Bias Selector to first poll the sleeper future

Previously, `lightning-background-processor`'s `Selector` would poll all
other futures *before* finally polling the sleeper and returning the
`exit` flag if it's ready. This could lead to scenarios where we
infinitely keep processing background events and never respect the
`exit` flag, as long as any of other futures keep being ready.

Here, we instead bias the `Selector` to always *first* poll the sleeper
future, and hence have us act on the `exit` flag immediately if is set.

Author: tnull

lightning-background-processor/src/lib.rs

@@ -509,6 +509,14 @@ pub(crate) mod futures_util {
 		fn poll(
 			mut self: Pin<&mut Self>, ctx: &mut core::task::Context<'_>,
 		) -> Poll<SelectorOutput> {
+			// Bias the selector so it first polls the sleeper future, allowing to exit immediately
+			// if the flag is set.
+			match Pin::new(&mut self.e).poll(ctx) {
+				Poll::Ready(res) => {
+					return Poll::Ready(SelectorOutput::E(res));
+				},
+				Poll::Pending => {},
+			}
 			match Pin::new(&mut self.a).poll(ctx) {
 				Poll::Ready(()) => {
 					return Poll::Ready(SelectorOutput::A);
@@ -533,12 +541,6 @@ pub(crate) mod futures_util {
 				},
 				Poll::Pending => {},
 			}
-			match Pin::new(&mut self.e).poll(ctx) {
-				Poll::Ready(res) => {
-					return Poll::Ready(SelectorOutput::E(res));
-				},
-				Poll::Pending => {},
-			}
 			Poll::Pending
 		}
 	}