diff -r 2e38b5ec4753 -r d9175daf6522 runtime/Worker.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/runtime/Worker.py	Tue Aug 21 16:11:02 2018 +0200
@@ -0,0 +1,110 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+# This file is part of Beremiz runtime.
+#
+# Copyright (C) 2018: Edouard TISSERANT
+#
+# See COPYING.Runtime file for copyrights details.
+
+from __future__ import absolute_import
+import thread
+from threading import Lock, Condition
+
+class job(object):
+    """
+    job to be executed by a worker
+    """
+    def __init__(self, call, *args, **kwargs):
+        self.job = (call, args, kwargs)
+        self.result = None
+        self.success = False
+        self.exc_info = None
+
+    def do(self):
+        """
+        do the job by executing the call, and deal with exceptions
+        """
+        try:
+            call, args, kwargs = self.job
+            self.result = call(*args, **kwargs)
+            self.success = True
+        except Exception:
+            self.success = False
+            self.exc_info = sys.exc_info()
+
+
+class worker(object):
+    """
+    serialize main thread load/unload of PLC shared objects
+    """
+    def __init__(self):
+        # Only one job at a time
+        self._finish = False
+        self._threadID = None
+        self.mutex = Lock()
+        self.todo = Condition(self.mutex)
+        self.done = Condition(self.mutex)
+        self.free = Condition(self.mutex)
+        self.job = None
+
+    def runloop(self, *args, **kwargs):
+        """
+        meant to be called by worker thread (blocking)
+        """
+        self._threadID = thread.get_ident()
+        if args or kwargs:
+            job(*args, **kwargs).do()
+            # result is ignored
+        self.mutex.acquire()
+        while not self._finish:
+            self.todo.wait()
+            if self.job is not None:
+                self.job.do()
+                self.done.notify()
+            else:
+                self.free.notify()
+        self.mutex.release()
+
+    def call(self, *args, **kwargs):
+        """
+        creates a job, execute it in worker thread, and deliver result.
+        if job execution raise exception, re-raise same exception
+        meant to be called by non-worker threads, but this is accepted.
+        blocking until job done
+        """
+
+        _job = job(*args, **kwargs)
+
+        if self._threadID == thread.get_ident() or self._threadID is None:
+            # if caller is worker thread execute immediately
+            _job.do()
+        else:
+            # otherwise notify and wait for completion
+            self.mutex.acquire()
+
+            while self.job is not None:
+                self.free.wait()
+
+            self.job = _job
+            self.todo.notify()
+            self.done.wait()
+            _job = self.job
+            self.job = None
+            self.mutex.release()
+
+        if _job.success:
+            return _job.result
+        else:
+            raise _job.exc_info[0], _job.exc_info[1], _job.exc_info[2]
+
+    def quit(self):
+        """
+        unblocks main thread, and terminate execution of runloop()
+        """
+        # mark queue
+        self._finish = True
+        self.mutex.acquire()
+        self.job = None
+        self.todo.notify()
+        self.mutex.release()