runtime: Add ModelStop exception to signal simulation stop

In the runtime, a simulator that is not operational after a process
will also throw the ModelStop exception.

engine/src/simulation.cpp

@@ -202,6 +202,9 @@ class SimulationMachine
       } catch (cloe::ModelReset& e) {
         logger()->error("Unhandled reset request in {} state: {}", id, e.what());
         this->push_interrupt(RESET);
+      } catch (cloe::ModelStop& e) {
+        logger()->error("Unhandled stop request in {} state: {}", id, e.what());
+        this->push_interrupt(STOP);
       } catch (cloe::ModelAbort& e) {
         logger()->error("Unhandled abort request in {} state: {}", id, e.what());
         this->push_interrupt(ABORT);
@@ -832,12 +835,16 @@ StateId SimulationMachine::StepSimulators::impl(SimulationContext& ctx) {
   ctx.foreach_simulator([&ctx](cloe::Simulator& simulator) {
     try {
       cloe::Duration sim_time = simulator.process(ctx.sync);
+      if (!simulator.is_operational()) {
+        throw cloe::ModelStop("simulator {} no longer operational", simulator.name());
+      }
       if (sim_time != ctx.sync.time()) {
-
         throw cloe::ModelError("simulator {} did not progress to required time: got {}ms, expected {}ms", simulator.name(), sim_time.count()/1'000'000, ctx.sync.time().count()/1'000'000);
       }
     } catch (cloe::ModelReset& e) {
       throw;
+    } catch (cloe::ModelStop& e) {
+      throw;
     } catch (cloe::ModelAbort& e) {
       throw;
     } catch (cloe::ModelError& e) {
@@ -915,6 +922,10 @@ StateId SimulationMachine::StepControllers::impl(SimulationContext& ctx) {
       this->logger()->error("Controller {} reset: {}", ctrl.name(), e.what());
       this->state_machine()->reset();
       return false;
+    } catch (cloe::ModelStop& e) {
+      this->logger()->error("Controller {} stop: {}", ctrl.name(), e.what());
+      this->state_machine()->stop();
+      return false;
     } catch (cloe::ModelAbort& e) {
       this->logger()->error("Controller {} abort: {}", ctrl.name(), e.what());
       this->state_machine()->abort();

runtime/include/cloe/model.hpp

@@ -40,6 +40,10 @@
  * ModelReset
  *   Error used when a model needs to *signal* that it desires a simulation
  *   reset.
+ *
+ * ModelStop
+ *   Error used when a model needs to *signal* that it desires a simulation
+ *   stop.
  */
 
 #pragma once
@@ -125,6 +129,29 @@ class ModelReset : public ModelError {
   }
 };
 
+/**
+ * ModelStop indicates that the model has encountered a request that causes
+ * it to believe that the simulation should be stopped.
+ */
+class ModelStop : public ModelError {
+ public:
+  using ModelError::ModelError;
+  virtual ~ModelStop() noexcept = default;
+
+  const std::string& explanation() const { return Error::explanation(); }
+
+  ModelError explanation(const std::string& explanation) && {
+    this->set_explanation(explanation);
+    return std::move(*this);
+  }
+
+  template <typename... Args>
+  ModelError explanation(const char* format, const Args&... args) && {
+    this->set_explanation(fmt::format(format, args...));
+    return std::move(*this);
+  }
+};
+
 /**
  * The Model class serves as an interface which Controller and Simulator can
  * inherit from.