Abuse `esp_mesh_scan_get_ap_record` to reduce stack use of `scan_results`

[bugadani]

Based on #221. I suspect relying on weird internal implementation details (i.e. that we can just call a mesh api function without actually using wifi mesh) might not be very elegant, but it works. We probably will need to flush the scan results still, I just wanted to upload the initial findings :)

It looks like C2 will need to remain on the non-mesh code.

My main worry is that any new binary blob may break this. Now, the binaries esp-wifi uses are in our hands, so it's possible to test for such breakage, but it is probably additional work on each blob update.

[bjoernQ]

Interesting but .... mhhhh - I'm not super convinced to be honest

The fact it doesn't work for all chips already doubles that. As you already said "My main worry is that any new binary blob may break this." - I agree we can handle that but it potentially adds additional trouble when updating the blobs (which sometimes is already enough trouble)

But the main thing is that it doesn't get us too much. With 10 records requested it looks like this

<esp_wifi::wifi::WifiController as embedded_svc::wifi::Wifi>::scan_n::hc2359ba1e996a506
code=1862 stack=Some(1504)

Given scan probably isn't used too much after wifi-provisioning (if at all) and the fact that there are much worse functions (I plan to work on that soon) I'm not sure if it's worth the potential trouble this might cause

[bugadani]

Well I'm not fighting for this, though I really would like to see a non-heapless::Vec-bound "give me the results as an iterator" API eventually. This PR kind of reflects what I have in mind, if only we had a stable API from the wifi libs.

Oh well, one can hope :)

Also, I think this isn't too much trouble to keep rebased, so I might just do that in case someone wants it.

~stack=Some(1504) is pretty substantial, given that we have 8kB of stack space defined in the .lds.~ This problem has been solved since writing this comment.

[bugadani]

@MabezDev have you been able to find out anything about this? (i.e. if this function is planned to be further restricted or if the relevant team could make it instead more generally available for "normal wifi" and, e.g. for the C2?)

[MabezDev]

Good news, with the latest WiFi blobs the API is available. The relevant pull request is yet to land in esp-idf, but the signature of the function is as follows:

esp_err_t esp_wifi_scan_get_ap_record(wifi_ap_record_t *ap_record);

The next time we update the blobs we can modify this PR and merge it :).

[MabezDev]

The blobs have been updated and the API is now available.

[bugadani]

Noted; I'm a bit swamped right now but if nobody jumps in I will get to this... eventually

[bjoernQ]

Thanks! esp-wifi is moved into the esp-hal repository. If you want to continue working on this please consider re-opening the PR there