ipn: remove the last traces of Prefs.AllowSingleHosts

We stopped reading this field nearly two years ago, with a TODO comment
to remove it sometime in 2025.

It is now 2026.

Updates #12058

Change-Id: I8ddf1c2e4c3c428e8d45a6491d3899368ec52c30
Signed-off-by: Alex Chan <alexc@tailscale.com>

ipn/ipn_clone.go

@@ -104,7 +104,6 @@ var _PrefsCloneNeedsRegeneration = Prefs(struct {
 	DriveShares                []*drive.Share
 	RelayServerPort            *uint16
 	RelayServerStaticEndpoints []netip.AddrPort
-	AllowSingleHosts           marshalAsTrueInJSON
 	Persist                    *persist.Persist
 }{})
 

ipn/ipn_view.go

@@ -453,16 +453,6 @@ func (v PrefsView) RelayServerStaticEndpoints() views.Slice[netip.AddrPort] {
 	return views.SliceOf(v.ж.RelayServerStaticEndpoints)
 }
 
-// AllowSingleHosts was a legacy field that was always true
-// for the past 4.5 years. It controlled whether Tailscale
-// peers got /32 or /128 routes for each other.
-// As of 2024-05-17 we're starting to ignore it, but to let
-// people still downgrade Tailscale versions and not break
-// all peer-to-peer networking we still write it to disk (as JSON)
-// so it can be loaded back by old versions.
-// TODO(bradfitz): delete this in 2025 sometime. See #12058.
-func (v PrefsView) AllowSingleHosts() marshalAsTrueInJSON { return v.ж.AllowSingleHosts }
-
 // The Persist field is named 'Config' in the file for backward
 // compatibility with earlier versions.
 // TODO(apenwarr): We should move this out of here, it's not a pref.
@@ -506,7 +496,6 @@ var _PrefsViewNeedsRegeneration = Prefs(struct {
 	DriveShares                []*drive.Share
 	RelayServerPort            *uint16
 	RelayServerStaticEndpoints []netip.AddrPort
-	AllowSingleHosts           marshalAsTrueInJSON
 	Persist                    *persist.Persist
 }{})
 

ipn/prefs.go

@@ -291,16 +291,6 @@ type Prefs struct {
 	// non-nil.
 	RelayServerStaticEndpoints []netip.AddrPort `json:",omitempty"`
 
-	// AllowSingleHosts was a legacy field that was always true
-	// for the past 4.5 years. It controlled whether Tailscale
-	// peers got /32 or /128 routes for each other.
-	// As of 2024-05-17 we're starting to ignore it, but to let
-	// people still downgrade Tailscale versions and not break
-	// all peer-to-peer networking we still write it to disk (as JSON)
-	// so it can be loaded back by old versions.
-	// TODO(bradfitz): delete this in 2025 sometime. See #12058.
-	AllowSingleHosts marshalAsTrueInJSON
-
 	// The Persist field is named 'Config' in the file for backward
 	// compatibility with earlier versions.
 	// TODO(apenwarr): We should move this out of here, it's not a pref.
@@ -331,13 +321,6 @@ func (au1 AutoUpdatePrefs) Equals(au2 AutoUpdatePrefs) bool {
 		ok1 == ok2
 }
 
-type marshalAsTrueInJSON struct{}
-
-var trueJSON = []byte("true")
-
-func (marshalAsTrueInJSON) MarshalJSON() ([]byte, error) { return trueJSON, nil }
-func (*marshalAsTrueInJSON) UnmarshalJSON([]byte) error  { return nil }
-
 // AppConnectorPrefs are the app connector settings for the node agent.
 type AppConnectorPrefs struct {
 	// Advertise specifies whether the app connector subsystem is advertising

ipn/prefs_test.go

@@ -70,7 +70,6 @@ func TestPrefsEqual(t *testing.T) {
 		"DriveShares",
 		"RelayServerPort",
 		"RelayServerStaticEndpoints",
-		"AllowSingleHosts",
 		"Persist",
 	}
 	if have := fieldsOf(reflect.TypeFor[Prefs]()); !reflect.DeepEqual(have, prefsHandles) {
@@ -714,7 +713,7 @@ func TestMaskedPrefsFields(t *testing.T) {
 	have := map[string]bool{}
 	for _, f := range fieldsOf(reflect.TypeFor[Prefs]()) {
 		switch f {
-		case "Persist", "AllowSingleHosts":
+		case "Persist":
 			// These can't be edited.
 			continue
 		}
@@ -1222,27 +1221,6 @@ func TestNotifyPrefsJSONRoundtrip(t *testing.T) {
 	}
 }
 
-// Verify that our Prefs type writes out an AllowSingleHosts field so we can
-// downgrade to older versions that require it.
-func TestPrefsDowngrade(t *testing.T) {
-	var p Prefs
-	j, err := json.Marshal(p)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	type oldPrefs struct {
-		AllowSingleHosts bool
-	}
-	var op oldPrefs
-	if err := json.Unmarshal(j, &op); err != nil {
-		t.Fatal(err)
-	}
-	if !op.AllowSingleHosts {
-		t.Fatal("AllowSingleHosts should be true")
-	}
-}
-
 func TestParseAutoExitNodeString(t *testing.T) {
 	tests := []struct {
 		name       string