plugin XEP-0045: don't fail on item-not-found with MAM:

Some servers don't store permanently the whole history of MUC with MAM. As a result, an
"item-not-found" stanza error can be received when message id used as reference doesn't exist
anymore on the server.
When this case happens, the history is retrieved in the same way as for a newly joined room
(i.e. last 50 messages are requested).

sat/plugins/plugin_xep_0045.py

@@ -20,9 +20,9 @@
 from sat.core.i18n import _, D_
 from sat.core.constants import Const as C
 from sat.core.log import getLogger
-log = getLogger(__name__)
 from twisted.internet import defer
 from twisted.words.protocols.jabber import jid
+from twisted.words.protocols.jabber import error as xmpp_error
 from twisted.python import failure
 
 from sat.core import exceptions
@@ -41,6 +41,9 @@ from wokkel import rsm
 from wokkel import mam
 
 
+log = getLogger(__name__)
+
+
 PLUGIN_INFO = {
     C.PI_NAME: "XEP-0045 Plugin",
     C.PI_IMPORT_NAME: "XEP-0045",
@@ -979,8 +982,22 @@ class SatMUCClient(muc.MUCClient):
         complete = False
         count = 0
         while not complete:
-            mam_data = yield self._mam.getArchives(client, mam_req,
-                                                   service=room_jid)
+            try:
+                mam_data = yield self._mam.getArchives(client, mam_req,
+                                                       service=room_jid)
+            except xmpp_error.StanzaError as e:
+                if last_mess and e.condition == 'item-not-found':
+                    log.info(
+                        f"requested item (with id {stanza_id!r}) can't be found in "
+                        f"history of {room_jid}, history has probably been purged on "
+                        f"server.")
+                    # we get last items like for a new room
+                    rsm_req = rsm.RSMRequest(max_=50, before='')
+                    mam_req = mam.MAMRequest(rsm_=rsm_req)
+                    no_loop=True
+                    continue
+                else:
+                    raise e
             elt_list, rsm_response, mam_response = mam_data
             complete = True if no_loop else mam_response["complete"]
             # we update MAM request for next iteration