Callbacks are called from other threads

[daurnimator]

When in async mode, callbacks seem to be called when mosquitto feels like it, which is entirely undefined behaviour.

local mqtt = require "mosquitto"
local gettid do
    local ffi = require "ffi" -- use luajit or https://github.com/facebookarchive/luaffifb
    ffi.cdef[[int syscall(int);]]
    gettid = function()
        return ffi.C.syscall(186)
    end
end
local client = mqtt.new()
client:callback_set("ON_CONNECT", function()
    print("connected", gettid())
    client:subscribe("$SYS/#")
end)
client:callback_set("ON_MESSAGE", function(mid, topic, payload)
    print("message", gettid(), mid, topic, payload)
end)
local broker = arg[1] or "localhost"
assert(client:connect_async(broker))
assert(client:loop_start())

client:loop_read()
client:loop_read()
client:loop_write()
client:loop_misc()
client:loop_read()
client:loop_read()
client:loop_write()
client:loop_misc()
client:loop_read()
client:loop_read()
client:loop_write()

[karlp]

your code here is completely wrong. loop_start starts a new thread. you then do not keep trying to call loop_read/write/misc yourself. the callback is called from the loop thread yes, this is well explained in the mosquitto reference

[daurnimator]

your code here is completely wrong. loop_start starts a new thread. you then do not keep trying to call loop_read/write/misc yourself. the callback is called from the loop thread yes, this is well explained in the mosquitto reference

Ah oops. I misread what :loop_start was for. Looks like I just call :loop_read/write/misc as required.

[daurnimator]

So it appears that :loop_start is always unsafe to use in the style that lua-mosquitto binds it. Could you remove it from the API?

[karlp]

You're perfectly free to implement your own locking and access controls, and there are totally uses where you don't need anything shared, where having it called from other threads is completely ok. I don't see any reason to just go and flat remove the binding, that seems overreaching.