Respect write-locks in synchronous Websocket events (#1170)

panel/io/server.py

@@ -76,9 +76,12 @@ def unlocked():
         events = []
         for conn in connections:
             socket = conn._socket
+            if hasattr(socket, 'write_lock') and socket.write_lock._block._value == 0:
+                state._locks.add(socket)
+            locked = socket in state._locks
             for event in curdoc._held_events:
                 if (isinstance(event, ModelChangedEvent) and event not in old_events
-                    and hasattr(socket, 'write_message')):
+                    and hasattr(socket, 'write_message') and not locked):
                     msg = conn.protocol.create('PATCH-DOC', [event])
                     WebSocketHandler.write_message(socket, msg.header_json)
                     WebSocketHandler.write_message(socket, msg.metadata_json)

panel/io/state.py

@@ -5,6 +5,8 @@
 
 import threading
 
+from weakref import WeakSet
+
 import param
 
 from bokeh.document import Document
@@ -49,6 +51,9 @@ class _state(param.Parameterized):
     # Jupyter display handles
     _handles = {}
 
+    # Stores a set of locked Websockets, reset after every change event
+    _locks = WeakSet()
+
     def __repr__(self):
         server_info = []
         for server, panel, docs in self._servers.values():

panel/viewable.py

@@ -772,6 +772,7 @@ def _comm_change(self, msg, ref=None):
             self._changing = {}
 
     def _server_change(self, doc, ref, attr, old, new):
+        state._locks.clear()
         self._events.update({attr: new})
         if not self._processing:
             self._processing = True