Implement detached signal sender

Allow signals to be sent detached from the channel struct.

This is done using a higher-order function and by cloning the sender so
it can be used independently of the `Channel`.

This is useful to be able to spawn a cancelation handler separately from
the main `Channel` recieve loop.

Signed-off-by: Joe Grund <[email protected]>
f08f74

Signed-off-by: Joe Grund <[email protected]>

russh/src/channels/mod.rs

@@ -1,5 +1,6 @@
-use std::sync::Arc;
+use std::{pin::Pin, sync::Arc};
 
+use futures::{Future, FutureExt as _};
 use russh_cryptovec::CryptoVec;
 use tokio::io::{AsyncRead, AsyncWrite};
 use tokio::sync::mpsc::{Sender, UnboundedReceiver};
@@ -318,6 +319,26 @@ impl<S: From<(ChannelId, ChannelMsg)> + Send + Sync + 'static> Channel<S> {
     pub async fn close(&self) -> Result<(), Error> {
         self.send_msg(ChannelMsg::Close).await
     }
+    /// Get a `FnOnce` that can be used to send a signal through this channel
+    pub fn get_signal_sender(
+        &self,
+    ) -> impl FnOnce(Sig) -> Pin<Box<dyn Future<Output = Result<(), Error>> + std::marker::Send>>
+    {
+        let sender = self.sender.clone();
+        let id = self.id;
+
+        move |signal| {
+            async move {
+                sender
+                    .send((id, ChannelMsg::Signal { signal }).into())
+                    .await
+                    .map_err(|_| Error::SendError)?;
+
+                Ok(())
+            }
+            .boxed()
+        }
+    }
 
     async fn send_msg(&self, msg: ChannelMsg) -> Result<(), Error> {
         self.sender