tailcfg, control/controlclient: make nil MapResponse.DNSConfig mean unchanged (mapver15)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -717,6 +717,7 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*netm
 		}
 	}()
 
+	var lastDNSConfig = new(tailcfg.DNSConfig)
 	var lastDERPMap *tailcfg.DERPMap
 	var lastUserProfile = map[tailcfg.UserID]tailcfg.UserProfile{}
 	var lastParsedPacketFilter []filter.Match
@@ -820,6 +821,9 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*netm
 		if pf := resp.PacketFilter; pf != nil {
 			lastParsedPacketFilter = c.parsePacketFilter(pf)
 		}
+		if c := resp.DNSConfig; c != nil {
+			lastDNSConfig = c
+		}
 
 		if v, ok := resp.CollectServices.Get(); ok {
 			collectServices = v
@@ -846,7 +850,7 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*netm
 			User:            resp.Node.User,
 			UserProfiles:    make(map[tailcfg.UserID]tailcfg.UserProfile),
 			Domain:          resp.Domain,
-			DNS:             resp.DNSConfig,
+			DNS:             *lastDNSConfig,
 			Hostinfo:        resp.Node.Hostinfo,
 			PacketFilter:    lastParsedPacketFilter,
 			CollectServices: collectServices,

tailcfg/tailcfg.go

@@ -37,7 +37,8 @@ import (
 //    12: 2021-03-04: client understands PingRequest
 //    13: 2021-03-19: client understands FilterRule.IPProto
 //    14: 2021-04-07: client understands DNSConfig.Routes and DNSConfig.Resolvers
-const CurrentMapRequestVersion = 14
+//    15: 2021-04-12: client treats nil MapResponse.DNSConfig as meaning unchanged
+const CurrentMapRequestVersion = 15
 
 type StableID string
 
@@ -875,11 +876,8 @@ type MapResponse struct {
 	SearchPaths []string `json:",omitempty"`
 
 	// DNSConfig contains the DNS settings for the client to use.
-	//
-	// TODO(bradfitz): make this a pointer and conditionally sent
-	// only if changed, like DERPMap, PacketFilter, etc. It's
-	// small, though.
-	DNSConfig DNSConfig `json:",omitempty"`
+	// A nil value means no change from an earlier non-nil value.
+	DNSConfig *DNSConfig `json:",omitempty"`
 
 	// Domain is the name of the network that this node is
 	// in. It's either of the form "example.com" (for user