Protect the timer to avoid "cancel" and "refresh" happens at the same time.

Signed-off-by: Tao He <sighingnow@gmail.com>

etcd/KeepAlive.hpp

@@ -5,6 +5,7 @@
 #include <chrono>
 #include <exception>
 #include <functional>
+#include <mutex>
 #include <string>
 #include <thread>
 
@@ -117,6 +118,9 @@ namespace etcd
 
     int ttl;
     int64_t lease_id;
+
+    // protect the initializing status of `timer`.
+    std::mutex mutex_for_refresh_;
     std::atomic_bool continue_next;
 
     // grpc timeout in `refresh()`

src/KeepAlive.cpp

@@ -124,6 +124,7 @@ etcd::KeepAlive::~KeepAlive()
 
 void etcd::KeepAlive::Cancel()
 {
+  std::lock_guard<std::mutex> scope_lock(mutex_for_refresh_);
   if (!continue_next.exchange(false)) {
     return;
   }
@@ -142,6 +143,7 @@ void etcd::KeepAlive::Check() {
 
 void etcd::KeepAlive::refresh()
 {
+  std::lock_guard<std::mutex> scope_lock(mutex_for_refresh_);
   if (!continue_next.load()) {
     return;
   }