Make sim_elapsed function safe to call outside of simulation

src/lib.rs

@@ -182,11 +182,17 @@ pub fn elapsed() -> Duration {
 
 /// Returns how long the simulation has been executing for in virtual time.
 ///
-/// Must be called from within a Turmoil simulation. Will return None if the
-/// duration is not available, typically because there is no currently executing
-/// host.
+/// Will return None if the duration is not available, typically because
+/// there is no currently executing host or world.
 pub fn sim_elapsed() -> Option<Duration> {
-    World::try_current(|world| world.current_host().timer.sim_elapsed()).ok()
+    World::try_current(|world| {
+        world
+            .try_current_host()
+            .map(|host| host.timer.sim_elapsed())
+            .ok()
+    })
+    .ok()
+    .flatten()
 }
 
 /// Lookup an IP address by host name.

src/sim.rs

@@ -414,7 +414,7 @@ mod test {
     };
 
     use crate::{
-        elapsed, hold,
+        elapsed, hold, World,
         net::{TcpListener, TcpStream},
         sim_elapsed, Builder, Result,
     };
@@ -560,6 +560,21 @@ mod test {
         Ok(())
     }
 
+    /// This is a regression test to ensure it is safe to call sim_elapsed
+    /// if current world of host is not set. 
+    #[test]
+    fn sim_elapsed_time() -> Result {
+        // Safe to call outside of simution while there 
+        // is no current world set
+        assert!(sim_elapsed().is_none());
+
+        let sim = Builder::new().build();
+        // Safe to call while there is no current host set
+        World::enter(&sim.world, ||  assert!(sim_elapsed().is_none()));
+
+        Ok(())
+    }
+
     #[test]
     fn hold_release_peers() -> Result {
         let global = Duration::from_millis(2);

src/world.rs

@@ -101,6 +101,11 @@ impl World {
         self.hosts.get(&addr).expect("host missing")
     }
 
+    pub(crate) fn try_current_host(&self) -> TurmoilResult<&Host> {
+        let addr = self.current.ok_or("current host missing")?;
+        self.hosts.get(&addr).ok_or_else(|| "host missing".into())
+    }
+
     pub(crate) fn lookup(&mut self, host: impl ToIpAddr) -> IpAddr {
         self.dns.lookup(host)
     }