With Lua/NodeMCU, how can I wait until >1 mqtt publish calls have been made before running a block of code?

Question

My application involves a battery-powered ESP8266 running NodeMCU for the purpose of updating sensor values periodically over MQTT.

To save battery life, I want to call dsleep() as soon as I'm done with my work. That work might involve more than 1 call to mqqt.Client.publish(). This brings us to the problem I'm facing.

I'm a Lua newbie, but as I understand, the right way to run some code after publish() has finished is to give it a PUBACK callback:

m = mqtt.Client(...)
m.publish("/my/topic", "some message", 1, 0, callback_func)

And in a simple case like above, that works great - even though the actual sending of the MQTT message is async with regard to the publish() call (see a good discussion of this here), callback_func() in the above example will only be called when publish() is done.

But when I have more than 1 call to publish() and want my callback to a) be called after they're all complete, and b) only be called once, I'm stuck.

The naive approach to this would be to put the callback (which is optional) only on the Nth publish() call:

m = mqtt.Client(...)
m.publish("/my/topic", "some message", 1, 0)
m.publish("/another/topic", "unrelated message", 1, 0, callback_func)

But this will not do what's expected. As documented:

NOTE: When calling publish() more than once, the last callback function defined will be called for ALL publish commands.

So in the above example, callback_func() ends up getting called twice (once for each successful publish().

I could combine the multiple publish() calls into a single call, but that feels like an ugly hack, and would have other adverse implications. If my two messages are conceptually distinct, this approach would push logic to separate them into the subscriber - yuck. And if they needed to go to different topics, this would be even worse. There must be a better way.

I thought perhaps mqqt.Client.close() would wait for my different publish() calls to finish, but it doesn't.

I'm out of ideas, and hoping someone with more Lua and/or NodeMCU+mqqt experience can give me a nudge in the right direction.

Here's my actual code, if it helps:

-- prior to this, we've gotten on the wifi network and acquired an IP
dofile("temp.lua") -- provides get_temp()

m = mqtt.Client("clientid", 120, "8266test", "password")

function mainloop(client) 
    print("connected - at top of loop")
    m:publish("uptime",tmr.time(),1,0, function(client) print("sent uptime") end) 
    temp, humi = get_temp()
    if (temp ~= nil) then 
        print(string.format("temp: %d", temp))
        print(string.format("humi: %d", humi))
        m:publish("temp",temp,1,0)
        m:publish("humi",humi,1,0, function(client) -- note: this callback will be used for all publish() calls
            rtctime.dsleep(SLEEP_USEC)
            end)
    end
end

m:on("connect", mainloop)
m:on("offline", function(client) is_connected = false print ("offline") end)
m:connect(MQQT_SVR, 1883, 0, mainloop,
    function(client, reason) print("failed reason: "..reason) end)

Answer 1

Option 1: publish all data at once, then go to sleep.

Option 2: split your callback into two parts. the first part checks if you are done, the second part goes to sleep if you are done. Of course you can solve this differently, count how many are left, count how many you have sent, send and remove items from a list until the list is empty,...

Of course there are more options but these are simple and sufficient.

Edit: example as requested

local totalItemCount = 5

function publishCallback()
  itemsPublished = (itemsPublished or 0) + 1
  print("item published")
  if itemsPublished == totalItemCount then
    print("I'm done, good night!")
  end
end   

for i = 1, totalItemCount do
  publishCallback()
end

---

item published

item published

item published

item published

item published

I'm done, good night!