clientupdate: implement updates for Synology (#8858)

Implement naive update for Synology packages, using latest versions from
pkgs.tailscale.com. This is naive because we completely trust
pkgs.tailscale.com to give us a safe package. We should switch this to
some better signing mechanism later.

I've only tested this on one DS218 box, so all the CPU architecture
munging is purely based on docs.

Updates #6995

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

clientupdate/clientupdate_test.go

@@ -4,9 +4,12 @@
 package clientupdate
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"testing"
+
+	"tailscale.com/tailcfg"
 )
 
 func TestUpdateDebianAptSourcesListBytes(t *testing.T) {
@@ -440,3 +443,44 @@ tailscale installed size:
 		})
 	}
 }
+
+func TestSynoArch(t *testing.T) {
+	tests := []struct {
+		goarch  string
+		model   string
+		want    string
+		wantErr bool
+	}{
+		{goarch: "amd64", model: "DS224+", want: "x86_64"},
+		{goarch: "arm64", model: "DS124", want: "armv8"},
+		{goarch: "386", model: "DS415play", want: "i686"},
+		{goarch: "arm", model: "DS213air", want: "88f6281"},
+		{goarch: "arm", model: "NVR1218", want: "hi3535"},
+		{goarch: "arm", model: "DS1517", want: "alpine"},
+		{goarch: "arm", model: "DS216se", want: "armada370"},
+		{goarch: "arm", model: "DS115", want: "armada375"},
+		{goarch: "arm", model: "DS419slim", want: "armada38x"},
+		{goarch: "arm", model: "RS815", want: "armadaxp"},
+		{goarch: "arm", model: "DS414j", want: "comcerto2k"},
+		{goarch: "arm", model: "DS216play", want: "monaco"},
+		{goarch: "riscv64", model: "DS999", wantErr: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(fmt.Sprintf("%s-%s", tt.goarch, tt.model), func(t *testing.T) {
+			got, err := synoArch(&tailcfg.Hostinfo{GoArch: tt.goarch, DeviceModel: tt.model})
+			if err != nil {
+				if !tt.wantErr {
+					t.Fatalf("got unexpected error %v", err)
+				}
+				return
+			}
+			if tt.wantErr {
+				t.Fatalf("got %q, expected an error", got)
+			}
+			if got != tt.want {
+				t.Errorf("got %q, want %q", got, tt.want)
+			}
+		})
+	}
+}