fix(mqtt_bridge): ensure `server` key is a binary

Fixes https://emqx.atlassian.net/browse/EMQX-10461

So that it can be JSON encoded correctly.

```
2023-06-30T02:00:41.160110+00:00 [debug] msg: publish_to, mfa: emqx_trace:publish/1, line: 73, topic: b/$, payload: {"topic":"t/1","server":[49,48,46,52,50,46,51,46,49,56,48,58,49,56,56,51],"retain":false,"qos":1,"pub_props":{},"payload":"{\"msg\": \"hello\"}","message_received_at":1688090441159,"id":"0005FF4F2F181488103417000C2E0000","dup":false}, tag: PUBLISH
```

apps/emqx_bridge_mqtt/src/emqx_bridge_mqtt.app.src

@@ -1,7 +1,7 @@
 %% -*- mode: erlang -*-
 {application, emqx_bridge_mqtt, [
     {description, "EMQX MQTT Broker Bridge"},
-    {vsn, "0.1.1"},
+    {vsn, "0.1.2"},
     {registered, []},
     {applications, [
         kernel,

apps/emqx_bridge_mqtt/src/emqx_bridge_mqtt_ingress.erl

@@ -221,7 +221,7 @@ import_msg(
 ) ->
     #{
         id => emqx_guid:to_hexstr(emqx_guid:gen()),
-        server => Server,
+        server => to_bin(Server),
         payload => Payload,
         topic => Topic,
         qos => QoS,
@@ -272,3 +272,6 @@ to_broker_msg(#{dup := Dup} = Msg, Local, Props) ->
             emqx_message:make(bridge, QoS, Topic, Payload)
         )
     ).
+
+to_bin(B) when is_binary(B) -> B;
+to_bin(Str) when is_list(Str) -> iolist_to_binary(Str).

apps/emqx_bridge_mqtt/test/emqx_bridge_mqtt_SUITE.erl

@@ -248,6 +248,50 @@ t_mqtt_conn_bridge_ingress(_) ->
 
     ok.
 
+t_mqtt_conn_bridge_ingress_full_context(_Config) ->
+    User1 = <<"user1">>,
+    IngressConf =
+        emqx_utils_maps:deep_merge(
+            ?INGRESS_CONF,
+            #{<<"local">> => #{<<"payload">> => <<"${.}">>}}
+        ),
+    {ok, 201, _Bridge} = request(
+        post,
+        uri(["bridges"]),
+        ?SERVER_CONF(User1)#{
+            <<"type">> => ?TYPE_MQTT,
+            <<"name">> => ?BRIDGE_NAME_INGRESS,
+            <<"ingress">> => IngressConf
+        }
+    ),
+
+    RemoteTopic = <<?INGRESS_REMOTE_TOPIC, "/1">>,
+    LocalTopic = <<?INGRESS_LOCAL_TOPIC, "/", RemoteTopic/binary>>,
+    Payload = <<"hello">>,
+    emqx:subscribe(LocalTopic),
+    timer:sleep(100),
+    %% PUBLISH a message to the 'remote' broker, as we have only one broker,
+    %% the remote broker is also the local one.
+    emqx:publish(emqx_message:make(RemoteTopic, Payload)),
+    %% we should receive a message on the local broker, with specified topic
+    #message{payload = EncodedPayload} = assert_mqtt_msg_received(LocalTopic),
+    ?assertMatch(
+        #{
+            <<"dup">> := false,
+            <<"id">> := _,
+            <<"message_received_at">> := _,
+            <<"payload">> := <<"hello">>,
+            <<"pub_props">> := #{},
+            <<"qos">> := 0,
+            <<"retain">> := false,
+            <<"server">> := <<"127.0.0.1:1883">>,
+            <<"topic">> := <<"ingress_remote_topic/1">>
+        },
+        emqx_utils_json:decode(EncodedPayload, [return_maps])
+    ),
+
+    ok.
+
 t_mqtt_conn_bridge_ingress_shared_subscription(_) ->
     PoolSize = 4,
     Ns = lists:seq(1, 10),

changes/ce/fix-11174.en.md

@@ -0,0 +1,3 @@
+Fixed the encoding of the `server` key coming from an ingress MQTT bridge.
+
+Before the fix, it was being encoded as a list of integers corresponding to the ASCII characters of the server string.