Core dump only on debug.level=3 when setting anything via index.html RPC

I’ve noticed a very particular situation where my device core dumps and I’m trying to work out why.
Under normal operation (debug.level=2) it all operates as expected, which is:

  1. Device boots in AP mode with http server enabled
  2. I connect to the device and enter some network credentials
  3. I click “save” which triggers a XMLHttpRequest() that sends wifi.sta data via RPC endpoint
  4. Device reboots and connects to the network

The problem comes when I set debug.level=3. The device core dumps at step 3.

I’ve started with the demo-c project in an attempt to rule out any of the code I’ve added.
main.c is unchanged, mos.yml and index.html changes look like this:

  - ["wifi.ap.pass", ""]
  - ["wifi.ap.enable", true]
  - ["debug.level", 3]
<!DOCTYPE html>

  <button class="scan" id="save">

    save.onclick = function () {
      var request = new XMLHttpRequest();
      var data = JSON.stringify({
        config: {
          wifi: {
            sta: {
              "enable": true,
              "ssid": "test",
              "pass": "1234"
      });'POST', '/rpc/Config.Set', true);
      request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
      request.onload = function () {


Clicking the save button from the webpage with debug.level=3 results in a core dump with no real info:

mg_rpc.c:499            0x3ffc8960 CHAN OPEN (HTTP
[May  8 20:40:13.593] mg_rpc.c:526            0x3ffc8960 GOT PARSED FRAME: '' -> '' 64109454 args '{"config":{"wifi":{"sta":{"enable":true,"ssid":"test","pass":"1234"}}}}'
[May  8 20:40:13.607] mg_rpc.c:314            Config.Set via HTTP
[May  8 20:40:13.613] Guru Meditation Error: Core  1 panic'ed (LoadProhibited). Exception was unhandled.
[May  8 20:40:13.620] 
[May  8 20:40:13.620] Core  1 register dump:

Then debugging the core dump:

Dump contains FreeRTOS task info
Loaded core dump from last snippet in  /core
0x40080400 in ?? ()
Unmapped addr 0xdeadbee3
#0  0x40080400 in ?? ()

When debug.level=2 everything works as expected:

[May  8 22:29:26.455] main.c:12               Tick uptime: 47.19, RAM: 253320, 162256 free
[May  8 22:29:27.400] mg_rpc.c:314            Config.Set via HTTP
[May  8 22:29:27.425] mg_rpc.c:314            Config.Save via HTTP
[May  8 22:29:27.454] main.c:12               Tock uptime: 48.19, RAM: 253312, 160696 free

Any ideas??

I can confirm this for mos 2.20.0. Issue is not present in previous versions. It is not possible to set config when debug is set to 3.

@nliviu any idea how we can make progress with this one?

I’ve attempted to look in the cesanta/mongoose codebase but see no reference to anything like what is getting pumped out in the logs. I’m guessing it’s part of something not open source?

The error comes from here when key==NULL.