Add a DirectedGossip struct (paritytech#6803)

* Add a DirectedGossip struct

* Move protocol from prototype::new to biuld

* More traits impls

* Explain ordering

* Apply suggestions from code review

Co-authored-by: Toralf Wittner <tw@dtex.org>

* Address concerns

* Add basic test

* Concerns

* More concerns

* Remove QueueSenderPrototype

* Rename

* Apply suggestions from code review

Co-authored-by: Max Inden <mail@max-inden.de>

Co-authored-by: Toralf Wittner <tw@dtex.org>
Co-authored-by: parity-processbot <>
Co-authored-by: Max Inden <mail@max-inden.de>

client/network/Cargo.toml

@@ -16,6 +16,7 @@ targets = ["x86_64-unknown-linux-gnu"]
 prost-build = "0.6.1"
 
 [dependencies]
+async-std = { version = "1.6.2", features = ["unstable"] }
 bitflags = "1.2.0"
 bs58 = "0.3.1"
 bytes = "0.5.0"
@@ -66,7 +67,6 @@ default-features = false
 features = ["identify", "kad", "mdns", "mplex", "noise", "ping", "tcp-async-std", "websocket", "yamux"]
 
 [dev-dependencies]
-async-std = "1.6.2"
 assert_matches = "1.3"
 env_logger = "0.7.0"
 libp2p = { version = "0.23.0", default-features = false, features = ["secio"] }

