Fix message dropped

Prior to this change, when store_qos0 option is set to false, the hook
`message.dropped` would not be triggered. It is wrong design and when
the ignore_loop is enabled or nl is set to 1, the hook
`message.dropped` also would not be triggered.

src/emqx_mqueue.erl

@@ -124,8 +124,8 @@ stats(#mqueue{max_len = MaxLen, dropped = Dropped} = MQ) ->
 
 %% @doc Enqueue a message.
 -spec(in(message(), mqueue()) -> {maybe(message()), mqueue()}).
-in(#message{qos = ?QOS_0}, MQ = #mqueue{store_qos0 = false}) ->
+in(Msg = #message{qos = ?QOS_0}, MQ = #mqueue{store_qos0 = false}) ->
-    {_Dropped = undefined, MQ};
+    {_Dropped = Msg, MQ};
 in(Msg = #message{topic = Topic}, MQ = #mqueue{default_p = Dp,
                                                p_table = PTab,
                                                q = Q,

src/emqx_session.erl

@@ -812,7 +812,11 @@ is_awaiting_full(#state{awaiting_rel = AwaitingRel,
 %% Dispatch messages
 %%------------------------------------------------------------------------------
 
-handle_dispatch(Msgs, State = #state{inflight = Inflight, subscriptions = SubMap}) ->
+handle_dispatch(Msgs, State = #state{inflight = Inflight,
+                                     client_id = ClientId,
+                                     username = Username,
+                                     subscriptions = SubMap}) ->
+    SessProps = #{client_id => ClientId, username => Username},
     %% Drain the mailbox and batch deliver
     Msgs1 = drain_m(batch_n(Inflight), Msgs),
     %% Ack the messages for shared subscription
@@ -823,7 +827,9 @@ handle_dispatch(Msgs, State = #state{inflight = Inflight, subscriptions = SubMap
                       SubOpts = find_subopts(Topic, SubMap),
                       case process_subopts(SubOpts, Msg, State) of
                           {ok, Msg1} -> [Msg1|Acc];
-                          ignore -> Acc
+                          ignore ->
+                              emqx_hooks:run('message.dropped', [SessProps, Msg]),
+                              Acc
                       end
               end, [], Msgs2),
     NState = batch_process(Msgs3, State),
@@ -957,7 +963,7 @@ enqueue_msg(Msg, State = #state{mqueue = Q, client_id = ClientId, username = Use
     if
         Dropped =/= undefined ->
             SessProps = #{client_id => ClientId, username => Username},
-            ok = emqx_hooks:run('message.dropped', [SessProps, Msg]);
+            ok = emqx_hooks:run('message.dropped', [SessProps, Dropped]);
         true -> ok
     end,
     State#state{mqueue = NewQ}.