cmd/tailscale: improve exit node menu for location based exit nodes

This change provides minor improvements to the exit node menu when there
are location based exit nodes present. It will ensure that non location
based exit nodes are displayed at the top of the list, followed by a
the best node for a country/city combination, and followed by all
location based exit nodes.

Updates tailscale/tailscale#9421

Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

cmd/tailscale/main.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"cmp"
 	"context"
 	"crypto/rand"
 	"crypto/sha1"
@@ -31,6 +32,7 @@ import (
 	"gioui.org/io/system"
 	"gioui.org/layout"
 	"gioui.org/op"
+	"golang.org/x/exp/maps"
 	"inet.af/netaddr"
 
 	"github.com/tailscale/tailscale-android/jni"
@@ -135,9 +137,11 @@ const (
 )
 
 type Peer struct {
-	Label  string
-	Online bool
-	ID     tailcfg.StableNodeID
+	Label             string
+	Online            bool
+	ID                tailcfg.StableNodeID
+	Location          *tailcfg.Location
+	PreferredExitNode bool
 }
 
 type BackendState struct {
@@ -675,10 +679,19 @@ func (s *BackendState) updateExitNodes() {
 		myExit := p.StableID() == exitID
 		hasMyExit = hasMyExit || myExit
 		exit := Peer{
-			Label:  p.DisplayName(true),
-			Online: canRoute,
-			ID:     p.StableID(),
+			Label:    p.DisplayName(true),
+			Online:   canRoute,
+			ID:       p.StableID(),
+			Location: p.Hostinfo().Location(),
 		}
+
+		if exit.Location != nil {
+			// We want to shorten what the users sees here,
+			// so override the display name with the computed
+			// name.
+			exit.Label = p.ComputedName()
+		}
+
 		if myExit {
 			s.Exit = exit
 			if canRoute {
@@ -689,9 +702,80 @@ func (s *BackendState) updateExitNodes() {
 			s.Exits = append(s.Exits, exit)
 		}
 	}
+
+	locationBasedExitPeersMap := make(map[string]Peer)
+	var nonLocationBasedExitPeers []Peer
+	var allLocationBasedExitPeers []Peer
+	for _, peer := range s.Exits {
+		if peer.Location != nil {
+			countryCityLocation, ok := locationBasedExitPeersMap[fmt.Sprintf("%s (%s)", peer.Location.Country, peer.Location.City)]
+			if !ok {
+				// If we have not seen the country/city combination, add it to the
+				// map.
+				locationBasedExitPeersMap[fmt.Sprintf("%s (%s)", peer.Location.Country, peer.Location.City)] = peer
+				continue
+			}
+
+			if countryCityLocation.Location.Priority < peer.Location.Priority {
+				// If the priority for the location based exit node is higher than
+				// the current option, replace it.
+				locationBasedExitPeersMap[fmt.Sprintf("%s (%s)", peer.Location.Country, peer.Location.City)] = peer
+			}
+
+			allLocationBasedExitPeers = append(allLocationBasedExitPeers, peer)
+			continue
+		}
+		nonLocationBasedExitPeers = append(nonLocationBasedExitPeers, peer)
+	}
+
+	// We want to order the exit nodes to be display to the user in
+	// the order of non location based exit nodes, the best exit
+	// node per location, and then all of the location based exit nodes.
+
+	// Non location based exit nodes.
+	s.Exits = nonLocationBasedExitPeers
 	sort.Slice(s.Exits, func(i, j int) bool {
 		return s.Exits[i].Label < s.Exits[j].Label
 	})
+
+	// Best location based exit nodes
+	locationBasedExitPeersMapValues := maps.Values(locationBasedExitPeersMap)
+	if len(locationBasedExitPeersMapValues) > 0 {
+		var preferredLocationBasedExitPeers []Peer
+		for _, peer := range locationBasedExitPeersMapValues {
+			peerCopy := peer
+			peerCopy.PreferredExitNode = true
+			peerCopy.Label = fmt.Sprintf("%s - %s (%s)", peerCopy.Location.Country, peerCopy.Location.City, peerCopy.Label)
+
+			preferredLocationBasedExitPeers = append(preferredLocationBasedExitPeers, peerCopy)
+		}
+
+		sort.Slice(preferredLocationBasedExitPeers, func(i, j int) bool {
+			// Sort the order by country, and cities.
+			res := cmp.Compare(preferredLocationBasedExitPeers[i].Location.Country, preferredLocationBasedExitPeers[j].Location.Country)
+
+			switch res {
+			case -1:
+				return true
+			case 1:
+				return false
+			default:
+				// If the two peers have the same country, sort by city.
+				return preferredLocationBasedExitPeers[i].Location.City < preferredLocationBasedExitPeers[j].Location.City
+			}
+		})
+		s.Exits = append(s.Exits, preferredLocationBasedExitPeers...)
+	}
+
+	if len(allLocationBasedExitPeers) > 0 {
+		// All location based exit nodes at the end.
+		sort.Slice(allLocationBasedExitPeers, func(i, j int) bool {
+			// Sort the order by label
+			return allLocationBasedExitPeers[i].Label < allLocationBasedExitPeers[j].Label
+		})
+		s.Exits = append(s.Exits, allLocationBasedExitPeers...)
+	}
+
 	if !hasMyExit {
 		// Insert node missing from netmap.
 		s.Exit = Peer{Label: "Unknown device", ID: exitID}

cmd/tailscale/ui.go

@@ -1054,7 +1054,7 @@ func (ui *UI) layoutExitNodeDialog(gtx layout.Context, sysIns system.Insets, exi
 									Bottom: unit.Dp(16),
 								}.Layout(gtx, btn.Layout)
 							}
-							node := Peer{Label: "None", Online: true}
+							node := Peer{Label: "None", Online: true, Location: nil}
 							if idx >= 2 {
 								node = exits[idx-2]
 							}