ipn: another controlplane synonym

This one doesn't bother me so much, as long term we want a synonym here.

Fixes #2384
Fixes #2386

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -735,7 +735,7 @@ func TestUpdatePrefs(t *testing.T) {
 			wantSimpleUp:   true,
 			wantJustEditMP: &ipn.MaskedPrefs{WantRunningSet: true},
 		},
-		/* TODO(crawshaw): fix, #2384 {
+		{
 			name:  "control_synonym",
 			flags: []string{},
 			curPrefs: &ipn.Prefs{
@@ -745,7 +745,22 @@ func TestUpdatePrefs(t *testing.T) {
 			env:            upCheckEnv{backendState: "Running"},
 			wantSimpleUp:   true,
 			wantJustEditMP: &ipn.MaskedPrefs{WantRunningSet: true},
-		},*/
+		},
+		{
+			name:  "change_login_server",
+			flags: []string{"--login-server=https://localhost:1000"},
+			curPrefs: &ipn.Prefs{
+				ControlURL:       "https://login.tailscale.com",
+				Persist:          &persist.Persist{LoginName: "crawshaw.github"},
+				AllowSingleHosts: true,
+				CorpDNS:          true,
+				NetfilterMode:    preftype.NetfilterOn,
+			},
+			env:            upCheckEnv{backendState: "Running"},
+			wantSimpleUp:   true,
+			wantJustEditMP: &ipn.MaskedPrefs{WantRunningSet: true},
+			wantErrSubtr:   "can't change --login-server without --force-reauth",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

cmd/tailscale/cli/up.go

@@ -259,7 +259,8 @@ func updatePrefs(prefs, curPrefs *ipn.Prefs, env upCheckEnv) (simpleUp bool, jus
 		}
 	}
 
-	controlURLChanged := curPrefs.ControlURL != prefs.ControlURL
+	controlURLChanged := curPrefs.ControlURL != prefs.ControlURL &&
+		!(ipn.IsLoginServerSynonym(curPrefs.ControlURL) && ipn.IsLoginServerSynonym(prefs.ControlURL))
 	if controlURLChanged && env.backendState == ipn.Running.String() && !env.upArgs.forceReauth {
 		return false, nil, fmt.Errorf("can't change --login-server without --force-reauth")
 	}
@@ -399,7 +400,7 @@ func runUp(ctx context.Context, args []string) error {
 				startLoginInteractive()
 			case ipn.NeedsMachineAuth:
 				printed = true
-				fmt.Fprintf(os.Stderr, "\nTo authorize your machine, visit (as admin):\n\n\t%s/admin/machines\n\n", upArgs.server)
+				fmt.Fprintf(os.Stderr, "\nTo authorize your machine, visit (as admin):\n\n\t%s\n\n", prefs.AdminPageURL())
 			case ipn.Starting, ipn.Running:
 				// Done full authentication process
 				if printed {
@@ -608,7 +609,7 @@ func checkForAccidentalSettingReverts(newPrefs, curPrefs *ipn.Prefs, env upCheck
 		if reflect.DeepEqual(valCur, valNew) {
 			continue
 		}
-		if flagName == "login-server" && isLoginServerSynonym(valCur) && isLoginServerSynonym(valNew) {
+		if flagName == "login-server" && ipn.IsLoginServerSynonym(valCur) && ipn.IsLoginServerSynonym(valNew) {
 			continue
 		}
 		missing = append(missing, fmtFlagValueArg(flagName, valCur))
@@ -657,10 +658,6 @@ func applyImplicitPrefs(prefs, oldPrefs *ipn.Prefs, curUser string) {
 	}
 }
 
-func isLoginServerSynonym(val interface{}) bool {
-	return val == "https://login.tailscale.com" || val == "https://controlplane.tailscale.com"
-}
-
 func flagAppliesToOS(flag, goos string) bool {
 	switch flag {
 	case "netfilter-mode", "snat-subnet-routes":

ipn/handle.go

@@ -156,7 +156,7 @@ func (h *Handle) Expiry() time.Time {
 }
 
 func (h *Handle) AdminPageURL() string {
-	return h.prefsCache.ControlURLOrDefault() + "/admin/machines"
+	return h.prefsCache.AdminPageURL()
 }
 
 func (h *Handle) StartLoginInteractive() {

ipn/prefs.go

@@ -25,11 +25,17 @@ import (
 
 //go:generate go run tailscale.com/cmd/cloner -type=Prefs -output=prefs_clone.go
 
-// DefaultControlURL returns the URL base of the control plane
+// DefaultControlURL is the URL base of the control plane
 // ("coordination server") for use when no explicit one is configured.
 // The default control plane is the hosted version run by Tailscale.com.
 const DefaultControlURL = "https://controlplane.tailscale.com"
 
+// IsLoginServerSynonym reports whether a URL is a drop-in replacement
+// for the primary Tailscale login server.
+func IsLoginServerSynonym(val interface{}) bool {
+	return val == "https://login.tailscale.com" || val == "https://controlplane.tailscale.com"
+}
+
 // Prefs are the user modifiable settings of the Tailscale node agent.
 type Prefs struct {
 	// ControlURL is the URL of the control server to use.
@@ -405,6 +411,16 @@ func (p *Prefs) ControlURLOrDefault() string {
 	return DefaultControlURL
 }
 
+// AdminPageURL returns the admin web site URL for the current ControlURL.
+func (p *Prefs) AdminPageURL() string {
+	url := p.ControlURLOrDefault()
+	if IsLoginServerSynonym(url) {
+		// TODO(crawshaw): In future release, make this https://console.tailscale.com
+		url = "https://login.tailscale.com"
+	}
+	return url + "/admin/machines"
+}
+
 // PrefsFromBytes deserializes Prefs from a JSON blob. If
 // enforceDefaults is true, Prefs.RouteAll and Prefs.AllowSingleHosts
 // are forced on.