sim-lib: only use start delay if explicitly specified

As is, we'll run with a 0 second start delay for:
- Random activities
- Defined activities with no `start_secs` specified

This very likely isn't the intent for defined activities (people should
set `start_secs=0` if they want their payments to fire immediately),
and doesn't work for random activity because we just fire all payments
on start.

Instead, we switch over to using our payment wait as the start delay in
the absence of this field being set for defined activities and always
for random activities.

sim-lib/src/defined_activity.rs

@@ -7,7 +7,7 @@ use tokio::time::Duration;
 #[derive(Clone)]
 pub struct DefinedPaymentActivity {
     destination: NodeInfo,
-    start: Duration,
+    start: Option<Duration>,
     count: Option<u64>,
     wait: ValueOrRange<u16>,
     amount: ValueOrRange<u64>,
@@ -16,7 +16,7 @@ pub struct DefinedPaymentActivity {
 impl DefinedPaymentActivity {
     pub fn new(
         destination: NodeInfo,
-        start: Duration,
+        start: Option<Duration>,
         count: Option<u64>,
         wait: ValueOrRange<u16>,
         amount: ValueOrRange<u64>,
@@ -48,7 +48,7 @@ impl DestinationGenerator for DefinedPaymentActivity {
 }
 
 impl PaymentGenerator for DefinedPaymentActivity {
-    fn payment_start(&self) -> Duration {
+    fn payment_start(&self) -> Option<Duration> {
         self.start
     }
 
@@ -77,7 +77,6 @@ impl PaymentGenerator for DefinedPaymentActivity {
 #[cfg(test)]
 mod tests {
     use super::DefinedPaymentActivity;
-    use super::*;
     use crate::test_utils::{create_nodes, get_random_keypair};
     use crate::{DestinationGenerator, PaymentGenerationError, PaymentGenerator};
 
@@ -91,7 +90,7 @@ mod tests {
 
         let generator = DefinedPaymentActivity::new(
             node.clone(),
-            Duration::from_secs(0),
+            None,
             None,
             crate::ValueOrRange::Value(60),
             crate::ValueOrRange::Value(payment_amt),

sim-lib/src/lib.rs

@@ -182,8 +182,7 @@ pub struct ActivityParser {
     #[serde(with = "serializers::serde_node_id")]
     pub destination: NodeId,
     /// The time in the simulation to start the payment.
-    #[serde(default)]
-    pub start_secs: u16,
+    pub start_secs: Option<u16>,
     /// The number of payments to send over the course of the simulation.
     #[serde(default)]
     pub count: Option<u64>,
@@ -204,7 +203,7 @@ pub struct ActivityDefinition {
     /// The destination of the payment.
     pub destination: NodeInfo,
     /// The time in the simulation to start the payment.
-    pub start_secs: u16,
+    pub start_secs: Option<u16>,
     /// The number of payments to send over the course of the simulation.
     pub count: Option<u64>,
     /// The interval of the event, as in every how many seconds the payment is performed.
@@ -316,7 +315,7 @@ pub struct PaymentGenerationError(String);
 
 pub trait PaymentGenerator: Display + Send {
     /// Returns the time that the payments should start
-    fn payment_start(&self) -> Duration;
+    fn payment_start(&self) -> Option<Duration>;
 
     /// Returns the number of payments that should be made
     fn payment_count(&self) -> Option<u64>;
@@ -773,7 +772,9 @@ impl Simulation {
             for description in self.activity.iter() {
                 let activity_generator = DefinedPaymentActivity::new(
                     description.destination.clone(),
-                    Duration::from_secs(description.start_secs.into()),
+                    description
+                        .start_secs
+                        .map(|start| Duration::from_secs(start.into())),
                     description.count,
                     description.interval_secs,
                     description.amount_msat,
@@ -1037,14 +1038,15 @@ async fn produce_events<N: DestinationGenerator + ?Sized, A: PaymentGenerator +
             }
         }
 
-        let wait: Duration = if current_count == 0 {
-            let start = node_generator.payment_start();
-            if start != Duration::from_secs(0) {
-                log::debug!(
-                    "Using a start delay. The first payment for {source} will be at {:?}.",
-                    start
-                );
-            }
+        // On our first run, add a one-time start delay if set for our payment.
+        let opt_start = node_generator.payment_start();
+        let wait = if current_count == 0 && opt_start.is_some() {
+            let start = opt_start.unwrap(); // Safe due to is_some check above.
+            log::debug!(
+                "Using a start delay. The first payment for {source} will be at {:?}.",
+                start
+            );
+
             start
         } else {
             log::debug!(

sim-lib/src/random_activity.rs

@@ -195,8 +195,8 @@ fn events_per_month(source_capacity_msat: u64, multiplier: f64, expected_payment
 
 impl PaymentGenerator for RandomPaymentActivity {
     /// Returns the time that the payments should start. This will always be 0 for the RandomPaymentActivity type.
-    fn payment_start(&self) -> Duration {
-        Duration::from_secs(0)
+    fn payment_start(&self) -> Option<Duration> {
+        None
     }
 
     /// Returns the number of payments that should be made. This will always be None for the RandomPaymentActivity type.