wgengine/magicsock: send out disco keys over TSMP periodically (#19212)

Instead of sending out disco keys via TSMP once, send them out in
intervals of 60+ seconds. The trigger is still callmemaaybe and the keys
will not be send if no direct connection needs to be established.

This fixes a case where a node can have stale keys but have communicated
with the other peer before, leading to an infinite DERP state.

Updates #12639

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -40,6 +40,11 @@ import (
 var mtuProbePingSizesV4 []int
 var mtuProbePingSizesV6 []int
 
+// discoKeyAdvertisementInterval tells how often a disco update via TSMP can
+// happen. The update is triggered via enqueueCallMeMaybe, and thus it will
+// only be sent if the magicsock is in a state to send out CallMeMaybe.
+const discoKeyAdvertisementInterval = time.Second * 60
+
 func init() {
 	for _, m := range tstun.WireMTUsToProbe {
 		mtuProbePingSizesV4 = append(mtuProbePingSizesV4, pktLenToPingSize(m, false))
@@ -80,7 +85,7 @@ type endpoint struct {
 	lastSendAny               mono.Time      // last time there were outgoing packets sent this peer from any trigger, internal or external to magicsock
 	lastFullPing              mono.Time      // last time we pinged all disco or wireguard only endpoints
 	lastUDPRelayPathDiscovery mono.Time      // last time we ran UDP relay path discovery
-	sentDiscoKeyAdvertisement bool           // whether we sent a TSMPDiscoAdvertisement or not to this endpoint
+	lastDiscoKeyAdvertisement mono.Time      // last time we sent a TSMPDiscoAdvertisement or not to this endpoint
 	derpAddr                  netip.AddrPort // fallback/bootstrap path, if non-zero (non-zero for well-behaved clients)
 
 	bestAddr           addrQuality // best non-DERP path; zero if none; mutate via setBestAddrLocked()