webrtc: Avoid future being dropped when channel is full

.gitignore

@@ -1 +1,2 @@
 /target
+.idea

src/transport/webrtc/substream.rs

@@ -26,7 +26,7 @@ use crate::{
 use bytes::{Buf, BufMut, BytesMut};
 use futures::{Future, Stream};
 use parking_lot::Mutex;
-use tokio::sync::mpsc::{channel, Receiver, Sender};
+use tokio::sync::mpsc::{channel, error::TrySendError, Receiver, Sender};
 
 use std::{
     pin::Pin,
@@ -198,17 +198,20 @@ impl tokio::io::AsyncWrite for Substream {
             return Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into()));
         }
 
-        // TODO: try to coalesce multiple calls to `poll_write()` into single `Event::Message`
-
-        let num_bytes = std::cmp::min(MAX_FRAME_SIZE, buf.len());
-        let future = self.tx.reserve();
-        futures::pin_mut!(future);
-
-        let permit = match futures::ready!(future.poll(cx)) {
-            Err(_) => return Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into())),
+        let permit = match self.tx.try_reserve() {
             Ok(permit) => permit,
+            Err(err) =>
+                return match err {
+                    TrySendError::Full(_) => {
+                        cx.waker().wake_by_ref();
+                        Poll::Pending
+                    },
+                    TrySendError::Closed(_) =>
+                        Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into())),
+                },
         };
 
+        let num_bytes = std::cmp::min(MAX_FRAME_SIZE, buf.len());
         let frame = buf[..num_bytes].to_vec();
         permit.send(Event::Message(frame));