Replace the implementation of oneshot_to_stream in the async runtime …

…using the flatten_stream library function

src/async_runtime.rs

@@ -6,6 +6,7 @@ use std::sync::Arc;
 use futures::future::join_all;
 use futures::stream;
 use futures::stream::BoxStream;
+use futures::FutureExt;
 use futures::StreamExt;
 use std::fmt::Debug;
 use tokio::select;
@@ -24,40 +25,21 @@ use crate::core::MonitoringSemantics;
 use crate::core::Specification;
 use crate::core::{OutputStream, StreamContext, StreamData, VarName};
 
-/* A struct containing a stream of type S which is lazily provided via a
- * oneshot channel. This is used to allow a stream to be provided lazily to a
- * consumer.
- *
- * This is either in the state Arrived(S) if the stream has already arrived, or
- * Waiting(oneshot::Receiver<S>) when we are still waiting to receive the
- * stream on the channel.
- */
-enum WaitingStream<S> {
-    Arrived(S),
-    Waiting(oneshot::Receiver<S>),
-}
-
 /* This function takes a receiver of a stream and returns a new stream which
  * first waits for the stream to be received on the channel before the first
  * element is supplied. This is useful for providing code which depend on a
  * stream with this stream before it has been defined, and make it possible to
  * define multiple mutually recursive streams.
  */
-fn oneshot_to_stream<T: 'static>(receiver: oneshot::Receiver<OutputStream<T>>) -> OutputStream<T> {
-    Box::pin(stream::unfold(
-        WaitingStream::Waiting(receiver),
-        |waiting_stream| async move {
-            match waiting_stream {
-                WaitingStream::Arrived(mut stream) => {
-                    Some((stream.next().await?, WaitingStream::Arrived(stream)))
-                }
-                WaitingStream::Waiting(receiver) => match receiver.await {
-                    Ok(mut stream) => Some((stream.next().await?, WaitingStream::Arrived(stream))),
-                    Err(_) => None,
-                },
-            }
-        },
-    ))
+fn oneshot_to_stream<T: StreamData>(
+    receiver: oneshot::Receiver<OutputStream<T>>,
+) -> OutputStream<T> {
+    let empty_stream = Box::pin(stream::empty());
+    Box::pin(
+        receiver
+            .map(|res| res.unwrap_or(empty_stream))
+            .flatten_stream()
+    )
 }
 
 /* Wrap a stream in a drop guard to ensure that the associated cancellation