Merge branch 'blocking-sockets-fixes'

Closes #17937.

include/wx/msw/private/sockmsw.h

@@ -51,9 +51,25 @@ private:
 
     virtual void UnblockAndRegisterWithEventLoop() wxOVERRIDE
     {
-        // no need to make the socket non-blocking, Install_Callback() will do
-        // it
-        wxSocketManager::Get()->Install_Callback(this);
+        if ( GetSocketFlags() & wxSOCKET_BLOCK )
+        {
+            // Counter-intuitively, we make the socket non-blocking even in
+            // this case as it is necessary e.g. for Read() to return
+            // immediately if there is no data available. However we must not
+            // install a callback for it as blocking sockets don't use any
+            // events and generating them would actually be harmful (and not
+            // just useless) as they would be dispatched by the main thread
+            // while this blocking socket can be used from a worker one, so it
+            // would result in data races and other unpleasantness.
+            unsigned long trueArg = 1;
+            ioctlsocket(m_fd, FIONBIO, &trueArg);
+        }
+        else
+        {
+            // No need to make the socket non-blocking, Install_Callback() will
+            // do it as a side effect of calling WSAAsyncSelect().
+            wxSocketManager::Get()->Install_Callback(this);
+        }
     }
 
     int m_msgnumber;