guarantee protected turns actually get first dibs in the scheduler

map_model/src/turn.rs

@@ -71,7 +71,8 @@ pub struct Turn {
     // TODO Some turns might not actually have geometry. Currently encoded by two equal points.
     // Represent more directly?
     pub geom: PolyLine,
-    // Empty except for TurnType::Crosswalk.
+    // Empty except for TurnType::Crosswalk. Usually just one other ID, except for the case of 4
+    // duplicates at a degenerate intersection.
     pub other_crosswalk_ids: BTreeSet<TurnID>,
 
     // Just for convenient debugging lookup.

sim/src/mechanics/intersection.rs

@@ -159,12 +159,19 @@ impl IntersectionSimState {
             unreachable!()
         };
 
-        protected.extend(yielding);
         for req in protected {
             // Use update because multiple agents could finish a turn at the same time, before the
             // waiting one has a chance to try again.
             scheduler.update(now, Command::update_agent(req.agent));
         }
+        // Make sure the protected group gets first dibs. The scheduler arbitrarily (but
+        // deterministically) orders commands with the same time.
+        for req in yielding {
+            scheduler.update(
+                now + Duration::seconds(0.1),
+                Command::update_agent(req.agent),
+            );
+        }
     }
 
     // This is only triggered for traffic signals.

sim/src/scheduler.rs

@@ -70,6 +70,7 @@ impl Ord for Item {
         if ord != Ordering::Equal {
             return ord;
         }
+        // This is important! The tie-breaker if time is the same is ARBITRARY!
         self.cmd_type.cmp(&other.cmd_type)
     }
 }