Improve latest model version lookup (#240)

Author: GuanLuo

src/model_lifecycle.cc

@@ -344,18 +344,17 @@ ModelLifeCycle::GetModel(
 
     // The case where the request is asking for latest version
     int64_t latest = -1;
-    for (auto& version_model : mit->second) {
-      if (version_model.first > latest) {
-        std::lock_guard<std::mutex> lock(version_model.second->mtx_);
-        if (version_model.second->state_ == ModelReadyState::READY) {
-          latest = version_model.first;
-          // Tedious, but have to set handle for any "latest" version
-          // at the moment to avoid edge case like the following:
-          // "versions : 1 3 2", version 3 is latest but is requested
-          // to be unloaded when the iterator is examining version 2,
-          // then 'model' will ensure version 3 is still valid
-          *model = version_model.second->model_;
-        }
+    static_assert(
+        std::is_same<
+            decltype(mit->second)::key_compare, std::less<int64_t>>::value,
+        "Below assume versions are sorted in specific order");
+    for (auto version_model = mit->second.rbegin();
+         version_model != mit->second.rend(); ++version_model) {
+      std::lock_guard<std::mutex> lock(version_model->second->mtx_);
+      if (version_model->second->state_ == ModelReadyState::READY) {
+        latest = version_model->first;
+        *model = version_model->second->model_;
+        break;
       }
     }
     if (latest == -1) {