Drop tokio/macros dependency in lightning-net-tokio, fix MSRV

The `tokio` `macros` feature depends on `proc-macro2`, which
recently broke its MSRV in a patch version. Such crates aren't
reasonable for us to have as dependencies, so instead we replace
the one trivial use we have of `tokio::select!()` with our own
manual future.

Author: TheBlueMatt

lightning-background-processor/Cargo.toml

@@ -25,7 +25,7 @@ lightning = { version = "0.0.116-rc1", path = "../lightning", default-features =
 lightning-rapid-gossip-sync = { version = "0.0.116-rc1", path = "../lightning-rapid-gossip-sync", default-features = false }
 
 [dev-dependencies]
-tokio = { version = "1.14", features = [ "macros", "rt", "rt-multi-thread", "sync", "time" ] }
+tokio = { version = "1.14", features = [ "rt", "rt-multi-thread", "sync", "time" ] }
 lightning = { version = "0.0.116-rc1", path = "../lightning", features = ["_test_utils"] }
 lightning-invoice = { version = "0.24.0-rc1", path = "../lightning-invoice" }
 lightning-persister = { version = "0.0.116-rc1", path = "../lightning-persister" }

lightning-net-tokio/Cargo.toml

@@ -17,7 +17,7 @@ rustdoc-args = ["--cfg", "docsrs"]
 [dependencies]
 bitcoin = "0.29.0"
 lightning = { version = "0.0.116-rc1", path = "../lightning" }
-tokio = { version = "1.0", features = [ "io-util", "macros", "rt", "sync", "net", "time" ] }
+tokio = { version = "1.0", features = [ "io-util", "rt", "sync", "net", "time" ] }
 
 [dev-dependencies]
 tokio = { version = "1.14", features = [ "io-util", "macros", "rt", "rt-multi-thread", "sync", "net", "time" ] }

lightning-net-tokio/src/lib.rs

@@ -42,16 +42,79 @@ use lightning::ln::peer_handler::APeerManager;
 use lightning::ln::msgs::NetAddress;
 
 use std::ops::Deref;
-use std::task;
+use std::task::{self, Poll};
+use std::future::Future;
 use std::net::SocketAddr;
 use std::net::TcpStream as StdTcpStream;
 use std::sync::{Arc, Mutex};
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::time::Duration;
+use std::pin::Pin;
 use std::hash::Hash;
 
 static ID_COUNTER: AtomicU64 = AtomicU64::new(0);
 
