&self) -> &T {
+    pub fn topic(&self) -> &T {
         &self.topic
     }
 
     /// Returns the event id for this event listener.
-    pub async fn event_id(&self) -> &String {
+    pub fn event_id(&self) -> &String {
         &self.event_id
     }
 }
diff --git a/core/src/pubsub.rs b/core/src/pubsub.rs
index bcc24ef..09b62ea 100644
--- a/core/src/pubsub.rs
+++ b/core/src/pubsub.rs
@@ -1,11 +1,14 @@
 use std::{collections::HashMap, sync::Arc};
 
+use futures_util::stream::{FuturesUnordered, StreamExt};
 use log::error;
 
-use crate::{async_runtime::lock::Mutex, util::random_16, Result};
+use crate::{async_runtime::lock::Mutex, util::random_32, Result};
+
+const CHANNEL_BUFFER_SIZE: usize = 1000;
 
 pub type ArcPublisher<T> = Arc<Publisher<T>>;
-pub type SubscriptionID = u16;
+pub type SubscriptionID = u32;
 
 /// A simple publish-subscribe system.
 // # Example
@@ -28,27 +31,46 @@ pub type SubscriptionID = u16;
 /// ```
 pub struct Publisher<T> {
     subs: Mutex<HashMap<SubscriptionID, async_channel::Sender<T>>>,
+    subscription_buffer_size: usize,
 }
 
 impl<T: Clone> Publisher<T> {
-    /// Creates a new Publisher
+    /// Creates a new [`Publisher`]
     pub fn new() -> ArcPublisher<T> {
         Arc::new(Self {
             subs: Mutex::new(HashMap::new()),
+            subscription_buffer_size: CHANNEL_BUFFER_SIZE,
+        })
+    }
+
+    /// Creates a new [`Publisher`] with the provided buffer size for the
+    /// [`Subscription`] channel.
+    ///
+    /// This is important to control the memory used by the [`Subscription`] channel.
+    /// If the subscriber can't keep up with the new messages coming, then the
+    /// channel buffer will fill with new messages, and if the buffer is full,
+    /// the emit function will block until the subscriber starts to process
+    /// the buffered messages.
+    ///
+    /// If `size` is zero, this function will panic.
+    pub fn with_buffer_size(size: usize) -> ArcPublisher<T> {
+        Arc::new(Self {
+            subs: Mutex::new(HashMap::new()),
+            subscription_buffer_size: size,
         })
     }
 
-    /// Subscribe and return a Subscription
+    /// Subscribes and return a [`Subscription`]
     pub async fn subscribe(self: &Arc<Self>) -> Subscription<T> {
         let mut subs = self.subs.lock().await;
 
-        let chan = async_channel::unbounded();
+        let chan = async_channel::bounded(self.subscription_buffer_size);
 
-        let mut sub_id = random_16();
+        let mut sub_id = random_32();
 
-        // While the SubscriptionID already exists, generate a new one
+        // Generate a new one if sub_id already exists
         while subs.contains_key(&sub_id) {
-            sub_id = random_16();
+            sub_id = random_32();
         }
 
         let sub = Subscription::new(sub_id, self.clone(), chan.1);
@@ -57,22 +79,30 @@ impl<T: Clone> Publisher<T> {
         sub
     }
 
-    /// Unsubscribe from the Publisher
+    /// Unsubscribes from the publisher
     pub async fn unsubscribe(self: &Arc<Self>, id: &SubscriptionID) {
         self.subs.lock().await.remove(id);
     }
 
-    /// Notify all subscribers
+    /// Notifies all subscribers
     pub async fn notify(self: &Arc<Self>, value: &T) {
         let mut subs = self.subs.lock().await;
+
+        let mut results = FuturesUnordered::new();
         let mut closed_subs = vec![];
 
         for (sub_id, sub) in subs.iter() {
-            if let Err(err) = sub.send(value.clone()).await {
-                error!("failed to notify {}: {}", sub_id, err);
-                closed_subs.push(*sub_id);
+            let result = async { (*sub_id, sub.send(value.clone()).await) };
+            results.push(result);
+        }
+
+        while let Some((id, fut_err)) = results.next().await {
+            if let Err(err) = fut_err {
+                error!("failed to notify {}: {}", id, err);
+                closed_subs.push(id);
             }
         }
+        drop(results);
 
         for sub_id in closed_subs.iter() {
             subs.remove(sub_id);