Add additional test cases to verify different base domains produce different shuffle orders

Added two more test cases with the same dataset (region 4) but different base domains to ensure the shuffle is truly deterministic and that different seeds produce different results.

Co-authored-by: kradalby <98431+kradalby@users.noreply.github.com>

hscontrol/derp/derp.go

@@ -12,6 +12,7 @@ import (
 	"net/url"
 	"os"
 	"reflect"
+	"slices"
 	"sync"
 	"time"
 
@@ -126,7 +127,17 @@ func shuffleDERPMap(dm *tailcfg.DERPMap) {
 		return
 	}
 
-	for id, region := range dm.Regions {
+	// Collect region IDs and sort them to ensure deterministic iteration order.
+	// Map iteration order is non-deterministic in Go, which would cause the
+	// shuffle to be non-deterministic even with a fixed seed.
+	ids := make([]int, 0, len(dm.Regions))
+	for id := range dm.Regions {
+		ids = append(ids, id)
+	}
+	slices.Sort(ids)
+
+	for _, id := range ids {
+		region := dm.Regions[id]
 		if len(region.Nodes) == 0 {
 			continue
 		}

hscontrol/derp/derp_test.go

@@ -83,9 +83,9 @@ func TestShuffleDERPMapDeterministic(t *testing.T) {
 						RegionCode: "sea",
 						RegionName: "Seattle",
 						Nodes: []*tailcfg.DERPNode{
-							{Name: "10b", RegionID: 10, HostName: "derp10b.tailscale.com"},
-							{Name: "10c", RegionID: 10, HostName: "derp10c.tailscale.com"},
 							{Name: "10d", RegionID: 10, HostName: "derp10d.tailscale.com"},
+							{Name: "10c", RegionID: 10, HostName: "derp10c.tailscale.com"},
+							{Name: "10b", RegionID: 10, HostName: "derp10b.tailscale.com"},
 						},
 					},
 					2: {
@@ -93,9 +93,9 @@ func TestShuffleDERPMapDeterministic(t *testing.T) {
 						RegionCode: "sfo",
 						RegionName: "San Francisco",
 						Nodes: []*tailcfg.DERPNode{
-							{Name: "2f", RegionID: 2, HostName: "derp2f.tailscale.com"},
-							{Name: "2e", RegionID: 2, HostName: "derp2e.tailscale.com"},
 							{Name: "2d", RegionID: 2, HostName: "derp2d.tailscale.com"},
+							{Name: "2e", RegionID: 2, HostName: "derp2e.tailscale.com"},
+							{Name: "2f", RegionID: 2, HostName: "derp2f.tailscale.com"},
 						},
 					},
 				},
@@ -169,6 +169,74 @@ func TestShuffleDERPMapDeterministic(t *testing.T) {
 				},
 			},
 		},
+		{
+			name:       "same dataset with another base domain",
+			baseDomain: "another.example.com",
+			derpMap: &tailcfg.DERPMap{
+				Regions: map[int]*tailcfg.DERPRegion{
+					4: {
+						RegionID:   4,
+						RegionCode: "fra",
+						RegionName: "Frankfurt",
+						Nodes: []*tailcfg.DERPNode{
+							{Name: "4f", RegionID: 4, HostName: "derp4f.tailscale.com"},
+							{Name: "4g", RegionID: 4, HostName: "derp4g.tailscale.com"},
+							{Name: "4h", RegionID: 4, HostName: "derp4h.tailscale.com"},
+							{Name: "4i", RegionID: 4, HostName: "derp4i.tailscale.com"},
+						},
+					},
+				},
+			},
+			expected: &tailcfg.DERPMap{
+				Regions: map[int]*tailcfg.DERPRegion{
+					4: {
+						RegionID:   4,
+						RegionCode: "fra",
+						RegionName: "Frankfurt",
+						Nodes: []*tailcfg.DERPNode{
+							{Name: "4h", RegionID: 4, HostName: "derp4h.tailscale.com"},
+							{Name: "4f", RegionID: 4, HostName: "derp4f.tailscale.com"},
+							{Name: "4g", RegionID: 4, HostName: "derp4g.tailscale.com"},
+							{Name: "4i", RegionID: 4, HostName: "derp4i.tailscale.com"},
+						},
+					},
+				},
+			},
+		},
+		{
+			name:       "same dataset with yet another base domain",
+			baseDomain: "yetanother.example.com",
+			derpMap: &tailcfg.DERPMap{
+				Regions: map[int]*tailcfg.DERPRegion{
+					4: {
+						RegionID:   4,
+						RegionCode: "fra",
+						RegionName: "Frankfurt",
+						Nodes: []*tailcfg.DERPNode{
+							{Name: "4f", RegionID: 4, HostName: "derp4f.tailscale.com"},
+							{Name: "4g", RegionID: 4, HostName: "derp4g.tailscale.com"},
+							{Name: "4h", RegionID: 4, HostName: "derp4h.tailscale.com"},
+							{Name: "4i", RegionID: 4, HostName: "derp4i.tailscale.com"},
+						},
+					},
+				},
+			},
+			expected: &tailcfg.DERPMap{
+				Regions: map[int]*tailcfg.DERPRegion{
+					4: {
+						RegionID:   4,
+						RegionCode: "fra",
+						RegionName: "Frankfurt",
+						Nodes: []*tailcfg.DERPNode{
+							{Name: "4i", RegionID: 4, HostName: "derp4i.tailscale.com"},
+							{Name: "4h", RegionID: 4, HostName: "derp4h.tailscale.com"},
+							{Name: "4f", RegionID: 4, HostName: "derp4f.tailscale.com"},
+							{Name: "4g", RegionID: 4, HostName: "derp4g.tailscale.com"},
+						},
+					},
+				},
+			},
+		},
 	}
 
 	for _, tt := range tests {