L2SS-465: Lookup nodes in bulk during OPC-UA device initialisation.

Closes L2SS-465

devices/clients/opcua_client.py

@@ -45,6 +45,9 @@ class OPCUAConnection(AsyncCommClient):
         # prefix path to all nodes with this. this allows the user to switch trees more easily.
         self.node_path_prefix = []
 
+        # cache of looked up child node lists for each comma-separated parent path
+        self._node_cache = {}
+
         super().__init__(fault_func, event_loop)
 
     def _servername(self):
@@ -117,13 +120,41 @@ class OPCUAConnection(AsyncCommClient):
 
         return path
 
+    async def get_node(self, path):
+        """ Retrieve an OPC-UA node from either the cache, or the server. """
+
+        if not path:
+            return self.obj
+
+        cache_key = ",".join(path)
+
+        # lookup in cache
+        if cache_key in self._node_cache:
+            return self._node_cache[cache_key]
+
+        # cache it and all of its siblings to save us the round trips for them later on.
+        parent_path = path[:-1]
+        parent_node = await self.obj.get_child(parent_path) if parent_path else self.obj
+        child_nodes = await parent_node.get_children_descriptions()
+
+        for child_node in child_nodes:
+            # add node to the cache
+            child_path = parent_path + [f"{self.name_space_index}:{child_node.DisplayName.Text}"]
+            self._node_cache[",".join(child_path)] = self.client.get_node(child_node.NodeId)
+
+        # lookup in cache again. if the name is valid, it should be in there.
+        if cache_key in self._node_cache:
+            return self._node_cache[cache_key]
+
+        # we couldnt find the requested child, ask server directly to get the appropriate error
+        return await self.obj.get_child(path)
 
     async def setup_attribute(self, annotation, attribute):
         # process the annotation
         path = self.get_node_path(annotation)
 
         try:
-            node = await self.obj.get_child(path)
+            node = await self.get_node(path)
         except Exception as e:
             logger.exception("Could not get node: %s on server %s", path, self._servername())
             raise Exception("Could not get node: %s on server %s", path, self._servername()) from e
@@ -155,7 +186,7 @@ class OPCUAConnection(AsyncCommClient):
 
 
     async def call_method(self, method_path, *args):
-        node = await self.obj.get_child(method_path[:-1])
+        node = await self.get_node(method_path[:-1])
         return await node.call_method(method_path[-1], *args)