plugin pubsub cache: new resync argument to force resynchronisation in synchronize

sat/plugins/plugin_pubsub_cache.py

@@ -707,16 +707,17 @@ class PubsubCache:
         self,
         client: SatXMPPEntity,
         service: jid.JID,
-        node: str
+        node: str,
+        resync: bool = True
     ) -> None:
         """Synchronise a node with a pubsub service
 
-        If the node is already synchronised, it will be resynchronised (all items will be
-        deleted and re-downloaded).
-
         The node will be synchronised even if there is no matching analyser.
 
         Note that when a node is synchronised, it is automatically subscribed.
+        @param resync: if True and the node is already synchronised, it will be
+            resynchronised (all items will be deleted and re-downloaded).
+
         """
         pubsub_node = await self.host.memory.storage.getPubsubNode(
             client, service, node
@@ -735,6 +736,9 @@ class PubsubCache:
                 analyser=analyse.get("name"),
                 type_=analyse.get("type"),
             )
+        elif not resync:
+                # the node exists, nothing to do
+                return
 
         if ((pubsub_node.sync_state == SyncState.IN_PROGRESS
              or (service, node) in self.in_progress)):