feat: honor cache flush

[lyager]

According to RFC 6762, when the cache-flush bit is set on a RR, all previously cache entries should be set to expire 1 second in the future.

This bit was previously called 'UNIQUE` which might have the same meaning, but cache-flush seems more in line with the RFC

[lyager]

I've had issues with embedded devices changing IP's and appearing with both the previous and current IP. I'm guessing this goes for other records too?

[keepsimple1]

Thanks for your PR! Please see comments inline. And could you please add tests for the new feature?

[lyager]

Thanks for the response, I'll dive into it 👍

[lyager]

I had some trouble constructing a proper test. My idea was to start browsing, similar to the test service_with_temporarily_invalidated_ptr, but I had trouble constructing response packets. I'll give it another shot, but if you have some test code where MDNS response packets are constructed and send out, please let me know :)

[lyager]

Broke a test putting into draft while looking at it.

[keepsimple1]

I had some trouble constructing a proper test. My idea was to start browsing, similar to the test service_with_temporarily_invalidated_ptr, but I had trouble constructing response packets. I'll give it another shot, but if you have some test code where MDNS response packets are constructed and send out, please let me know :)

service_with_temporarily_invalidated_ptr is a good sample to start. The response packet is constructed in to_packet_data called by broadcast_dns_on_intf . And you want to add CLASS_CACHE_FLUSH when creating a record (valid record instead of the invalid one in below):

mdns-sd/src/service_daemon.rs

Lines 2775 to 2784 in 1b9647d

	let invalidate_ptr_packet = DnsPointer::new(
	my_service.get_type(),
	TYPE_PTR,
	CLASS_IN,
	0,
	my_service.get_fullname().to_string(),
	);
	let mut packet_buffer = DnsOutgoing::new(FLAGS_QR_RESPONSE | FLAGS_AA);
	packet_buffer.add_additional_answer(Box::new(invalidate_ptr_packet));

And a different, possible simpler approach is to construct a DnsCache directly, and then call its add_or_update and verify if the changes are applied.

[lyager]

I fixed the already existing test. Thanks for the help with understanding, I'll attempt writing a specific cache-flush test now.

[lyager]

Test added for cache flush - hope it's ok.

[keepsimple1]

Thanks for your updates. Some minor comments inline.

[lyager]

Thanks for the feedback, I've stoppede squashing the commit, if you prefer me to, let me know.

[keepsimple1]

LGTM, thanks !