Replace futures select with our own select enum to fix MSRV

`futures` recently broke our MSRV by bumping the `syn` major
version in a patch release. This makes it impractical for us to
use, instead here we replace the usage of its `select_biased` macro
with a trivial enum.

Given its simplicity we likely should have done this without ever
taking the dependency.

Author: TheBlueMatt

lightning-background-processor/Cargo.toml

@@ -23,7 +23,7 @@ default = ["std"]
 bitcoin = { version = "0.29.0", default-features = false }
 lightning = { version = "0.0.114", path = "../lightning", default-features = false }
 lightning-rapid-gossip-sync = { version = "0.0.114", path = "../lightning-rapid-gossip-sync", default-features = false }
-futures-util = { version = "0.3", default-features = false, features = ["async-await-macro"], optional = true }
+futures-util = { version = "0.3", default-features = false, optional = true }
 
 [dev-dependencies]
 lightning = { version = "0.0.114", path = "../lightning", features = ["_test_utils"] }

lightning-background-processor/src/lib.rs

@@ -384,6 +384,40 @@ macro_rules! define_run_body {
 	} }
 }
 
+#[cfg(feature = "futures")]
+use core::future::Future;
+#[cfg(feature = "futures")]
+use core::task::Poll;
+#[cfg(feature = "futures")]
+use core::pin::Pin;
+#[cfg(feature = "futures")]
+use core::marker::Unpin;
+#[cfg(feature = "futures")]
+struct Selecter<A: Future<Output=()> + Unpin, B: Future<Output=bool> + Unpin> {
+	a: A,
+	b: B,
+}
+#[cfg(feature = "futures")]
+enum SelecterOutput {
+	A, B(bool),
+}
+
+#[cfg(feature = "futures")]
+impl<A: Future<Output=()> + Unpin, B: Future<Output=bool> + Unpin> Future for Selecter<A, B> {
+	type Output = SelecterOutput;
+	fn poll(mut self: Pin<&mut Self>, ctx: &mut core::task::Context<'_>) -> Poll<SelecterOutput> {
+		match Pin::new(&mut self.a).poll(ctx) {
+			Poll::Ready(()) => { return Poll::Ready(SelecterOutput::A); },
+			Poll::Pending => {},
+		}
+		match Pin::new(&mut self.b).poll(ctx) {
+			Poll::Ready(res) => { return Poll::Ready(SelecterOutput::B(res)); },
+			Poll::Pending => {},
+		}
+		Poll::Pending
+	}
+}
+
 /// Processes background events in a future.
 ///
 /// `sleeper` should return a future which completes in the given amount of time and returns a
@@ -470,9 +504,13 @@ where
 		chain_monitor, chain_monitor.process_pending_events_async(async_event_handler).await,
 		channel_manager, channel_manager.process_pending_events_async(async_event_handler).await,
 		gossip_sync, peer_manager, logger, scorer, should_break, {
-			select_biased! {
-				_ = channel_manager.get_persistable_update_future().fuse() => true,
-				exit = sleeper(Duration::from_millis(100)).fuse() => {
+			let fut = Selecter {
+				a: channel_manager.get_persistable_update_future(),
+				b: sleeper(Duration::from_millis(100)),
+			};
+			match fut.await {
+				SelecterOutput::A => true,
+				SelecterOutput::B(exit) => {
 					should_break = exit;
 					false
 				}