multi: allow empty activity description and validate random activity

Allow parsing of empty activity descriptions to facilitate default run
with random activity generation. We still use a vector in the
Simulation struct to allow a path where we have a combination of
random and specific activity.

This commit also adds validation that we have at least two nodes when
running random activity. This is required because destination nodes
are chosen from within the nodes that we have execution on (to prevent
liquidity draining and eventual death spirals).

sim-lib/src/lib.rs

@@ -62,7 +62,7 @@ pub struct ClnConnection {
 #[derive(Debug, Serialize, Deserialize, Clone)]
 pub struct Config {
     pub nodes: Vec<NodeConnection>,
-    pub activity: Vec<ActivityDefinition>,
+    pub activity: Option<Vec<ActivityDefinition>>,
 }
 
 #[derive(Debug, Clone, Copy, Serialize, Deserialize)]
@@ -284,14 +284,14 @@ const DEFAULT_PRINT_BATCH_SIZE: u32 = 500;
 impl Simulation {
     pub fn new(
         nodes: HashMap<PublicKey, Arc<Mutex<dyn LightningNode + Send>>>,
-        activity: Vec<ActivityDefinition>,
+        activity: Option<Vec<ActivityDefinition>>,
         total_time: Option<u32>,
         print_batch_size: Option<u32>,
     ) -> Self {
         let (shutdown_trigger, shutdown_listener) = triggered::trigger();
         Self {
             nodes,
-            activity,
+            activity: activity.unwrap_or_default(),
             shutdown_trigger,
             shutdown_listener,
             total_time: total_time.map(|x| Duration::from_secs(x as u64)),
@@ -302,8 +302,15 @@ impl Simulation {
     }
 
     /// validate_activity validates that the user-provided activity description is achievable for the network that
-    /// we're working with.
+    /// we're working with. If no activity description is provided, then it ensures that we have configured a network
+    /// that is suitable for random activity generation.
     async fn validate_activity(&self) -> Result<(), LightningError> {
+        if self.activity.is_empty() && self.nodes.len() <= 1 {
+            return Err(LightningError::ValidationError(
+                "At least two nodes required for random activity generation.".to_string(),
+            ));
+        }
+
         for payment_flow in self.activity.iter() {
             // We need every source node that is configured to execute some activity to be included in our set of
             // nodes so that we can execute events on it.