client/network/src/gossip.rs

@@ -0,0 +1,245 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2017-2020 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! Helper for sending rate-limited gossip messages.
+//!
+//! # Context
+//!
+//! The [`NetworkService`] struct provides a way to send notifications to a certain peer through
+//! the [`NetworkService::notification_sender`] method. This method is quite low level and isn't
+//! expected to be used directly.
+//!
+//! The [`QueuedSender`] struct provided by this module is built on top of
+//! [`NetworkService::notification_sender`] and provides a cleaner way to send notifications.
+//!
+//! # Behaviour
+//!
+//! An instance of [`QueuedSender`] is specific to a certain combination of `PeerId` and
+//! protocol name. It maintains a buffer of messages waiting to be sent out. The user of this API
+//! is able to manipulate that queue, adding or removing obsolete messages.
+//!
+//! Creating a [`QueuedSender`] also returns a opaque `Future` whose responsibility it to
+//! drain that queue and actually send the messages. If the substream with the given combination
+//! of peer and protocol is closed, the queue is silently discarded. It is the role of the user
+//! to track which peers we are connected to.
+//!
+//! In normal situations, messages sent through a [`QueuedSender`] will arrive in the same
+//! order as they have been sent.
+//! It is possible, in the situation of disconnects and reconnects, that messages arrive in a
+//! different order. See also https://github.com/paritytech/substrate/issues/6756.
+//! However, if multiple instances of [`QueuedSender`] exist for the same peer and protocol, or
+//! if some other code uses the [`NetworkService`] to send notifications to this combination or
+//! peer and protocol, then the notifications will be interleaved in an unpredictable way.
+//!
+
+use crate::{ExHashT, NetworkService};
+
+use async_std::sync::{Condvar, Mutex, MutexGuard};
+use futures::prelude::*;
+use libp2p::PeerId;
+use sp_runtime::{traits::Block as BlockT, ConsensusEngineId};
+use std::{
+	collections::VecDeque,
+	fmt,
+	sync::{atomic, Arc},
+	time::Duration,
+};
+
+#[cfg(test)]
+mod tests;
+
+/// Notifications sender for a specific combination of network service, peer, and protocol.
+pub struct QueuedSender<M> {
+	/// Shared between the front and the back task.
+	shared: Arc<Shared<M>>,
+}
+
+impl<M> QueuedSender<M> {
+	/// Returns a new [`QueuedSender`] containing a queue of message for this specific
+	/// combination of peer and protocol.
+	///
+	/// In addition to the [`QueuedSender`], also returns a `Future` whose role is to drive
+	/// the messages sending forward.
+	pub fn new<B, H, F>(
+		service: Arc<NetworkService<B, H>>,
+		peer_id: PeerId,
+		protocol: ConsensusEngineId,
+		queue_size_limit: usize,
+		messages_encode: F
+	) -> (Self, impl Future<Output = ()> + Send + 'static)
+	where
+		M: Send + 'static,
+		B: BlockT + 'static,
+		H: ExHashT,
+		F: Fn(M) -> Vec<u8> + Send + 'static,
+	{
+		let shared = Arc::new(Shared {
+			stop_task: atomic::AtomicBool::new(false),
+			condvar: Condvar::new(),
+			queue_size_limit,
+			messages_queue: Mutex::new(VecDeque::with_capacity(queue_size_limit)),
+		});
+
+		let task = spawn_task(
+			service,
+			peer_id,
+			protocol,
+			shared.clone(),
+			messages_encode
+		);
+
+		(QueuedSender { shared }, task)
+	}
+
+	/// Locks the queue of messages towards this peer.
+	///
+	/// The returned `Future` is expected to be ready quite quickly.
+	pub async fn lock_queue<'a>(&'a self) -> QueueGuard<'a, M> {
+		QueueGuard {
+			messages_queue: self.shared.messages_queue.lock().await,
+			condvar: &self.shared.condvar,
+			queue_size_limit: self.shared.queue_size_limit,
+		}
+	}
+
+	/// Pushes a message to the queue, or discards it if the queue is full.
+	///
+	/// The returned `Future` is expected to be ready quite quickly.
+	pub async fn queue_or_discard(&self, message: M)
+	where
+		M: Send + 'static
+	{
+		self.lock_queue().await.push_or_discard(message);
+	}
+}
+
+impl<M> fmt::Debug for QueuedSender<M> {
+	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+		f.debug_struct("QueuedSender").finish()
+	}
+}
+
+impl<M> Drop for QueuedSender<M> {
+	fn drop(&mut self) {
+		// The "clean" way to notify the `Condvar` here is normally to first lock the `Mutex`,
+		// then notify the `Condvar` while the `Mutex` is locked. Unfortunately, the `Mutex`
+		// being asynchronous, it can't reasonably be locked from within a destructor.
+		// See also the corresponding code in the background task.
+		self.shared.stop_task.store(true, atomic::Ordering::Release);
+		self.shared.condvar.notify_all();
+	}
+}
+
+/// Locked queue of messages to the given peer.
+///
+/// As long as this struct exists, the background task is asleep and the owner of the [`QueueGuard`]
+/// is in total control of the buffer. Messages can only ever be sent out after the [`QueueGuard`]
+/// is dropped.
+#[must_use]
+pub struct QueueGuard<'a, M> {
+	messages_queue: MutexGuard<'a, VecDeque<M>>,
+	condvar: &'a Condvar,
+	/// Same as [`Shared::queue_size_limit`].
+	queue_size_limit: usize,
+}
+
+impl<'a, M: Send + 'static> QueueGuard<'a, M> {
+	/// Pushes a message to the queue, or discards it if the queue is full.
+	///
+	/// The message will only start being sent out after the [`QueueGuard`] is dropped.
+	pub fn push_or_discard(&mut self, message: M) {
+		if self.messages_queue.len() < self.queue_size_limit {
+			self.messages_queue.push_back(message);
+		}
+	}
+
+	/// Calls `filter` for each message in the queue, and removes the ones for which `false` is
+	/// returned.
+	///
+	/// > **Note**: The parameter of `filter` is a `&M` and not a `&mut M` (which would be
+	/// >           better) because the underlying implementation relies on `VecDeque::retain`.
+	pub fn retain(&mut self, filter: impl FnMut(&M) -> bool) {
+		self.messages_queue.retain(filter);
+	}
+}
+
+impl<'a, M> Drop for QueueGuard<'a, M> {
+	fn drop(&mut self) {
+		// We notify the `Condvar` in the destructor in order to be able to push multiple
+		// messages and wake up the background task only once afterwards.
+		self.condvar.notify_one();
+	}
+}
+
+#[derive(Debug)]
+struct Shared<M> {
+	/// Read by the background task after locking `locked`. If true, the task stops.
+	stop_task: atomic::AtomicBool,
+	/// Queue of messages waiting to be sent out.
+	messages_queue: Mutex<VecDeque<M>>,
+	/// Must be notified every time the content of `locked` changes.
+	condvar: Condvar,
+	/// Maximum number of elements in `messages_queue`.
+	queue_size_limit: usize,
+}
+
+async fn spawn_task<B: BlockT, H: ExHashT, M, F: Fn(M) -> Vec<u8>>(
+	service: Arc<NetworkService<B, H>>,
+	peer_id: PeerId,
+	protocol: ConsensusEngineId,
+	shared: Arc<Shared<M>>,
+	messages_encode: F,
+) {
+	loop {
+		let next_message = 'next_msg: loop {
+			let mut queue = shared.messages_queue.lock().await;
+
+			loop {
+				if shared.stop_task.load(atomic::Ordering::Acquire) {
+					return;
+				}
+
+				if let Some(msg) = queue.pop_front() {
+					break 'next_msg msg;
+				}
+
+				// It is possible that the destructor of `QueuedSender` sets `stop_task` to
+				// true and notifies the `Condvar` after the background task loads `stop_task`
+				// and before it calls `Condvar::wait`.
+				// See also the corresponding comment in `QueuedSender::drop`.
+				// For this reason, we use `wait_timeout`. In the worst case scenario,
+				// `stop_task` will always be checked again after the timeout is reached.
+				queue = shared.condvar.wait_timeout(queue, Duration::from_secs(10)).await.0;
+			}
+		};
+
+		// Starting from below, we try to send the message. If an error happens when sending,
+		// the only sane option we have is to silently discard the message.
+		let sender = match service.notification_sender(peer_id.clone(), protocol) {
+			Ok(s) => s,
+			Err(_) => continue,
+		};
+
+		let ready = match sender.ready().await {
+			Ok(r) => r,
+			Err(_) => continue,
+		};
+
+		let _ = ready.send(messages_encode(next_message));
+	}
+}