control/controlclient,ipn/ipnlocal,wgengine: avoid restarting wireguard when key is learned via tsmp (#19142)

When disco keys are learned on a node that is connected to control and
has a mapSession, wgengine will see the key as having changed, and
assume that any existing connections will need to be reset.

For keys learned via TSMP, the connection should not be reset as that
key is learned via an active wireguard connection. If wgengine resets
that connetion, a 15s timeout will occur.

This change adds a map to track new keys coming in via TSMP, and removes
them from the list of keys that needs to trigger wireguard resets. This
is done with an interface chain from controlclient down via localBackend
to userspaceEngine via the watchdog.

Once a key has been actively used for preventing a wireguard reset, the
key is removed from the map.

If mapSession becomes a long lived process instead of being dependent on
having a connection to control. This interface chain can be removed, and
the event sequence from wrap->controlClient->userspaceEngine, can be
changed to wrap->userspaceEngine->controlClient as we know the map will
not be gunked up with stale TSMP entries.

Updates #12639

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

control/controlclient/auto.go

@@ -478,6 +478,27 @@ func (mrs mapRoutineState) UpdateNetmapDelta(muts []netmap.NodeMutation) bool {
 	return err == nil && ok
 }
 
+var _ patchDiscoKeyer = mapRoutineState{}
+
+func (mrs mapRoutineState) PatchDiscoKey(pub key.NodePublic, disco key.DiscoPublic) {
+	c := mrs.c
+	c.mu.Lock()
+	goodState := c.loggedIn && c.inMapPoll
+	dun, ok := c.observer.(patchDiscoKeyer)
+	c.mu.Unlock()
+
+	if !goodState || !ok {
+		return
+	}
+
+	ctx, cancel := context.WithTimeout(c.mapCtx, 2*time.Second)
+	defer cancel()
+
+	c.observerQueue.RunSync(ctx, func() {
+		dun.PatchDiscoKey(pub, disco)
+	})
+}
+
 // mapRoutine is responsible for keeping a read-only streaming connection to the
 // control server, and keeping the netmap up to date.
 func (c *Auto) mapRoutine() {