Make the getNextLoop method multi-thread safe (#274)

Author: an-tao

trantor/net/EventLoopThreadPool.cc

@@ -47,10 +47,9 @@ EventLoop *EventLoopThreadPool::getNextLoop()
 {
     if (loopThreadVector_.size() > 0)
     {
-        EventLoop *loop = loopThreadVector_[loopIndex_]->getLoop();
-        ++loopIndex_;
-        if (loopIndex_ >= loopThreadVector_.size())
-            loopIndex_ = 0;
+        size_t index = loopIndex_.fetch_add(1, std::memory_order_relaxed);
+        EventLoop *loop =
+            loopThreadVector_[index % loopThreadVector_.size()]->getLoop();
         return loop;
     }
     return nullptr;

trantor/net/EventLoopThreadPool.h

@@ -18,6 +18,7 @@
 #include <trantor/exports.h>
 #include <vector>
 #include <memory>
+#include <atomic>
 
 namespace trantor
 {
@@ -87,6 +88,6 @@ class TRANTOR_EXPORT EventLoopThreadPool : NonCopyable
 
   private:
     std::vector<std::shared_ptr<EventLoopThread>> loopThreadVector_;
-    size_t loopIndex_;
+    std::atomic<size_t> loopIndex_{0};
 };
 }  // namespace trantor