make AggregatorSet close to zero cost when only 1 aggregator asked for

backend/common/src/either_sink.rs

@@ -0,0 +1,57 @@
+use futures::sink::Sink;
+use pin_project_lite::pin_project;
+
+pin_project! {
+    #[project = EitherSinkInner]
+    pub enum EitherSink<A, B> {
+        A { #[pin] inner: A },
+        B { #[pin] inner: B }
+    }
+}
+
+/// A simple enum that delegates implementation to one of
+/// the two possible sinks contained within.
+impl <A, B> EitherSink<A, B> {
+    pub fn a(val: A) -> Self {
+        EitherSink::A { inner: val }
+    }
+    pub fn b(val: B) -> Self {
+        EitherSink::B { inner: val }
+    }
+}
+
+impl <Item, Error, A, B> Sink<Item> for EitherSink<A, B>
+where
+    A: Sink<Item, Error = Error>,
+    B: Sink<Item, Error = Error>
+{
+    type Error = Error;
+
+    fn poll_ready(self: std::pin::Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> std::task::Poll<Result<(), Self::Error>> {
+        match self.project() {
+            EitherSinkInner::A{ inner } => inner.poll_ready(cx),
+            EitherSinkInner::B{ inner } => inner.poll_ready(cx)
+        }
+    }
+
+    fn start_send(self: std::pin::Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
+        match self.project() {
+            EitherSinkInner::A{ inner } => inner.start_send(item),
+            EitherSinkInner::B{ inner } => inner.start_send(item)
+        }
+    }
+
+    fn poll_flush(self: std::pin::Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> std::task::Poll<Result<(), Self::Error>> {
+        match self.project() {
+            EitherSinkInner::A{ inner } => inner.poll_flush(cx),
+            EitherSinkInner::B{ inner } => inner.poll_flush(cx)
+        }
+    }
+
+    fn poll_close(self: std::pin::Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> std::task::Poll<Result<(), Self::Error>> {
+        match self.project() {
+            EitherSinkInner::A{ inner } => inner.poll_close(cx),
+            EitherSinkInner::B{ inner } => inner.poll_close(cx)
+        }
+    }
+}