Add ability to run hosts in random order (#173)

src/builder.rs

@@ -170,6 +170,14 @@ impl Builder {
         self
     }
 
+    /// Enables running of nodes in random order. This allows exploration
+    /// of extra state space in multi-node simulations where race conditions
+    /// may arise based on message send/receive order.
+    pub fn enable_random_order(&mut self) -> &mut Self {
+        self.config.random_node_order = true;
+        self
+    }
+
     /// Build a simulation with the settings from the builder.
     ///
     /// This will use default rng with entropy from the device running.

src/config.rs

@@ -39,6 +39,10 @@ pub(crate) struct Config {
 
     /// Enables tokio IO driver
     pub(crate) enable_tokio_io: bool,
+
+    /// Enables running of host/client code in random order at each
+    /// simulation step
+    pub(crate) random_node_order: bool,
 }
 
 /// Configures link behavior.
@@ -91,6 +95,7 @@ impl Default for Config {
             tcp_capacity: 64,
             udp_capacity: 64,
             enable_tokio_io: false,
+            random_node_order: false,
         }
     }
 }

src/sim.rs

@@ -1,3 +1,4 @@
+use rand::seq::SliceRandom;
 use std::cell::RefCell;
 use std::future::Future;
 use std::net::IpAddr;
@@ -347,13 +348,18 @@ impl<'a> Sim<'a> {
         // Tick each host runtimes with running software. If the software
         // completes, extract the result and return early if an error is
         // encountered.
-        for (&addr, rt) in self
+
+        let mut running: Vec<_> = self
             .rts
             .iter_mut()
             .filter(|(_, rt)| rt.is_software_running())
-        {
-            let _span_guard = tracing::span!(Level::INFO, "node", name = &*rt.nodename).entered();
+            .collect();
+        if self.config.random_node_order {
+            running.shuffle(&mut self.world.borrow_mut().rng);
+        }
 
+        for (&addr, rt) in running {
+            let _span_guard = tracing::span!(Level::INFO, "node", name = &*rt.nodename,).entered();
             {
                 let mut world = self.world.borrow_mut();
                 // We need to move deliverable messages off the network and