cmd/tailscale: add start of "debug derp" subcommand

Updates #6526

Change-Id: I84e440a8bd837c383000ce0cec4ff36b24249e8b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -37,6 +37,7 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/must"
 	"tailscale.com/util/strs"
 )
 
@@ -160,6 +161,11 @@ var debugCmd = &ffcli.Command{
 				return fs
 			})(),
 		},
+		{
+			Name:      "derp",
+			Exec:      runDebugDERP,
+			ShortHelp: "test a DERP configuration",
+		},
 	},
 }
 
@@ -610,3 +616,15 @@ func runDevStoreSet(ctx context.Context, args []string) error {
 	}
 	return localClient.SetDevStoreKeyValue(ctx, key, val)
 }
+
+func runDebugDERP(ctx context.Context, args []string) error {
+	if len(args) != 1 {
+		return errors.New("usage: debug derp <region>")
+	}
+	st, err := localClient.DebugDERPRegion(ctx, args[0])
+	if err != nil {
+		return err
+	}
+	fmt.Printf("%s\n", must.Get(json.MarshalIndent(st, "", " ")))
+	return nil
+}

cmd/tailscale/depaware.txt

@@ -108,6 +108,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/util/lineread                                  from tailscale.com/net/interfaces+
         tailscale.com/util/mak                                       from tailscale.com/net/netcheck+
         tailscale.com/util/multierr                                  from tailscale.com/control/controlhttp
+        tailscale.com/util/must                                      from tailscale.com/cmd/tailscale/cli
         tailscale.com/util/quarantine                                from tailscale.com/cmd/tailscale/cli
         tailscale.com/util/singleflight                              from tailscale.com/net/dnscache
         tailscale.com/util/strs                                      from tailscale.com/hostinfo+