make sure shutdown is thread safe and can only be called once

Fixes #34

include/reactor-cpp/environment.hh

@@ -48,6 +48,8 @@ private:
   void build_dependency_graph(Reactor* reactor);
   void calculate_indexes();
 
+  std::mutex shutdown_mutex_{};
+
 public:
   explicit Environment(unsigned int num_workers, bool run_forever = default_run_forever,
                        bool fast_fwd_execution = default_fast_fwd_execution, const Duration& timeout = Duration::max())

lib/environment.cc

@@ -88,10 +88,20 @@ void Environment::build_dependency_graph(Reactor* reactor) { // NOLINT
 }
 
 void Environment::sync_shutdown() {
-  validate(this->phase() == Phase::Execution, "sync_shutdown() may only be called during execution phase!");
-  phase_ = Phase::Shutdown;
+  {
+    std::lock_guard<std::mutex> lock(shutdown_mutex_);
 
-  log::Info() << "Terminating the execution";
+    if (phase_ >= Phase::Shutdown) {
+      // sync_shutdown() was already called -> abort
+      return;
+    }
+
+    validate(phase_ == Phase::Execution, "sync_shutdown() may only be called during execution phase!");
+    phase_ = Phase::Shutdown;
+  }
+
+  // the following will only be executed once
+  log::Debug() << "Terminating the execution";
 
   for (auto* reactor : top_level_reactors_) {
     reactor->shutdown();