gokrazy, clientupdate: add start of Gokrazy auto-updates, tests

This adds support for Gokrazy GAF (Gokrazy Archive Format) zip
auto-updates, starting to wire up Tailscale's clientupdate mechanism
to Gokrazy's update mechanism.

Currently there's just a CLI command to update from a GAF URL,
with an --unsigned flag for use in a new natlab vmtest.

Next step would be publishing unstable track GAF files on
pkgs.tailscale.com, with detached signatures, and then making the
clientupdate mechanism also download those and check signatures.

Updates #20002

Change-Id: Ib03c56f17a57f8a4638398ef83549dac4813323d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/update.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"io"
 	"runtime"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
@@ -67,8 +68,19 @@ var updateArgs struct {
 	version string // explicit version; empty means auto
 }
 
+const gokrazyUpdateFromURLMagicArg = "--gokrazy-update-from-url"
+
 func runUpdate(ctx context.Context, args []string) error {
 	if len(args) > 0 {
+		if runtime.GOOS == "linux" && distro.Get() == distro.Gokrazy {
+			gokArgs, err := gokrazyUpdateArgsFromMagicArg(args)
+			if err != nil {
+				return err
+			}
+			if gokArgs != nil {
+				return clientupdate.GokrazyUpdateFromURL.Get()(ctx, *gokArgs)
+			}
+		}
 		return flag.ErrHelp
 	}
 	if updateArgs.version != "" && updateArgs.track != "" {
@@ -102,3 +114,39 @@ func confirmUpdate(ver string) bool {
 	msg := fmt.Sprintf("This will update Tailscale from %v to %v. Continue?", version.Short(), ver)
 	return prompt.YesNo(msg, true)
 }
+
+// gokrazyUpdateArgsFromMagicArg parses the Gokrazy update-from-URL command-line
+// flow. It returns nil if args do not select that flow. A non-nil result means
+// the caller may safely invoke clientupdate.GokrazyUpdateFromURL.
+func gokrazyUpdateArgsFromMagicArg(args []string) (*clientupdate.GokrazyUpdateArgs, error) {
+	var updateURL string
+	var unsigned bool
+
+	fs := flag.NewFlagSet("gokrazy-update", flag.ContinueOnError)
+	fs.SetOutput(io.Discard)
+	// This flag path is exercised end-to-end by TestGokrazyUpdatesItselfToSameImage.
+	fs.StringVar(&updateURL, gokrazyUpdateFromURLMagicArg[2:], "", "URL of the Gokrazy archive format file to install")
+	fs.BoolVar(&unsigned, "unsigned", false, "allow an unsigned GAF; for tests only")
+	if err := fs.Parse(args); err != nil {
+		return nil, err
+	}
+	if fs.NArg() != 0 {
+		return nil, nil
+	}
+	if updateURL == "" {
+		return nil, nil
+	}
+	if !unsigned {
+		return nil, errors.New("signed GAF verification is not implemented yet; see https://github.com/tailscale/tailscale/issues/20002; pass --unsigned for test updates")
+	}
+	if !clientupdate.GokrazyUpdateFromURL.IsSet() {
+		return nil, errors.New("gokrazy update support is not linked into this binary")
+	}
+	return &clientupdate.GokrazyUpdateArgs{
+		URL:           updateURL,
+		AllowUnsigned: unsigned,
+		Logf: func(format string, args ...any) {
+			printf(format+"\n", args...)
+		},
+	}, nil
+}

cmd/tailscale/depaware.txt

@@ -372,6 +372,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         vendor/golang.org/x/text/unicode/bidi                        from vendor/golang.org/x/net/idna+
         vendor/golang.org/x/text/unicode/norm                        from vendor/golang.org/x/net/idna
         archive/tar                                                  from tailscale.com/clientupdate
+   L    archive/zip                                                  from tailscale.com/clientupdate
         bufio                                                        from compress/flate+
         bytes                                                        from archive/tar+
         cmp                                                          from slices+

cmd/tailscaled/depaware.txt

@@ -572,6 +572,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         vendor/golang.org/x/text/unicode/bidi                        from vendor/golang.org/x/net/idna+
         vendor/golang.org/x/text/unicode/norm                        from vendor/golang.org/x/net/idna
         archive/tar                                                  from tailscale.com/clientupdate
+   L    archive/zip                                                  from tailscale.com/clientupdate
         bufio                                                        from compress/flate+
         bytes                                                        from archive/tar+
         cmp                                                          from slices+

cmd/tta/tta.go

@@ -213,6 +213,24 @@ func main() {
 		}
 		serveCmd(w, "tailscale", args...)
 	})
+	ttaMux.HandleFunc("/tailscale", func(w http.ResponseWriter, r *http.Request) {
+		serveCmd(w, "tailscale", r.URL.Query()["arg"]...)
+	})
+	ttaMux.HandleFunc("/gokrazy-root", func(w http.ResponseWriter, r *http.Request) {
+		cmdLine, err := os.ReadFile("/proc/cmdline")
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		for s := range strings.FieldsSeq(string(cmdLine)) {
+			if root, ok := strings.CutPrefix(s, "root="); ok {
+				w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+				io.WriteString(w, root+"\n")
+				return
+			}
+		}
+		http.Error(w, "no root= in /proc/cmdline", http.StatusInternalServerError)
+	})
 	ttaMux.HandleFunc("/ip", func(w http.ResponseWriter, r *http.Request) {
 		conn, ok := r.Context().Value(connContextKey).(net.Conn)
 		if !ok {