wgengine/magicsock,control/controlclient: do not overwrite discokey with old key (#18606)

When a client starts up without being able to connect to control, it
sends its discoKey to other nodes it wants to communicate with over
TSMP. This disco key will be a newer key than the one control knows
about.

If the client that can connect to control gets a full netmap, ensure
that the disco key for the node not connected to control is not
overwritten with the stale key control knows about.

This is implemented through keeping track of mapSession and use that for
the discokey injection if it is available. This ensures that we are not
constantly resetting the wireguard connection when getting the wrong
keys from control.

This is implemented as:
 - If the key is received via TSMP:
   - Set lastSeen for the peer to now()
   - Set online for the peer to false
 - When processing new keys, only accept keys where either:
   - Peer is online
   - lastSeen is newer than existing last seen

If mapSession is not available, as in we are not yet connected to
control, punt down the disco key injection to magicsock.

Ideally, we will want to have mapSession be long lived at some point in
the near future so we only need to inject keys in one location and then
also use that for testing and loading the cache, but that is a yak for
another PR.

Updates #12639

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

types/events/disco_update.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package events contains type used as eventbus topics in tailscaled.
+package events
+
+import (
+	"net/netip"
+
+	"tailscale.com/types/key"
+)
+
+// DiscoKeyAdvertisement is an event sent on the [eventbus.Bus] when a disco
+// key has been received over TSMP.
+//
+// Its publisher is [tstun.Wrapper]; its main subscriber is
+// [controlclient.Direct], that injects the received key into the netmap as if
+// it was a netmap update from control.
+type DiscoKeyAdvertisement struct {
+	Src netip.Addr // Src field is populated by the IP header of the packet, not from the payload itself.
+	Key key.DiscoPublic
+}
+
+// PeerDiscoKeyUpdate is an event sent on the [eventbus.Bus] when
+// [controlclient.Direct] deems that it cannot handle the key update.
+//
+// Its publisher is [controlclient.Direct]; its main subscriber is
+// [wgengine.userspaceengine], that injects the received key into its
+// [magicsock.Conn] in order to set up the key directly.
+type PeerDiscoKeyUpdate DiscoKeyAdvertisement