core/muxing: Remove the StreamMuxer::flush_all function (#2669)

`libp2p-core` provides the `StreamMuxer` abstraction so it can provide
functionality that abstracts over this trait.

We never use the `flush_all` function as part of our abstractions.
No one else is going to use it so we can remove it from the abstraction.

core/src/either.rs

@@ -352,13 +352,6 @@ where
             EitherOutput::Second(inner) => inner.poll_close(cx).map_err(|e| e.into()),
         }
     }
-
-    fn flush_all(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
-        match self {
-            EitherOutput::First(inner) => inner.flush_all(cx).map_err(|e| e.into()),
-            EitherOutput::Second(inner) => inner.flush_all(cx).map_err(|e| e.into()),
-        }
-    }
 }
 
 #[derive(Debug, Copy, Clone)]

core/src/muxing.rs

@@ -216,13 +216,6 @@ pub trait StreamMuxer {
     /// >           properly informing the remote, there is no difference between this and
     /// >           immediately dropping the muxer.
     fn poll_close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
-
-    /// Flush this `StreamMuxer`.
-    ///
-    /// This drains any write buffers of substreams and delivers any pending shutdown notifications
-    /// due to `shutdown_substream` or `close`. One may thus shutdown groups of substreams
-    /// followed by a final `flush_all` instead of having to do `flush_substream` for each.
-    fn flush_all(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
 }
 
 /// Event about a connection, reported by an implementation of [`StreamMuxer`].
@@ -609,11 +602,6 @@ impl StreamMuxer for StreamMuxerBox {
     fn poll_close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         self.inner.poll_close(cx)
     }
-
-    #[inline]
-    fn flush_all(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
-        self.inner.flush_all(cx)
-    }
 }
 
 struct Wrap<T>
@@ -750,9 +738,4 @@ where
     fn poll_close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         self.inner.poll_close(cx).map_err(|e| e.into())
     }
-
-    #[inline]
-    fn flush_all(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
-        self.inner.flush_all(cx).map_err(|e| e.into())
-    }
 }

core/src/muxing/singleton.rs

@@ -149,12 +149,7 @@ where
 
     fn destroy_substream(&self, _: Self::Substream) {}
 
-    fn poll_close(&self, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
-        // The `StreamMuxer` trait requires that `close()` implies `flush_all()`.
-        self.flush_all(cx)
-    }
-
-    fn flush_all(&self, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
-        AsyncWrite::poll_flush(Pin::new(&mut *self.inner.lock()), cx)
+    fn poll_close(&self, _cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        Poll::Ready(Ok(()))
     }
 }