khepri_tree: Ignore unknown props to return

[Why]
A future version of Khepri might introduce new props to return. If a
Khepri cluster is being upgraded and several versions of the Khepri
machine are running at the same time, an old node could be called with
props to return it doesn’t know.

[How]
We simply ignore unknown props.

In the process, we ignore `delete_reason` because it is set by another
part of the code anyway.

src/khepri_tree.erl

@@ -234,7 +234,9 @@ gather_node_props(#node{props = #{payload_version := PVersion,
               end;
           (raw_payload, Acc) ->
               Acc#{raw_payload => Payload};
-          (delete_reason, Acc) ->
+          (_Unknown, Acc) ->
+              %% We ignore props we don't know about. It might be a new one in
+              %% a future version of the machine.
               Acc
       end, #{}, WantedProps);
 gather_node_props(#node{}, _Options) ->

test/queries.erl

@@ -399,3 +399,17 @@ include_child_names_in_query_response_test() ->
                          child_list_version => 2,
                          child_names => [bar, quux]}}},
        Ret).
+
+use_an_unknown_prop_to_return_test() ->
+    Commands = [#put{path = [foo],
+                     payload = khepri_payload:data(value)}],
+    S0 = khepri_machine:init(?MACH_PARAMS(Commands)),
+    Tree = khepri_machine:get_tree(S0),
+    Ret = khepri_tree:find_matching_nodes(
+            Tree, [foo],
+            #{props_to_return => [payload,
+                                  future_prop_name]}),
+
+    ?assertEqual(
+       {ok, #{[foo] => #{data => value}}},
+       Ret).