cln-plugin: Make notification handlers asynchronous

plugins/src/lib.rs

@@ -71,10 +71,36 @@ where
         self
     }
 
-    /// Subscribe to notifications for the given `topic`.
-    pub fn subscribe(mut self, topic: &str, callback: NotificationCallback<S>) -> Builder<S, I, O> {
-        self.subscriptions
-            .insert(topic.to_string(), Subscription { callback });
+    /// Subscribe to notifications for the given `topic`. The handler
+    /// is an async function that takes a `Plugin<S>` and the
+    /// notification as a `serde_json::Value` as inputs. Since
+    /// notifications do not expect a result the handler should only
+    /// report errors while processing. Any error reported while
+    /// processing the notification will be logged in the cln logs.
+    ///
+    /// ```
+    /// use cln_plugin::{options, Builder, Error, Plugin};
+    ///
+    /// async fn connect_handler(_p: Plugin<()>, v: serde_json::Value) -> Result<(), Error> {
+    ///     println!("Got a connect notification: {}", v);
+    ///     Ok(())
+    /// }
+    ///
+    /// let b = Builder::new((), tokio::io::stdin(), tokio::io::stdout())
+    ///     .subscribe("connect", connect_handler);
+    /// ```
+    pub fn subscribe<C, F>(mut self, topic: &str, callback: C) -> Builder<S, I, O>
+    where
+        C: Send + Sync + 'static,
+        C: Fn(Plugin<S>, Request) -> F + 'static,
+        F: Future<Output = Result<(), Error>> + Send + Sync + 'static,
+    {
+        self.subscriptions.insert(
+            topic.to_string(),
+            Subscription {
+                callback: Box::new(move |p, r| Box::pin(callback(p, r))),
+            },
+        );
         self
     }
 
@@ -255,7 +281,11 @@ type Request = serde_json::Value;
 type Response = Result<serde_json::Value, Error>;
 type AsyncCallback<S> =
     Box<dyn Fn(Plugin<S>, Request) -> Pin<Box<dyn Future<Output = Response> + Send>> + Send + Sync>;
-type NotificationCallback<S> = Box<fn(Plugin<S>, &Request) -> Result<(), Error>>;
+type AsyncNotificationCallback<S> = Box<
+    dyn Fn(Plugin<S>, Request) -> Pin<Box<dyn Future<Output = Result<(), Error>> + Send>>
+        + Send
+        + Sync,
+>;
 
 /// A struct collecting the metadata required to register a custom
 /// rpcmethod with the main daemon upon init. It'll get deconstructed
@@ -273,7 +303,7 @@ struct Subscription<S>
 where
     S: Clone + Send,
 {
-    callback: NotificationCallback<S>,
+    callback: AsyncNotificationCallback<S>,
 }
 
 struct Hook<S>
@@ -312,7 +342,7 @@ where
 
     #[allow(dead_code)] // Unused until we fill in the Hook structs.
     hooks: HashMap<String, AsyncCallback<S>>,
-    subscriptions: HashMap<String, NotificationCallback<S>>,
+    subscriptions: HashMap<String, AsyncNotificationCallback<S>>,
 }
 
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
@@ -464,7 +494,7 @@ where
             method,
             params
         );
-        if let Err(e) = callback(plugin.clone(), params) {
+        if let Err(e) = callback(plugin.clone(), params.clone()).await {
             log::error!("Error in notification handler '{}': {}", method, e);
         }
         Ok(())