What to mock and what to fake

[ivmarkov]

rs-matter does contain code that depends on OS services which might be difficult to bring up for unit / integration testing.

Some examples, where rs-matter already does have either mocks, or fakes:

• UDP; The e2e (= end-to-end) framework fakes UDP with a zerocopy channel and enables unit-tests-as-integration-tests where two instances of rs-matter can talk to each other over the faked UDP for testing purposes
• BTP; The BTP protocol is using a manually-crafted GattPeripheral mock for testing

Places we might or might not need to fill-in with tests:

mDNS

• The built-in mDNS responder. I would say, rather unit tests here, for the mDNS packet parsing and assembly logic
• External mDNS providers (zeroconf, avahi-over-zbus, resolved-over-zbus). Questionable. The logic to be tested with mocks is maybe a bit too simple?

BT/BlueZ over zbus

• ? ~ 1000LOCs, relatively non-trivial but then most of the logic is around the complicated "register your own objects in dBus" that BlueZ wants from us

Wifi/WPA Supplicant over zbus

• ? ~ 500 LOCs

Wifi/Network Manager over zbus

• ? ~ 500 LOCs

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Utility/system clusters and the overall logic of farics, ACLs etc., which is not covered with unit tests or the e2e tests:

• I hope these would be covered by addressing [TESTS] End-to-end tests #187 which we anyway urgently need
• This is IMO top priority

CC @gmarcosb