cln-plugin: Handle --help invocations better

We now have ternary outcomes for `Builder.configure()` and
`Builder.start()`:

 - Ok(Some(p)) means we were configured correctly, and can continue
   with our work normally
 - Ok(None) means that `lightningd` was invoked with `--help`, we
   weren't configured (which is not an error since the `lightningd` just
   implicitly told us to shut down) and user code should clean up and
   exit as well
 - Err(e) something went wrong, user code may report an error and exit.

plugins/examples/cln-plugin-startup.rs

@@ -6,7 +6,7 @@ use cln_plugin::{options, Builder, Error, Plugin};
 use tokio;
 #[tokio::main]
 async fn main() -> Result<(), anyhow::Error> {
-    let plugin = Builder::new((), tokio::io::stdin(), tokio::io::stdout())
+    if let Some(plugin) = Builder::new((), tokio::io::stdin(), tokio::io::stdout())
         .option(options::ConfigOption::new(
             "test-option",
             options::Value::Integer(42),
@@ -16,8 +16,12 @@ async fn main() -> Result<(), anyhow::Error> {
         .subscribe("connect", connect_handler)
         .hook("peer_connected", peer_connected_handler)
         .start()
-        .await?;
-    plugin.join().await
+        .await?
+    {
+        plugin.join().await
+    } else {
+        Ok(())
+    }
 }
 
 async fn testmethod(_p: Plugin<()>, _v: serde_json::Value) -> Result<serde_json::Value, Error> {
@@ -29,7 +33,10 @@ async fn connect_handler(_p: Plugin<()>, v: serde_json::Value) -> Result<(), Err
     Ok(())
 }
 
-async fn peer_connected_handler(_p: Plugin<()>, v: serde_json::Value) -> Result<serde_json::Value, Error> {
+async fn peer_connected_handler(
+    _p: Plugin<()>,
+    v: serde_json::Value,
+) -> Result<serde_json::Value, Error> {
     log::info!("Got a connect hook call: {}", v);
     Ok(json!({"result": "continue"}))
 }

plugins/grpc-plugin/src/main.rs

@@ -28,19 +28,25 @@ async fn main() -> Result<()> {
         ca_cert,
     };
 
-    let plugin = Builder::new(state.clone(), tokio::io::stdin(), tokio::io::stdout())
+    let plugin = match Builder::new(state.clone(), tokio::io::stdin(), tokio::io::stdout())
         .option(options::ConfigOption::new(
             "grpc-port",
             options::Value::Integer(-1),
             "Which port should the grpc plugin listen for incoming connections?",
         ))
         .configure()
-        .await?;
+        .await?
+    {
+        Some(p) => p,
+        None => return Ok(()),
+    };
 
     let bind_port = match plugin.option("grpc-port") {
         Some(options::Value::Integer(-1)) => {
             log::info!("`grpc-port` option is not configured, exiting.");
-            plugin.disable("`grpc-port` option is not configured.").await?;
+            plugin
+                .disable("`grpc-port` option is not configured.")
+                .await?;
             return Ok(());
         }
         Some(options::Value::Integer(i)) => i,

plugins/src/lib.rs

@@ -139,7 +139,13 @@ where
         self
     }
 
-    pub async fn configure(mut self) -> Result<ConfiguredPlugin<S, I, O>, anyhow::Error> {
+    /// Communicate with `lightningd` to tell it about our options,
+    /// RPC methods and subscribe to hooks, and then process the
+    /// initialization, configuring the plugin.
+    ///
+    /// Returns `None` if we were invoked with `--help` and thus
+    /// should exit after this handshake
+    pub async fn configure(mut self) -> Result<Option<ConfiguredPlugin<S, I, O>>, anyhow::Error> {
         let mut input = FramedRead::new(self.input.take().unwrap(), JsonRpcCodec::default());
 
         // Sadly we need to wrap the output in a mutex in order to
@@ -189,7 +195,7 @@ where
                 // If we are being called with --help we will get
                 // disconnected here. That's expected, so don't
                 // complain about it.
-                0
+                return Ok(None);
             }
         };
 
@@ -213,7 +219,7 @@ where
 
         // Leave the `init` reply pending, so we can disable based on
         // the options if required.
-        Ok(ConfiguredPlugin {
+        Ok(Some(ConfiguredPlugin {
             // The JSON-RPC `id` field so we can reply correctly.
             init_id,
             input,
@@ -228,7 +234,7 @@ where
                 ),
             },
             plugin,
-        })
+        }))
     }
 
     /// Build and start the plugin loop. This performs the handshake
@@ -236,8 +242,17 @@ where
     /// Core Lightning and dispatches them to the handlers. It only
     /// returns after completing the handshake to ensure that the
     /// configuration and initialization was successfull.
-    pub async fn start(self) -> Result<Plugin<S>, anyhow::Error> {
-        self.configure().await?.start().await
+    ///
+    /// If `lightningd` was called with `--help` we won't get a
+    /// `Plugin` instance and return `None` instead. This signals that
+    /// we should exit, and not continue running. `start()` returns in
+    /// order to allow user code to perform cleanup if necessary.
+    pub async fn start(self) -> Result<Option<Plugin<S>>, anyhow::Error> {
+        if let Some(cp) = self.configure().await? {
+            Ok(Some(cp.start().await?))
+        } else {
+            Ok(None)
+        }
     }
 
     fn handle_get_manifest(