Add support for esp32 network driver scanning for access points

[UncleGrumpy]

Add network:wifi_scan/0,1 to esp32 network driver, giving the ability for devices configured for "station mode" (or with "station + access point mode") to scan for available access points.

note: these changes depend on PR #1181

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later

[petermm]

I struggled here:

I started out empty example and tried:

    scan = :network.wifi_scan()
    IO.inspect(scan)

This crashes with: {noproc,{gen_server,call,[network,get_config]}} - looks like there is a code path for handling the error but it's never reached

Then I read docs, about having to start the network, and tried:

 config = [
      {:sta,
       [
         {:ssid, "Wokwi-GUEST"},
         {:psk, ""},
         {:connected,
          fn ->
            IO.inspect("network CONNECTED")
            :ok
          end}
       ]}
    ]

    case :network.start(config) do
      {:ok, _pid} ->
        IO.inspect("network started")
      error ->
        :io.put_chars("\nAn error occurred starting network:")
        :erlang.display(error)
    end

    scan = :network.wifi_scan()
    IO.inspect(scan)

which crashed with: wifi:sta_scan: STA is connecting, scan are not allowed!

delaying things and scanning after connection is done made it further to a crash:

CRASH 
======
pid: <0.1.0>

Stacktrace:
undefined

cp: #CP<module: 11, label: 31, offset: 26>

x[0]: error
x[1]: badarg
x[2]: error

Stack 
------

#CP<module: 11, label: 32, offset: 26>
<<"0">>
#CP<module: 11, label: 24, offset: 20>
#CP<module: 10, label: 11, offset: 11>
{0,[{none,[{rssi,undefined},{authmode,undefined},{channel,undefined}]}]}

It also seems that connecting to a network not available, disallows scanning with: wifi:sta_scan: STA is connecting, scan are not allowed!

Would be great seeing an example that could be used in CI eg:

1. scan with no network configured
2. connect known good network - scan
3. config known unavailable network - scan
4. probably some ap stuff
   etc.

I'll work on getting wokwi CI going so this can be covered by CI.

[UncleGrumpy]

I did not add an example yet, my plan was to write a demonstration of “roaming” for atomvm_examples. It is a little difficult to use until #1181 is merged, that will allow starting WiFi without immediately starting a connection. Scans will always fail when a connection is in progress, so your application will need to start a scan after obtaining an IP address, or before any connection is started (which is not possible until after #1181).

[UncleGrumpy]

I wish your second example had a stacktrace! You can see the scan result did come back with no networks found… this can happen with default scans (the dwell time is very short by default - and can easily miss networks, in my experience). I think it’s just a bad match in your test that is crashing there.

[petermm]

I wish your second example had a stacktrace! You can see the scan result did come back with no networks found… this can happen with default scans (the dwell time is very short by default - and can easily miss networks, in my experience). I think it’s just a bad match in your test that is crashing there.

ohh the embarrassment lol - it's the IO.inspect call that crashes, will look into it - used to throwing anything at IO.inspect..

Makes sense with the other stuff, suppose some kind of WifiManager GenServer will materialize, while this PR is on driver primitives level..

I'll test #1181 and help land that first..

[UncleGrumpy]

My plan precisely! The PRs I have already submitted will provide all the necessary low level functionality, but I would like to create a higher level network_manager module that can simplify configuration and orchestration.

[UncleGrumpy]

As far as “dwell” time for the scan, I have found between 300-500ms will find all of the available networks, with the default (120ms) I do notice networks being frequently missed.

[UncleGrumpy]

it's the IO.inspect call that crashes, will look into it - used to throwing anything at IO.inspect..

That does sound like a bug, from my limmited understanding of Elixir you should be able to give it just about anything, much like erlang:display/1.

[UncleGrumpy]

{0,[{none,[{rssi,undefined},{authmode,undefined},{channel,undefined}]}]}

You did point me to a problem here, I started testing more boards and realized that even with maximum dwell time no network are being found. I cherry-picked these commits from a different local branch and must have missed something.

[UncleGrumpy]

When I split up the working branch I was testing new network features on I mistakenly submitted this PR first, but in order to use the scan function PR #1181 needs to be merged first, and this will need to be rebased.

[UncleGrumpy]

{0,[{none,[{rssi,undefined},{authmode,undefined},{channel,undefined}]}]}

I did find a bug that would cause the results to be empty if only a single network was found, during most of my testing I had multiple networks, so this didn't get caught. @petermm, thanks for testing and helping me correct this!