RPC responses with no dst get stuck in the outgoing queue

[andrey-hinkov]

How to reproduce:

Invoke a RPC method over MQTT but don't provide src key in the request frame, for example:

{ "id": 1, "method": "Sys.GetUID"}

The call will get executed, but since dst is unknown, mg_rpc_channel_mqtt_send_frame() will fail, as expected:

[Jun  7 14:21:35.338] mgos_rpc_channel_mq:129 Cannot reply to RPC over MQTT, no dst: [{"id":1,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}}]
[Jun  7 14:21:35.355] mg_rpc.c:636            0x3ffd6e3c SEND FRAME (109): {"id":1,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}} -> 0

The problem is that mg_rpc_dispatch_frame() will queue the response anyway, despite the fact it can actually never be sent, which means it'll get stuck in the queue:

[Jun 7 14:21:35.369] mg_rpc.c:655 QUEUED FRAME (109): {"id":1,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}}

Eventually, such bad incoming requests can fill the queue with "dead" responses:

[Jun  7 14:22:59.774] mgos_rpc_channel_mq:129 Cannot reply to RPC over MQTT, no dst: [{"id":1,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}}]
[Jun  7 14:22:59.788] mg_rpc.c:636            0x3ffd6e3c SEND FRAME (109): {"id":1,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}} -> 0
[Jun  7 14:22:59.804] mgos_rpc_channel_mq:129 Cannot reply to RPC over MQTT, no dst: [{"id":2,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}}]
[Jun  7 14:22:59.819] mg_rpc.c:636            0x3ffd6e3c SEND FRAME (109): {"id":2,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}} -> 0
[Jun  7 14:22:59.833] mgos_rpc_channel_mq:129 Cannot reply to RPC over MQTT, no dst: [{"id":3,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}}]
[Jun  7 14:22:59.849] mg_rpc.c:636            0x3ffd6e3c SEND FRAME (109): {"id":3,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}} -> 0
[Jun  7 14:22:59.864] mgos_rpc_channel_mq:129 Cannot reply to RPC over MQTT, no dst: [{"id":4,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}}]
[Jun  7 14:22:59.881] mg_rpc.c:636            0x3ffd6e3c SEND FRAME (109): {"id":4,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}} -> 0
[Jun  7 14:22:59.894] mgos_rpc_channel_mq:129 Cannot reply to RPC over MQTT, no dst: [{"id":5,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}}]
[Jun  7 14:22:59.908] mg_rpc.c:636            0x3ffd6e3c SEND FRAME (109): {"id":5,"src":"shelly4pro-f008d1db58a0","result":{"pid": "mos", "app": "FourPro", "uid": "0f3abe5700799b62"}} -> 0

[rojer]

interesting. i think we may not even want to admit a request via mqtt that requires a response (has id) but does not provide src

[iliapenev]

Please check pull request #5