ipn,wgengine: remove vestigial Prefs.AllowSingleHosts

It was requested by the first customer 4-5 years ago and only used
for a brief moment of time. We later added netmap visibility trimming
which removes the need for this.

It's been hidden by the CLI for quite some time and never documented
anywhere else.

This keeps the CLI flag, though, out of caution. It just returns an
error if it's set to anything but true (its default).

Fixes #12058

Change-Id: I7514ba572e7b82519b04ed603ff9f3bdbaecfda7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/prefs.go

@@ -75,18 +75,6 @@ type Prefs struct {
 	// controlled by ExitNodeID/IP below.
 	RouteAll bool
 
-	// AllowSingleHosts specifies whether to install routes for each
-	// node IP on the tailscale network, in addition to a route for
-	// the whole network.
-	// This corresponds to the "tailscale up --host-routes" value,
-	// which defaults to true.
-	//
-	// TODO(danderson): why do we have this? It dumps a lot of stuff
-	// into the routing table, and a single network route _should_ be
-	// all that we need. But when I turn this off in my tailscaled,
-	// packets stop flowing. What's up with that?
-	AllowSingleHosts bool
-
 	// ExitNodeID and ExitNodeIP specify the node that should be used
 	// as an exit node for internet traffic. At most one of these
 	// should be non-zero.
@@ -252,6 +240,16 @@ type Prefs struct {
 	// by name.
 	DriveShares []*drive.Share
 
+	// AllowSingleHosts was a legacy field that was always true
+	// for the past 4.5 years. It controlled whether Tailscale
+	// peers got /32 or /127 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.
@@ -282,6 +280,13 @@ 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
@@ -299,7 +304,6 @@ type MaskedPrefs struct {
 
 	ControlURLSet             bool                `json:",omitempty"`
 	RouteAllSet               bool                `json:",omitempty"`
-	AllowSingleHostsSet       bool                `json:",omitempty"`
 	ExitNodeIDSet             bool                `json:",omitempty"`
 	ExitNodeIPSet             bool                `json:",omitempty"`
 	InternalExitNodePriorSet  bool                `json:",omitempty"` // Internal; can't be set by LocalAPI clients
@@ -484,9 +488,6 @@ func (p *Prefs) pretty(goos string) string {
 	var sb strings.Builder
 	sb.WriteString("Prefs{")
 	fmt.Fprintf(&sb, "ra=%v ", p.RouteAll)
-	if !p.AllowSingleHosts {
-		sb.WriteString("mesh=false ")
-	}
 	fmt.Fprintf(&sb, "dns=%v want=%v ", p.CorpDNS, p.WantRunning)
 	if p.RunSSH {
 		sb.WriteString("ssh=true ")
@@ -579,7 +580,6 @@ func (p *Prefs) Equals(p2 *Prefs) bool {
 
 	return p.ControlURL == p2.ControlURL &&
 		p.RouteAll == p2.RouteAll &&
-		p.AllowSingleHosts == p2.AllowSingleHosts &&
 		p.ExitNodeID == p2.ExitNodeID &&
 		p.ExitNodeIP == p2.ExitNodeIP &&
 		p.InternalExitNodePrior == p2.InternalExitNodePrior &&
@@ -663,7 +663,6 @@ func NewPrefs() *Prefs {
 		ControlURL: "",
 
 		RouteAll:            true,
-		AllowSingleHosts:    true,
 		CorpDNS:             true,
 		WantRunning:         false,
 		NetfilterMode:       preftype.NetfilterOn,