wgengine/magicsock: fix the freshness of endpoints in callmemaybe

Endpoint storage and state updates are all over the place so the full
story is hard to track, but explicitly callmemaybe is executed by a
callback that is triggered before the write of new endpoints into the
storage that CallMeMaybe sends read from. This is now reordered, so that
CallMeMaybe should always observe the most recent endpoints.

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -634,16 +634,17 @@ func (c *Conn) setEndpoints(endpoints []tailcfg.Endpoint) (changed bool) {
 	}
 
 	c.lastEndpointsTime = time.Now()
+	if !endpointSetsEqual(endpoints, c.lastEndpoints) {
+		changed = true
+	}
+	// lastEndpoints must be updated before onEndpointsRefreshed,
+	// as CallMyMaybe sent via this callback must observe the new endpoints.
+	c.lastEndpoints = endpoints
 	for de, fn := range c.onEndpointRefreshed {
 		go fn()
 		delete(c.onEndpointRefreshed, de)
 	}
-
-	if endpointSetsEqual(endpoints, c.lastEndpoints) {
-		return false
-	}
-	c.lastEndpoints = endpoints
-	return true
+	return changed
 }
 
 // SetStaticEndpoints sets static endpoints to the provided value and triggers