rotate the participants in the cycle once

Rotate the participants in the cycle when the cycle is created
rather than doing it "on the fly" each time they are iterated.

src/lib.rs

@@ -667,12 +667,7 @@ impl Cycle {
     /// is arbitrary but deterministic, but the ordering is otherwise determined
     /// by the execution.
     pub fn participant_keys(&self) -> impl Iterator<Item = DatabaseKeyIndex> + '_ {
-        let min = self.participants.iter().min().unwrap();
-        let index = self.participants.iter().position(|p| p == min).unwrap();
-        self.participants[index..]
-            .iter()
-            .chain(self.participants[..index].iter())
-            .copied()
+        self.participants.iter().copied()
     }
 
     /// Returns a vector with the debug information for

src/runtime.rs

@@ -286,6 +286,18 @@ impl Runtime {
                 to_id,
                 |aq| v.push(aq.database_key_index),
             );
+
+            // We want to give the participants in a deterministic order
+            // (at least for this execution, not necessarily across executions),
+            // no matter where it started on the stack. Find the minimum
+            // key and rotate it to the front.
+            let min = v.iter().min().unwrap();
+            let index = v.iter().position(|p| p == min).unwrap();
+            v.rotate_left(index);
+
+            // No need to store extra memory.
+            v.shrink_to_fit();
+
             Cycle::new(Arc::new(v))
         };
         debug!("cycle {:?}", cycle.debug(db));