sc_utils/

pubsub.rs

// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// 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.

//! Provides means to implement a typical Pub/Sub mechanism.
//!
//! This module provides a type [`Hub`] which can be used both to subscribe,
//! and to send the broadcast messages.
//!
//! The [`Hub`] type is parametrized by two other types:
//! - `Message` — the type of a message that shall be delivered to the subscribers;
//! - `Registry` — implementation of the subscription/dispatch logic.
//!
//! A Registry is implemented by defining the following traits:
//! - [`Subscribe<K>`];
//! - [`Dispatch<M>`];
//! - [`Unsubscribe`].
//!
//! As a result of subscription `Hub::subscribe` method returns an instance of
//! [`Receiver<Message,Registry>`]. That can be used as a [`Stream`] to receive the messages.
//! Upon drop the [`Receiver<Message, Registry>`] shall unregister itself from the `Hub`.

use std::{
	collections::HashMap,
	pin::Pin,
	sync::{Arc, Weak},
	task::{Context, Poll},
};

use futures::stream::{FusedStream, Stream};
// use parking_lot::Mutex;
use parking_lot::ReentrantMutex;
use std::cell::RefCell;

use crate::{
	id_sequence::SeqID,
	mpsc::{TracingUnboundedReceiver, TracingUnboundedSender},
};

#[cfg(test)]
mod tests;

/// Unsubscribe: unregisters a previously created subscription.
pub trait Unsubscribe {
	/// Remove all registrations of the subscriber with ID `subs_id`.
	fn unsubscribe(&mut self, subs_id: SeqID);
}

/// Subscribe using a key of type `K`
pub trait Subscribe<K> {
	/// Register subscriber with the ID `subs_id` as having interest to the key `K`.
	fn subscribe(&mut self, subs_key: K, subs_id: SeqID);
}

/// Dispatch a message of type `M`.
pub trait Dispatch<M> {
	/// The type of the that shall be sent through the channel as a result of such dispatch.
	type Item;
	/// The type returned by the `dispatch`-method.
	type Ret;

	/// Dispatch the message of type `M`.
	///
	/// The implementation is given an instance of `M` and is supposed to invoke `dispatch` for
	/// each matching subscriber, with an argument of type `Self::Item` matching that subscriber.
	///
	/// Note that this does not have to be of the same type with the item that will be sent through
	/// to the subscribers. The subscribers will receive a message of type `Self::Item`.
	fn dispatch<F>(&mut self, message: M, dispatch: F) -> Self::Ret
	where
		F: FnMut(&SeqID, Self::Item);
}

/// A subscription hub.
///
/// Does the subscription and dispatch.
/// The exact subscription and routing behaviour is to be implemented by the Registry (of type `R`).
/// The Hub under the hood uses the channel defined in `crate::mpsc` module.
#[derive(Debug)]
pub struct Hub<M, R> {
	tracing_key: &'static str,
	shared: Arc<ReentrantMutex<RefCell<Shared<M, R>>>>,
}

/// The receiving side of the subscription.
///
/// The messages are delivered as items of a [`Stream`].
/// Upon drop this receiver unsubscribes itself from the [`Hub<M, R>`].
#[derive(Debug)]
pub struct Receiver<M, R>
where
	R: Unsubscribe,
{
	rx: TracingUnboundedReceiver<M>,

	shared: Weak<ReentrantMutex<RefCell<Shared<M, R>>>>,
	subs_id: SeqID,
}

#[derive(Debug)]
struct Shared<M, R> {
	id_sequence: crate::id_sequence::IDSequence,
	registry: R,
	sinks: HashMap<SeqID, TracingUnboundedSender<M>>,
}

impl<M, R> Hub<M, R>
where
	R: Unsubscribe,
{
	/// Provide access to the registry (for test purposes).
	pub fn map_registry_for_tests<MapF, Ret>(&self, map: MapF) -> Ret
	where
		MapF: FnOnce(&R) -> Ret,
	{
		let shared_locked = self.shared.lock();
		let shared_borrowed = shared_locked.borrow();
		map(&shared_borrowed.registry)
	}
}

impl<M, R> Drop for Receiver<M, R>
where
	R: Unsubscribe,
{
	fn drop(&mut self) {
		if let Some(shared) = self.shared.upgrade() {
			shared.lock().borrow_mut().unsubscribe(self.subs_id);
		}
	}
}

impl<M, R> Hub<M, R> {
	/// Create a new instance of Hub (with default value for the Registry).
	pub fn new(tracing_key: &'static str) -> Self
	where
		R: Default,
	{
		Self::new_with_registry(tracing_key, Default::default())
	}

