Comment 8 for bug 1886169

Supplementary information you may find useful.

The Wi-Fi module on Pi-4 is not able to act as an Access Point *and* scan for wi-fi addresses at the same time. On Raspbian, this is dealt with by (1) specifying a fixed channel; and (2) adding the following line to /etc/dhcpcd.conf:

   nohook wpa_supplicant

and (3) disabling (masking?) the wpa_supplicant service.

If the wpa_supplicant service is running, any attempt to scan for wi-fi devices will take down the access point connection.

For some reason, on Rasbiam (Debian derived), the network manager *fork*s wpa_supplicant in order to do a wifi scan when restarting, if the wpa_supplicant service has been disabled or masked, unless the nohook wpa_supplicant line is added. This happens early in the network manager restart, and can cause various unpleasant symptoms ranging from occasional drops all the way up to a fullblown deathspiral of all network services depending on timing.

I'm not sure what the equivalent fix is on Ubuntu; but my reading of the message is:

  "This device is not able to do wi-fi scans when operating as an access point".

I'm currently running Ubuntu 21.04, which uses a different network stack, I think (no wpa_supplicant, no hostapd), so I can't try workarounds myself. It sounds like 18.04 is using the old network stack, so the raspbian changes will probably work on Ubuntu 18.04 as well.

In the meantime, I'm going to resume my search for the equivalent fix for an almost identical problem on the new network stack used by 21.04. :-P