clientupdate,net/tstun: add support for OpenWrt 25.12.0 using apk (#18545)

OpenWrt is changing to using alpine like `apk` for package installation
over its previous opkg. Additionally, they are not using the same repo
files as alpine making installation fail.

Add support for the new repository files and ensure that the required
package detection system uses apk.

Updates #18535

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

clientupdate/clientupdate_test.go

@@ -6,9 +6,12 @@ package clientupdate
 import (
 	"archive/tar"
 	"compress/gzip"
+	"encoding/json"
 	"fmt"
 	"io/fs"
 	"maps"
+	"net/http"
+	"net/http/httptest"
 	"os"
 	"path/filepath"
 	"slices"
@@ -299,6 +302,127 @@ tailscale-1.58.2-r0 installed size:
 	}
 }
 
+func TestCheckOutdatedAlpineRepo(t *testing.T) {
+	anyToString := func(a any) string {
+		str, ok := a.(string)
+		if !ok {
+			panic("failed to parse param as string")
+		}
+		return str
+	}
+
+	tests := []struct {
+		name              string
+		fileContent       string
+		latestHTTPVersion string
+		latestApkVersion  string
+		wantHTTPVersion   string
+		wantApkVersion    string
+		wantAlpineVersion string
+		track             string
+	}{
+		{
+			name:              "Up to date",
+			fileContent:       "https://dl-cdn.alpinelinux.org/alpine/v3.20/main",
+			latestHTTPVersion: "1.95.3",
+			latestApkVersion:  "1.95.3",
+			track:             "unstable",
+		},
+		{
+			name:              "Behind unstable",
+			fileContent:       "https://dl-cdn.alpinelinux.org/alpine/v3.20/main",
+			latestHTTPVersion: "1.95.4",
+			latestApkVersion:  "1.95.3",
+			wantHTTPVersion:   "1.95.4",
+			wantApkVersion:    "1.95.3",
+			wantAlpineVersion: "v3.20",
+			track:             "unstable",
+		},
+		{
+			name:              "Behind stable",
+			fileContent:       "https://dl-cdn.alpinelinux.org/alpine/v2.40/main",
+			latestHTTPVersion: "1.94.3",
+			latestApkVersion:  "1.92.1",
+			wantHTTPVersion:   "1.94.3",
+			wantApkVersion:    "1.92.1",
+			wantAlpineVersion: "v2.40",
+			track:             "stable",
+		},
+		{
+			name:              "Nothing in dist file",
+			fileContent:       "",
+			latestHTTPVersion: "1.94.3",
+			latestApkVersion:  "1.92.1",
+			wantHTTPVersion:   "1.94.3",
+			wantApkVersion:    "1.92.1",
+			track:             "stable",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			dir, err := os.MkdirTemp("", "example")
+			if err != nil {
+				t.Fatalf("error creating temp dir: %v", err)
+			}
+			t.Cleanup(func() { os.RemoveAll(dir) }) // clean up
+
+			file := filepath.Join(dir, "distfile")
+			if err := os.WriteFile(file, []byte(tt.fileContent), 0o666); err != nil {
+				t.Fatalf("error creating dist file: %v", err)
+			}
+
+			testServ := httptest.NewServer(http.HandlerFunc(
+				func(w http.ResponseWriter, _ *http.Request) {
+					version := trackPackages{
+						MSIsVersion:     tt.latestHTTPVersion,
+						MacZipsVersion:  tt.latestHTTPVersion,
+						TarballsVersion: tt.latestHTTPVersion,
+						SPKsVersion:     tt.latestHTTPVersion,
+					}
+					jsonData, err := json.Marshal(version)
+					if err != nil {
+						t.Errorf("failed to marshal version string: %v", err)
+					}
+					w.Header().Set("Content-Type", "application/json")
+					if _, err := w.Write(jsonData); err != nil {
+						t.Errorf("failed to write json blob: %v", err)
+					}
+				},
+			))
+			defer testServ.Close()
+
+			oldEndpoint := tailscaleHTTPEndpoint
+			tailscaleHTTPEndpoint = testServ.URL
+			defer func() { tailscaleHTTPEndpoint = oldEndpoint }()
+
+			var paramLatest string
+			var paramApkVer string
+			var paramAlpineVer string
+			logf := func(_ string, params ...any) {
+				paramLatest = anyToString(params[0])
+				paramApkVer = anyToString(params[1])
+				if len(params) > 2 {
+					paramAlpineVer = anyToString(params[2])
+				}
+			}
+
+			err = checkOutdatedAlpineRepo(logf, []string{file}, tt.latestApkVersion, tt.track)
+			if err != nil {
+				t.Errorf("did not expect error, got: %v", err)
+			}
+			if paramLatest != tt.wantHTTPVersion {
+				t.Errorf("expected HTTP version '%s', got '%s'", tt.wantHTTPVersion, paramLatest)
+			}
+			if paramApkVer != tt.wantApkVersion {
+				t.Errorf("expected APK version '%s', got '%s'", tt.wantApkVersion, paramApkVer)
+			}
+			if paramAlpineVer != tt.wantAlpineVersion {
+				t.Errorf("expected alpine version '%s', got '%s'", tt.wantAlpineVersion, paramAlpineVer)
+			}
+		})
+	}
+}
+
 func TestSynoArch(t *testing.T) {
 	tests := []struct {
 		goarch         string