	/// Create a new instance of Hub over the initialized Registry.
	pub fn new_with_registry(tracing_key: &'static str, registry: R) -> Self {
		let shared =
			Shared { registry, sinks: Default::default(), id_sequence: Default::default() };
		let shared = Arc::new(ReentrantMutex::new(RefCell::new(shared)));
		Self { tracing_key, shared }
	}

	/// Subscribe to this Hub using the `subs_key: K`.
	///
	/// A subscription with a key `K` is possible if the Registry implements `Subscribe<K>`.
	pub fn subscribe<K>(&self, subs_key: K, queue_size_warning: usize) -> Receiver<M, R>
	where
		R: Subscribe<K> + Unsubscribe,
	{
		let shared_locked = self.shared.lock();
		let mut shared_borrowed = shared_locked.borrow_mut();

		let subs_id = shared_borrowed.id_sequence.next_id();

		// The order (registry.subscribe then sinks.insert) is important here:
		// assuming that `Subscribe<K>::subscribe` can panic, it is better to at least
		// have the sink disposed.
		shared_borrowed.registry.subscribe(subs_key, subs_id);

		let (tx, rx) = crate::mpsc::tracing_unbounded(self.tracing_key, queue_size_warning);
		assert!(shared_borrowed.sinks.insert(subs_id, tx).is_none(), "Used IDSequence to create another ID. Should be unique until u64 is overflowed. Should be unique.");

		Receiver { shared: Arc::downgrade(&self.shared), subs_id, rx }
	}

	/// Send the message produced with `Trigger`.
	///
	/// This is possible if the registry implements `Dispatch<Trigger, Item = M>`.
	pub fn send<Trigger>(&self, trigger: Trigger) -> <R as Dispatch<Trigger>>::Ret
	where
		R: Dispatch<Trigger, Item = M>,
	{
		let shared_locked = self.shared.lock();
		let mut shared_borrowed = shared_locked.borrow_mut();
		let (registry, sinks) = shared_borrowed.get_mut();

		registry.dispatch(trigger, |subs_id, item| {
			if let Some(tx) = sinks.get_mut(subs_id) {
				if let Err(send_err) = tx.unbounded_send(item) {
					log::warn!("Sink with SubsID = {} failed to perform unbounded_send: {} ({} as Dispatch<{}, Item = {}>::dispatch(...))", subs_id, send_err, std::any::type_name::<R>(),
					std::any::type_name::<Trigger>(),
					std::any::type_name::<M>());
				}
			} else {
				log::warn!(
					"No Sink for SubsID = {} ({} as Dispatch<{}, Item = {}>::dispatch(...))",
					subs_id,
					std::any::type_name::<R>(),
					std::any::type_name::<Trigger>(),
					std::any::type_name::<M>(),
				);
			}
		})
	}
}

impl<M, R> Shared<M, R> {
	fn get_mut(&mut self) -> (&mut R, &mut HashMap<SeqID, TracingUnboundedSender<M>>) {
		(&mut self.registry, &mut self.sinks)
	}

	fn unsubscribe(&mut self, subs_id: SeqID)
	where
		R: Unsubscribe,
	{
		// The order (sinks.remove then registry.unsubscribe) is important here:
		// assuming that `Unsubscribe::unsubscribe` can panic, it is better to at least
		// have the sink disposed.
		self.sinks.remove(&subs_id);
		self.registry.unsubscribe(subs_id);
	}
}

impl<M, R> Clone for Hub<M, R> {
	fn clone(&self) -> Self {
		Self { tracing_key: self.tracing_key, shared: self.shared.clone() }
	}
}

impl<M, R> Unpin for Receiver<M, R> where R: Unsubscribe {}

impl<M, R> Stream for Receiver<M, R>
where
	R: Unsubscribe,
{
	type Item = M;

	fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
		Pin::new(&mut self.get_mut().rx).poll_next(cx)
	}
}

impl<Ch, R> FusedStream for Receiver<Ch, R>
where
	R: Unsubscribe,
{
	fn is_terminated(&self) -> bool {
		self.rx.is_terminated()
	}
}