fix: improve behavior when deleting mirrored record from farmOS

in some cases (maybe just dev?) the record does not exist in farmOS;
if so we should silently ignore.

and there seemed to be a problem with the sequence of events:

- user clicks delete in WF
- record is deleted from WF DB
- delete request sent to farmOS API
- webhook on farmOS side calls back to WF webhook URI

somewhere in there, in practice things seemed to hang after user
clicks delete.  i suppose the thread handling user's request is "tied
up" somehow, such that the webhook receiver can't process that
request?  that doesn't exactly make sense to me, but if we split off
to a separate thread to request the farmOS deletion, things seem to
work okay.  so maybe that idea is more accurate than i'd expect

src/wuttafarm/web/views/master.py

@@ -23,6 +23,9 @@
 Base class for WuttaFarm master views
 """
 
+import threading
+
+import requests
 from webhelpers2.html import tags
 
 from wuttaweb.views import MasterView
@@ -145,10 +148,24 @@ class WuttaFarmMasterView(MasterView):
 
         # maybe delete from farmOS also
         if farmos_uuid:
+            client = get_farmos_client_for_user(self.request)
+            # nb. must use separate thread to avoid some kind of race
+            # condition (?) - seems as though maybe a "boomerang"
+            # effect is happening; this seems to help anyway
+            thread = threading.Thread(
+                target=self.delete_from_farmos, args=(client, farmos_uuid)
+            )
+            thread.start()
+
+    def delete_from_farmos(self, client, farmos_uuid):
         entity_type = self.get_farmos_entity_type()
         bundle = self.get_farmos_bundle()
-            client = get_farmos_client_for_user(self.request)
+        try:
             client.resource.delete(entity_type, bundle, farmos_uuid)
+        except requests.HTTPError as exc:
+            # ignore if record not found in farmOS
+            if exc.response.status_code != 404:
+                raise
 
 
 class TaxonomyMasterView(WuttaFarmMasterView):