Allow message id override on merge

When performing a broadcast merge it might be the
case that the application wants to return a new message
id that results from the merge and have that broadcast instead
of the originally received message id. Support this by handling
an extra return tuple from the callback.

Author: lrascao

src/plumtree_broadcast.erl

@@ -354,6 +354,8 @@ handle_broadcast(false, _MessageId, _Message, Mod, _Round, Root, From, State) ->
     State1 = add_lazy(From, Root, State),
     _ = send({prune, Root, myself()}, Mod, From),
     State1;
+handle_broadcast({true, MessageId}, _OldMessageId, Message, Mod, Round, Root, From, State) ->
+    handle_broadcast(true, MessageId, Message, Mod, Round, Root, From, State);
 handle_broadcast(true, MessageId, Message, Mod, Round, Root, From, State) -> %% valid msg
     %% remove sender from lazy and set as eager
     plumtree_util:log(debug, "moving peer ~p from lazy to eager on tree rooted at ~p",

src/plumtree_broadcast_handler.erl

@@ -24,7 +24,8 @@
 
 %% Given the message id and payload, merge the message in the local state.
 %% If the message has already been received return `false', otherwise return `true'
--callback merge(any(), any()) -> boolean().
+%% If a new message id is to be propagated after the merge return `{true, MessageId}`
+-callback merge(any(), any()) -> boolean() | {true, any()}.
 
 %% Return true if the message (given the message id) has already been received.
 %% `false' otherwise

test/plumtree_test_broadcast_handler.erl

@@ -133,7 +133,7 @@ merge({Key, _Context} = MsgId, RemoteObj) ->
             lager:info("merge object has ben reconciled to ~p",
                       [Reconciled]),
             dbwrite(Key, Reconciled),
-            true
+            {true, {Key, plumtree_test_object:context(Reconciled)}}
     end.
 
 %% Return true if the message (given the message id) has already been received.