avoid crashing in the multi-thread operation for std::vector

1. stl container is not thread safety
2. rm_fd() and loop() end to clear the retired event source will in multi-thread operation
3. may be crashed in reaper thread to delete the items in the std::vector as the source is nullptr

src/epoll.cpp

@@ -92,7 +92,9 @@ void zmq::epoll_t::rm_fd (handle_t handle_)
     int rc = epoll_ctl (epoll_fd, EPOLL_CTL_DEL, pe->fd, &pe->ev);
     errno_assert (rc != -1);
     pe->fd = retired_fd;
+    retired_sync.lock ();
     retired.push_back (pe);
+    retired_sync.unlock ();
 
     //  Decrease the load metric of the thread.
     adjust_load (-1);
@@ -180,10 +182,12 @@ void zmq::epoll_t::loop ()
         }
 
         //  Destroy retired event sources.
+        retired_sync.lock ();
         for (retired_t::iterator it = retired.begin (); it != retired.end (); ++it) {
             LIBZMQ_DELETE(*it);
         }
         retired.clear ();
+        retired_sync.unlock ();
     }
 }
 

src/epoll.hpp

@@ -41,6 +41,7 @@
 #include "fd.hpp"
 #include "thread.hpp"
 #include "poller_base.hpp"
+#include "mutex.hpp"
 
 namespace zmq
 {
@@ -102,6 +103,9 @@ namespace zmq
         //  Handle of the physical thread doing the I/O work.
         thread_t worker;
 
+        //  Synchronisation of retired event sources
+        mutex_t retired_sync;
+
         epoll_t (const epoll_t&);
         const epoll_t &operator = (const epoll_t&);
     };