Make sim_elapsed function safe to call outside of simulation (#170)

If customer set custom timer for tracing subscriber that uses
sim_elapsed to log simulation time, this function might be called
before world or host set. E.g. with DEBUG level logging set
this function panics on internal log lines in turmoil crate
code before host is set.

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)
     }