tailcfg: add Hostinfo.ExitNodeID to report the selected exit node (#16625)

When a client selects a particular exit node, Control may use that as
a signal for deciding other routes.

This patch causes the client to report whenever the current exit node
changes, through tailcfg.Hostinfo.ExitNodeID. It relies on a properly
set ipn.Prefs.ExitNodeID, which should already be resolved by
`tailscale set`.

Updates tailscale/corp#30536

Signed-off-by: Simon Law <sfllaw@tailscale.com>

tailcfg/tailcfg.go

@@ -166,7 +166,8 @@ type CapabilityVersion int
 //   - 119: 2025-07-10: Client uses Hostinfo.Location.Priority to prioritize one route over another.
 //   - 120: 2025-07-15: Client understands peer relay disco messages, and implements peer client and relay server functions
 //   - 121: 2025-07-19: Client understands peer relay endpoint alloc with [disco.AllocateUDPRelayEndpointRequest] & [disco.AllocateUDPRelayEndpointResponse]
-const CurrentCapabilityVersion CapabilityVersion = 121
+//   - 122: 2025-07-21: Client sends Hostinfo.ExitNodeID to report which exit node it has selected, if any.
+const CurrentCapabilityVersion CapabilityVersion = 122
 
 // ID is an integer ID for a user, node, or login allocated by the
 // control plane.
@@ -875,6 +876,7 @@ type Hostinfo struct {
 	UserspaceRouter opt.Bool       `json:",omitempty"` // if the client's subnet router is running in userspace (netstack) mode
 	AppConnector    opt.Bool       `json:",omitempty"` // if the client is running the app-connector service
 	ServicesHash    string         `json:",omitempty"` // opaque hash of the most recent list of tailnet services, change in hash indicates config should be fetched via c2n
+	ExitNodeID      StableNodeID   `json:",omitzero"`  // the client’s selected exit node, empty when unselected.
 
 	// Location represents geographical location data about a
 	// Tailscale host. Location is optional and only set if

tailcfg/tailcfg_clone.go

@@ -186,6 +186,7 @@ var _HostinfoCloneNeedsRegeneration = Hostinfo(struct {
 	UserspaceRouter opt.Bool
 	AppConnector    opt.Bool
 	ServicesHash    string
+	ExitNodeID      StableNodeID
 	Location        *Location
 	TPM             *TPMInfo
 	StateEncrypted  opt.Bool

tailcfg/tailcfg_test.go

@@ -67,6 +67,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"UserspaceRouter",
 		"AppConnector",
 		"ServicesHash",
+		"ExitNodeID",
 		"Location",
 		"TPM",
 		"StateEncrypted",
@@ -273,6 +274,21 @@ func TestHostinfoEqual(t *testing.T) {
 			&Hostinfo{IngressEnabled: true},
 			false,
 		},
+		{
+			&Hostinfo{ExitNodeID: "stable-exit"},
+			&Hostinfo{ExitNodeID: "stable-exit"},
+			true,
+		},
+		{
+			&Hostinfo{ExitNodeID: ""},
+			&Hostinfo{},
+			true,
+		},
+		{
+			&Hostinfo{ExitNodeID: ""},
+			&Hostinfo{ExitNodeID: "stable-exit"},
+			false,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equal(tt.b)

tailcfg/tailcfg_view.go

@@ -300,6 +300,7 @@ func (v HostinfoView) Userspace() opt.Bool                    { return v.ж.User
 func (v HostinfoView) UserspaceRouter() opt.Bool              { return v.ж.UserspaceRouter }
 func (v HostinfoView) AppConnector() opt.Bool                 { return v.ж.AppConnector }
 func (v HostinfoView) ServicesHash() string                   { return v.ж.ServicesHash }
+func (v HostinfoView) ExitNodeID() StableNodeID               { return v.ж.ExitNodeID }
 func (v HostinfoView) Location() LocationView                 { return v.ж.Location.View() }
 func (v HostinfoView) TPM() views.ValuePointer[TPMInfo]       { return views.ValuePointerOf(v.ж.TPM) }
 
@@ -345,6 +346,7 @@ var _HostinfoViewNeedsRegeneration = Hostinfo(struct {
 	UserspaceRouter opt.Bool
 	AppConnector    opt.Bool
 	ServicesHash    string
+	ExitNodeID      StableNodeID
 	Location        *Location
 	TPM             *TPMInfo
 	StateEncrypted  opt.Bool