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]>

russh/src/channels.rs

@@ -174,11 +174,39 @@ impl<Send: From<(ChannelId, ChannelMsg)>> Channel<Send> {
     }
 
     /// Signal a remote process.
-    pub async fn signal(&mut self, signal: Sig) -> Result<(), Error> {
-        self.send_msg(ChannelMsg::Signal { signal }).await?;
+    pub async fn signal(&self, signal: Sig) -> Result<(), Error> {
+        self.sender
+            .sender
+            .send(Msg::Signal {
+                id: self.sender.id,
+                signal,
+            })
+            .await
+            .map_err(|_| Error::SendError)?;
         Ok(())
     }
 
+    /// 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>> + Send>> {
+        let sender = self.sender.clone();
+        let id = self.sender.id;
+
+        move |signal| {
+            async move {
+                sender
+                    .sender
+                    .send(Msg::Signal { id, signal })
+                    .await
+                    .map_err(|_| Error::SendError)?;
+
+                Ok(())
+            }
+            .boxed()
+        }
+    }
+
     /// Request the start of a subsystem with the given name.
     pub async fn request_subsystem<A: Into<String>>(
         &mut self,