+// We only need to select over multiple futures in one place, and taking on the full `tokio/macros`
+// dependency tree in order to do so (which has broken our MSRV before) is excessive. Instead, we
+// define a trivial two- and three- select macro with the specific types we need and just use that.
+
+pub(crate) enum SelectorOutput {
+	A(Option<()>), B(Option<()>), C(tokio::io::Result<usize>),
+}
+
+pub(crate) struct TwoSelector<
+	A: Future<Output=Option<()>> + Unpin, B: Future<Output=Option<()>> + Unpin
+> {
+	pub a: A,
+	pub b: B,
+}
+
+impl<
+	A: Future<Output=Option<()>> + Unpin, B: Future<Output=Option<()>> + Unpin
+> Future for TwoSelector<A, B> {
+	type Output = SelectorOutput;
+	fn poll(mut self: Pin<&mut Self>, ctx: &mut task::Context<'_>) -> Poll<SelectorOutput> {
+		match Pin::new(&mut self.a).poll(ctx) {
+			Poll::Ready(res) => { return Poll::Ready(SelectorOutput::A(res)); },
+			Poll::Pending => {},
+		}
+		match Pin::new(&mut self.b).poll(ctx) {
+			Poll::Ready(res) => { return Poll::Ready(SelectorOutput::B(res)); },
+			Poll::Pending => {},
+		}
+		Poll::Pending
+	}
+}
+
+pub(crate) struct ThreeSelector<
+	A: Future<Output=Option<()>> + Unpin, B: Future<Output=Option<()>> + Unpin, C: Future<Output=tokio::io::Result<usize>> + Unpin
+> {
+	pub a: A,
+	pub b: B,
+	pub c: C,
+}
+
+impl<
+	A: Future<Output=Option<()>> + Unpin, B: Future<Output=Option<()>> + Unpin, C: Future<Output=tokio::io::Result<usize>> + Unpin
+> Future for ThreeSelector<A, B, C> {
+	type Output = SelectorOutput;
+	fn poll(mut self: Pin<&mut Self>, ctx: &mut task::Context<'_>) -> Poll<SelectorOutput> {
+		match Pin::new(&mut self.a).poll(ctx) {
+			Poll::Ready(res) => { return Poll::Ready(SelectorOutput::A(res)); },
+			Poll::Pending => {},
+		}
+		match Pin::new(&mut self.b).poll(ctx) {
+			Poll::Ready(res) => { return Poll::Ready(SelectorOutput::B(res)); },
+			Poll::Pending => {},
+		}
+		match Pin::new(&mut self.c).poll(ctx) {
+			Poll::Ready(res) => { return Poll::Ready(SelectorOutput::C(res)); },
+			Poll::Pending => {},
+		}
+		Poll::Pending
+	}
+}
+
 /// Connection contains all our internal state for a connection - we hold a reference to the
 /// Connection object (in an Arc<Mutex<>>) in each SocketDescriptor we create as well as in the
 /// read future (which is returned by schedule_read).
@@ -127,29 +190,44 @@ impl Connection {
 				}
 				us_lock.read_paused
 			};
-			tokio::select! {
-				v = write_avail_receiver.recv() => {
+			// TODO: Drop the Box'ing of the futures once Rust has pin-on-stack support.
+			let select_result = if read_paused {
+				TwoSelector {
+					a: Box::pin(write_avail_receiver.recv()),
+					b: Box::pin(read_wake_receiver.recv()),
+				}.await
+			} else {
+				ThreeSelector {
+					a: Box::pin(write_avail_receiver.recv()),
+					b: Box::pin(read_wake_receiver.recv()),
+					c: Box::pin(reader.read(&mut buf)),
+				}.await
+			};
+			match select_result {
+				SelectorOutput::A(v) => {
 					assert!(v.is_some()); // We can't have dropped the sending end, its in the us Arc!
 					if peer_manager.as_ref().write_buffer_space_avail(&mut our_descriptor).is_err() {
 						break Disconnect::CloseConnection;
 					}
 				},
-				_ = read_wake_receiver.recv() => {},
-				read = reader.read(&mut buf), if !read_paused => match read {
-					Ok(0) => break Disconnect::PeerDisconnected,
-					Ok(len) => {
-						let read_res = peer_manager.as_ref().read_event(&mut our_descriptor, &buf[0..len]);
-						let mut us_lock = us.lock().unwrap();
-						match read_res {
-							Ok(pause_read) => {
-								if pause_read {
-									us_lock.read_paused = true;
-								}
-							},
-							Err(_) => break Disconnect::CloseConnection,
-						}
-					},
-					Err(_) => break Disconnect::PeerDisconnected,
+				SelectorOutput::B(_) => {},
+				SelectorOutput::C(read) => {
+					match read {
+						Ok(0) => break Disconnect::PeerDisconnected,
+						Ok(len) => {
+							let read_res = peer_manager.as_ref().read_event(&mut our_descriptor, &buf[0..len]);
+							let mut us_lock = us.lock().unwrap();
+							match read_res {
+								Ok(pause_read) => {
+									if pause_read {
+										us_lock.read_paused = true;
+									}
+								},
+								Err(_) => break Disconnect::CloseConnection,
+							}
+						},
+						Err(_) => break Disconnect::PeerDisconnected,
+					}
 				},
 			}
 			let _ = event_waker.try_send(());