plugin XEP-0313: fallback to last N items in case of missing item:

If a reference item is missing (e.g., after server purge) we now fallback to last N items
(50 for now), instead of giving up.

libervia/backend/plugins/plugin_xep_0313.py

@@ -171,12 +171,27 @@ class XEP_0313:
             assert mam_req.rsm is not None
             complete = False
             count = 0
+            use_fallback = False
             while not complete:
                 try:
                     mam_data = await self.get_archives(
                         client, mam_req, service=client.jid.userhostJID()
                     )
                 except StanzaError as e:
+                    if (
+                        stanza_id is not None
+                        and not use_fallback
+                        and e.condition == "item-not-found"
+                    ):
+                        log.warning(
+                            f"Requested item (with stanza_id {stanza_id!r}) can't be "
+                            f"found in history, archive has probably been purged on "
+                            f"server. Falling back to last N items."
+                        )
+                        use_fallback = True
+                        rsm_req = rsm.RSMRequest(max_=50, before="")
+                        mam_req = mam.MAMRequest(rsm_=rsm_req)
+                        continue
                     log.warning(
                         f"Can't retrieve MAM archives: {e}\n"
                         f"{xml_tools.p_fmt_elt(mam_req.toElement())}\n"