Split the service initialisation up into seperate functions (#6332)

* Seperate out the complexity in ServiceBuilder::build_common into seperate functions

* Fix line widths

* Move some functions to their respective crates

substrate/primitives/utils/src/status_sinks.rs

@@ -0,0 +1,139 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate 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.
+
+// Substrate 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 Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+use futures::{Stream, stream::futures_unordered::FuturesUnordered};
+use std::time::Duration;
+use std::pin::Pin;
+use std::task::{Poll, Context};
+use futures_timer::Delay;
+use crate::mpsc::TracingUnboundedSender;
+
+/// Holds a list of `UnboundedSender`s, each associated with a certain time period. Every time the
+/// period elapses, we push an element on the sender.
+///
+/// Senders are removed only when they are closed.
+pub struct StatusSinks<T> {
+	entries: FuturesUnordered<YieldAfter<T>>,
+}
+
+struct YieldAfter<T> {
+	delay: Delay,
+	interval: Duration,
+	sender: Option<TracingUnboundedSender<T>>,
+}
+
+impl<T> StatusSinks<T> {
+	/// Builds a new empty collection.
+	pub fn new() -> StatusSinks<T> {
+		StatusSinks {
+			entries: FuturesUnordered::new(),
+		}
+	}
+
+	/// Adds a sender to the collection.
+	///
+	/// The `interval` is the time period between two pushes on the sender.
+	pub fn push(&mut self, interval: Duration, sender: TracingUnboundedSender<T>) {
+		self.entries.push(YieldAfter {
+			delay: Delay::new(interval),
+			interval,
+			sender: Some(sender),
+		})
+	}
+
+	/// Processes all the senders. If any sender is ready, calls the `status_grab` function and
+	/// pushes what it returns to the sender.
+	///
+	/// This function doesn't return anything, but it should be treated as if it implicitly
+	/// returns `Poll::Pending`. In particular, it should be called again when the task
+	/// is waken up.
+	///
+	/// # Panic
+	///
+	/// Panics if not called within the context of a task.
+	pub fn poll(&mut self, cx: &mut Context, mut status_grab: impl FnMut() -> T) {
+		loop {
+			match Pin::new(&mut self.entries).poll_next(cx) {
+				Poll::Ready(Some((sender, interval))) => {
+					let status = status_grab();
+					if sender.unbounded_send(status).is_ok() {
+						self.entries.push(YieldAfter {
+							// Note that since there's a small delay between the moment a task is
+							// waken up and the moment it is polled, the period is actually not
+							// `interval` but `interval + <delay>`. We ignore this problem in
+							// practice.
+							delay: Delay::new(interval),
+							interval,
+							sender: Some(sender),
+						});
+					}
+				}
+				Poll::Ready(None) |
+				Poll::Pending => break,
+			}
+		}
+	}
+}
+
+impl<T> futures::Future for YieldAfter<T> {
+	type Output = (TracingUnboundedSender<T>, Duration);
+
+	fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
+		let this = Pin::into_inner(self);
+
+		match Pin::new(&mut this.delay).poll(cx) {
+			Poll::Pending => Poll::Pending,
+			Poll::Ready(()) => {
+				let sender = this.sender.take()
+					.expect("sender is always Some unless the future is finished; qed");
+				Poll::Ready((sender, this.interval))
+			}
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::StatusSinks;
+	use futures::prelude::*;
+	use crate::mpsc::tracing_unbounded;
+	use std::time::Duration;
+	use std::task::Poll;
+
+	#[test]
+	fn works() {
+		// We're not testing that the `StatusSink` properly enforces an order in the intervals, as
+		// this easily causes test failures on busy CPUs.
+
+		let mut status_sinks = StatusSinks::new();
+
+		let (tx, rx) = tracing_unbounded("status_sink_test");
+		status_sinks.push(Duration::from_millis(100), tx);
+
+		let mut val_order = 5;
+
+		futures::executor::block_on(futures::future::select(
+			futures::future::poll_fn(move |cx| {
+				status_sinks.poll(cx, || { val_order += 1; val_order });
+				Poll::<()>::Pending
+			}),
+			Box::pin(async {
+				let items: Vec<i32> = rx.take(3).collect().await;
+				assert_eq!(items, [6, 7, 8]);
+			})
+		));
+	}
+}