cmd/tailscale/cli/up: "LoggedOut" pref is implicit.

There's no need to warn that it was not provided on the command line
after doing a sequence of up; logout; up --args. If you're asking for
tailscale to be up, you always mean that you prefer LoggedOut to become
false.

Fixes #1828

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -348,6 +348,25 @@ func TestCheckForAccidentalSettingReverts(t *testing.T) {
 			},
 			want: accidentalUpPrefix + " --hostname=newhostname --accept-routes --exit-node=100.64.5.6 --accept-dns --shields-up --advertise-tags=tag:foo,tag:bar --unattended --advertise-routes=10.0.0.0/16 --netfilter-mode=nodivert --operator=alice",
 		},
+		{
+			name:    "loggedout_is_implicit",
+			flagSet: f("advertise-exit-node"),
+			curPrefs: &ipn.Prefs{
+				ControlURL: ipn.DefaultControlURL,
+				LoggedOut:  true,
+			},
+			mp: &ipn.MaskedPrefs{
+				Prefs: ipn.Prefs{
+					ControlURL: ipn.DefaultControlURL,
+					AdvertiseRoutes: []netaddr.IPPrefix{
+						netaddr.MustParseIPPrefix("0.0.0.0/0"),
+					},
+				},
+				AdvertiseRoutesSet: true,
+			},
+			// not an error. LoggedOut is implicit.
+			want: "",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

cmd/tailscale/cli/up.go

@@ -552,9 +552,17 @@ func checkForAccidentalSettingReverts(flagSet map[string]bool, curPrefs *ipn.Pre
 	flagExplicitValue := map[string]interface{}{} // e.g. "accept-dns" => true (from flagSet)
 	for i := 0; i < prefType.NumField(); i++ {
 		prefName := prefType.Field(i).Name
+		// Persist is a legacy field used for storing keys, which
+		// probably should never have been part of Prefs. It's
+		// likely to migrate elsewhere eventually.
 		if prefName == "Persist" {
 			continue
 		}
+		// LoggedOut is a preference, but running the "up" command
+		// always implies that the user now prefers LoggedOut->false.
+		if prefName == "LoggedOut" {
+			continue
+		}
 		flagName, hasFlag := flagForPref[prefName]
 
 		// Special case for advertise-exit-node; which is a