fix possible race condition when creating instances with instancemode "single"

Author: irmen

Pyro5/server.py

@@ -249,6 +249,7 @@ def __init__(self, host=None, port=0, unixsocket=None, nathost=None, natport=Non
         self._pyroInstances = {}   # pyro objects for instance_mode=single (singletons, just one per daemon)
         self.streaming_responses = {}   # stream_id -> (client, creation_timestamp, linger_timestamp, stream)
         self.housekeeper_lock = threading.Lock()
+        self.create_single_instance_lock = threading.Lock()
         self.__mustshutdown.clear()
         self.methodcall_error_handler = _default_methodcall_error_handler
 
@@ -574,12 +575,13 @@ def createInstance(clazz, creator):
         instance_mode, instance_creator = clazz._pyroInstancing
         if instance_mode == "single":
             # create and use one singleton instance of this class (not a global singleton, just exactly one per daemon)
-            instance = self._pyroInstances.get(clazz)
-            if not instance:
-                log.debug("instancemode %s: creating new pyro object for %s", instance_mode, clazz)
-                instance = createInstance(clazz, instance_creator)
-                self._pyroInstances[clazz] = instance
-            return instance
+            with self.create_single_instance_lock:
+                instance = self._pyroInstances.get(clazz)
+                if not instance:
+                    log.debug("instancemode %s: creating new pyro object for %s", instance_mode, clazz)
+                    instance = createInstance(clazz, instance_creator)
+                    self._pyroInstances[clazz] = instance
+                return instance
         elif instance_mode == "session":
             # Create and use one instance for this proxy connection
             # the instances are kept on the connection object.

docs/source/changelog.rst

@@ -8,6 +8,7 @@ Change Log
 - added methodcall_error_handler to Daemon that allows you to provide a custom error handler,
   which is called when an exception occurs in the method call's user code
 - introduced ``api.serve`` / ``server.serve`` as a replacement for the static class method ``Daemon.serveSimple``
+- fix possible race condition when creating instances with instancemode "single"
 - introduced some more type hintings
 
 

docs/source/servercode.rst

@@ -605,7 +605,7 @@ There are three possible choices for the ``instance_mode`` parameter:
 
 - ``session``: (the default) a new instance is created for every new proxy connection, and is reused for
   all the calls during that particular proxy session. Other proxy sessions will deal with a different instance.
-- ``single``: a single instance will be created and used for all method calls, regardless what proxy
+- ``single``: a single instance will be created and used for all method calls (for this daemon), regardless what proxy
   connection we're dealing with. This is the same as creating and registering a single object yourself
   (the old style of registering code with the deaemon). Be aware that the methods on this object can be called
   from separate threads concurrently.