Fix issue #335

The CreateEvent function requests EVENT_ALL_ACCESS access rights
when the event object already exists. This causes problems
when the event object is created from a service.
The solution is to call OpenEvent function when the CreateEvent
failed due to access control.
The proper solution would be to use CreateEventEx function, but
this one is not available on Windows XP.

src/signaler.cpp

@@ -238,7 +238,13 @@ int zmq::signaler_t::make_fdpair (fd_t *r_, fd_t *w_)
     //  two instances of the library don't accidentally create signaler
     //  crossing the process boundary.
     //  We'll use named event object to implement the critical section.
-    HANDLE sync = CreateEvent (NULL, FALSE, TRUE, TEXT("zmq-signaler-port-sync"));
+    //  Note that if the event object already exists, the CreateEvent requests
+    //  EVENT_ALL_ACCESS access right. If this fails, we try to open
+    //  the event object asking for SYNCHRONIZE access only.
+    HANDLE sync = CreateEvent (NULL, FALSE, TRUE, TEXT ("zmq-signaler-port-sync"));
+    if (sync == NULL && GetLastError () == ERROR_ACCESS_DENIED)
+      sync = OpenEvent (SYNCHRONIZE, FALSE, TEXT ("zmq-signaler-port-sync"));
+
     win_assert (sync != NULL);
 
     //  Enter the critical section.