refactor: simplify passive keepalive #70
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Instead of checking whether we have received a data packet after sending an authenticated one, we can simplify only set `want_keepalive` when we actually receive a data packet.
Instead of modifying the `want_passive_keepalive` variable as we are reading it, we only check it here. It will get modified when we actually send a packet.
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WireGuard has a passive keepalive mechanism. To quote the spec:
This is currently implemented correctly in
boringtunbut it is somewhat convoluted.want_keepaliveboolean.want_keepaliveboolean is checked and directly set to false as part of our timer code. Combining these two things makes the code hard to reason about.We can simplify this greatly by directly tracking the timestamp, when a keepalive is due. The new
want_keepalive_attimer is set every time we receive a data packet and cleared every time we send a packet. Inupdate_timers_at, we simply check ifnowhas surpassed that timer and send a keepalive if that is the case.As a bonus, this functionality is now also unit-tested.