srsenb,phy: do not get reference to ul_grants, do a copy instead to protect concurrent access

srsenb/hdr/phy/phy_common.h

@@ -227,7 +227,7 @@ public:
   void set_mch_period_stop(uint32_t stop);
 
   // Getters and setters for ul grants which need to be shared between workers
-  const stack_interface_phy_lte::ul_sched_list_t& get_ul_grants(uint32_t tti);
+  const stack_interface_phy_lte::ul_sched_list_t get_ul_grants(uint32_t tti);
   void set_ul_grants(uint32_t tti, const stack_interface_phy_lte::ul_sched_list_t& ul_grants);
   void clear_grants(uint16_t rnti);
 

srsenb/src/phy/phy_common.cc

@@ -85,7 +85,7 @@ void phy_common::clear_grants(uint16_t rnti)
   }
 }
 
-const stack_interface_phy_lte::ul_sched_list_t& phy_common::get_ul_grants(uint32_t tti)
+const stack_interface_phy_lte::ul_sched_list_t phy_common::get_ul_grants(uint32_t tti)
 {
   std::lock_guard<std::mutex> lock(grant_mutex);
   return ul_grants[tti];