clientupdate: mention when Alpine system upgrade is needed (#12306)

Alpine APK repos are versioned, and contain different package sets.
Older APK releases and repos don't have the latest tailscale package.
When we report "no update available", check whether pkgs.tailscale.com
has a newer tarball release. If it does, it's possible that the system
is on an older Alpine release. Print additional messages to suggest the
user to upgrade their OS.

Fixes #11309

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

clientupdate/clientupdate.go

@@ -653,6 +653,9 @@ func (up *Updater) updateAlpineLike() (err error) {
 		return fmt.Errorf(`failed to parse latest version from "apk info tailscale": %w`, err)
 	}
 	if !up.confirm(ver) {
+		if err := checkOutdatedAlpineRepo(up.Logf, ver, up.Track); err != nil {
+			up.Logf("failed to check whether Alpine release is outdated: %v", err)
+		}
 		return nil
 	}
 
@@ -690,6 +693,37 @@ func parseAlpinePackageVersion(out []byte) (string, error) {
 	return "", errors.New("tailscale version not found in output")
 }
 
+var apkRepoVersionRE = regexp.MustCompile(`v[0-9]+\.[0-9]+`)
+
+func checkOutdatedAlpineRepo(logf logger.Logf, apkVer, track string) error {
+	latest, err := LatestTailscaleVersion(track)
+	if err != nil {
+		return err
+	}
+	if latest == apkVer {
+		// Actually on latest release.
+		return nil
+	}
+	f, err := os.Open("/etc/apk/repositories")
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	// Read the first repo line. Typically, there are multiple repos that all
+	// contain the same version in the path, like:
+	//   https://dl-cdn.alpinelinux.org/alpine/v3.20/main
+	//   https://dl-cdn.alpinelinux.org/alpine/v3.20/community
+	s := bufio.NewScanner(f)
+	if !s.Scan() {
+		return s.Err()
+	}
+	alpineVer := apkRepoVersionRE.FindString(s.Text())
+	if alpineVer != "" {
+		logf("The latest Tailscale release for Linux is %q, but your apk repository only provides %q.\nYour Alpine version is %q, you may need to upgrade the system to get the latest Tailscale version: https://wiki.alpinelinux.org/wiki/Upgrading_Alpine", latest, apkVer, alpineVer)
+	}
+	return nil
+}
+
 func (up *Updater) updateMacSys() error {
 	return errors.New("NOTREACHED: On MacSys builds, `tailscale update` is handled in Swift to launch the GUI updater")
 }