ipn/{ipnlocal,localapi}: add API to toggle use of exit node

This is primarily for GUIs, so they don't need to remember the most
recently used exit node themselves.

This adds some CLI commands, but they're disabled and behind the WIP
envknob, as we need to consider naming (on/off is ambiguous with
running an exit node, etc) as well as automatic exit node selection in
the future. For now the CLI commands are effectively developer debug
things to test the LocalAPI.

Updates tailscale/corp#18724

Change-Id: I9a32b00e3ffbf5b29bfdcad996a4296b5e37be7e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/exitnode.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/peterbourgon/ff/v3/ffcli"
 	xmaps "golang.org/x/exp/maps"
+	"tailscale.com/envknob"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/tailcfg"
 )
@@ -25,7 +26,10 @@ var exitNodeCmd = &ffcli.Command{
 	ShortUsage: "exit-node [flags]",
 	ShortHelp:  "Show machines on your tailnet configured as exit nodes",
 	LongHelp:   "Show machines on your tailnet configured as exit nodes",
-	Subcommands: []*ffcli.Command{
+	Exec: func(context.Context, []string) error {
+		return errors.New("exit-node subcommand required; run 'tailscale exit-node -h' for details")
+	},
+	Subcommands: append([]*ffcli.Command{
 		{
 			Name:       "list",
 			ShortUsage: "exit-node list [flags]",
@@ -36,17 +40,51 @@ var exitNodeCmd = &ffcli.Command{
 				fs.StringVar(&exitNodeArgs.filter, "filter", "", "filter exit nodes by country")
 				return fs
 			})(),
-		},
-	},
-	Exec: func(context.Context, []string) error {
-		return errors.New("exit-node subcommand required; run 'tailscale exit-node -h' for details")
-	},
+		}},
+		(func() []*ffcli.Command {
+			if !envknob.UseWIPCode() {
+				return nil
+			}
+			return []*ffcli.Command{
+				{
+					Name:       "connect",
+					ShortUsage: "exit-node connect",
+					ShortHelp:  "connect to most recently used exit node",
+					Exec:       exitNodeSetUse(true),
+				},
+				{
+					Name:       "disconnect",
+					ShortUsage: "exit-node disconnect",
+					ShortHelp:  "disconnect from current exit node, if any",
+					Exec:       exitNodeSetUse(false),
+				},
+			}
+		})()...),
 }
 
 var exitNodeArgs struct {
 	filter string
 }
 
+func exitNodeSetUse(wantOn bool) func(ctx context.Context, args []string) error {
+	return func(ctx context.Context, args []string) error {
+		if len(args) > 0 {
+			return errors.New("unexpected non-flag arguments")
+		}
+		err := localClient.SetUseExitNode(ctx, wantOn)
+		if err != nil {
+			if !wantOn {
+				pref, err := localClient.GetPrefs(ctx)
+				if err == nil && pref.ExitNodeID == "" {
+					// Two processes concurrently turned it off.
+					return nil
+				}
+			}
+		}
+		return err
+	}
+}
+
 // runExitNodeList returns a formatted list of exit nodes for a tailnet.
 // If the exit node has location and priority data, only the highest
 // priority node for each city location is shown to the user.