nodemcu-firmware: MQTT subscribe - large messages get ignored quietly

Expected behavior

NodeMCU should receive every message that has been published with a MQTT topic it is subscribed to.

Actual behavior

If the message is longer than 984 bytes (in my tests with 35 bytes long topic) then such message is not received, it gets ignored quietly. Maybe the real limit is actually sum of topic length and message length, and it seems like it’s less than 1024 bytes. FYI, the topic max size is 64 kB and message max size is 256 MB per MQTT spec. Of course I don’t want an ESP device with 20 kB free heap to process 256 MB message but a hidden/undocumented limit to less than 1 kB is not fun either.

The worst thing here is that no message is received at all, there’s no indication that a message got lost. If would be far better (though still not ideal) if it at least returned first part of the message that fit into an internal buffer…

Test code

mqtt_client = mqtt.Client(1, 120, name, passwd)
mqtt_client:on("message", function(client, topic, data) print(#data) end)
mqtt_client:connect(broker, 1883, 0, 0, function(client)  mqtt_client:subscribe({["#"]=1}) end)

$ printf '%*s\n' 1024 | tr ' ' 'a' |  mosquitto_pub -t x -l

NodeMCU version

current master

Hardware

plain ESP8266-07