Use multicast to wait for new scan results

[Sh3Rm4n]

This extends the example to wait for new scan results via the multicast object.

This might be to low level, but provides a usecase to abstract multicast attachment.

A proper abstraction shouldn't probably be part of this crate but rather genetlink I assume.

[cathay4t]

Nice work.

The iw code said we should use another socket for multicast event, even through I don't understand it:

	/*
	 * WARNING: DO NOT COPY THIS CODE INTO YOUR APPLICATION
	 *
	 * This code has a bug, which requires creating a separate
	 * nl80211 socket to fix:
	 * It is possible for a NL80211_CMD_NEW_SCAN_RESULTS or
	 * NL80211_CMD_SCAN_ABORTED message to be sent by the kernel
	 * before (!) we listen to it, because we only start listening
	 * after we send our scan request.
	 *
	 * Doing it the other way around has a race condition as well,
	 * if you first open the events socket you may get a notification
	 * for a previous scan.
	 *
	 * The only proper way to fix this would be to listen to events
	 * before sending the command, and for the kernel to send the
	 * scan request along with the event, so that you can match up
	 * whether the scan you requested was finished or aborted (this
	 * may result in processing a scan that another application
	 * requested, but that doesn't seem to be a problem).
	 *
	 * Alas, the kernel doesn't do that (yet).
	 */

It seems to me your example code is doing exactly what iw developer suggest not to do.

iw comment indicate we should not reuse the socket for both requesting scan and wait scan result.

[cathay4t]

For abstraction, you can do:

• genetlink::new_multicast_connection().
• wl_nl80211::new_multicast_connection().

[Sh3Rm4n]

Nice work.

The iw code said we should use another socket for multicast event, even through I don't understand it:

// ...

It seems to me your example code is doing exactly what iw developer suggest not to do.

I've also stumbled upon this comment. But I assume wl_nl80211 does not have the problem, as

1. The connection runs concurrently via the tokio::spawn, which is reponsable to push the messages to the UnboundReceiver (aka. messages) and
2. the scan is called after the socket (connection) is already polled.

From my experience with this approach: I seem to get the messages consistently.

For abstraction, you can do:

* `genetlink::new_multicast_connection()`.

* `wl_nl80211::new_multicast_connection()`.

I've only enabled the "nl80211" "scan" group for now. But there are many more groups available

Should I just enable all groups as it is done in iw event or should this function take an argument specifying which groups should be enabled?

Something similar to that?

enum Family {
  Nl80211,
  /// ...
}

enum Nl80211Groups {
  Config,
  Scan,
  Regulatory,
  // ...
}

wl_nl80211::new_multicast_connection(groups: HashSet<Nl80211Groups>) -> Result<...>;

genetlink::new_multicast_connection(family: Family, groups: GenlGroups<Nl80211Groups>) -> Result<...>;

[Sh3Rm4n]

This rust-netlink/genetlink#12 seems to be very useful to have before introducing a new_multicast_connection method.

[cathay4t]

Let's wait genetlink to provide multicast support there.