ipn/ipnlocal: add logging and locking to c2n /update

Log some progress info to make updates more debuggable. Also, track
whether an active update is already started and return an error if
a concurrent update is attempted.

Some planned future PRs:
* add JSON output to `tailscale update`
* use JSON output from `tailscale update` to provide a more detailed
  status of in-progress update (stage, download progress, etc)

Updates #6907

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

clientupdate/clientupdate.go

@@ -128,8 +128,8 @@ func NewUpdater(args Arguments) (*Updater, error) {
 			return nil, err
 		}
 	}
-	if args.PkgsAddr == "" {
-		args.PkgsAddr = "https://pkgs.tailscale.com"
+	if up.Arguments.PkgsAddr == "" {
+		up.Arguments.PkgsAddr = "https://pkgs.tailscale.com"
 	}
 	return &up, nil
 }

ipn/ipnlocal/c2n.go

@@ -4,6 +4,7 @@
 package ipnlocal
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -14,6 +15,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strconv"
+	"strings"
 	"time"
 
 	"tailscale.com/clientupdate"
@@ -112,8 +114,7 @@ func (b *LocalBackend) handleC2N(w http.ResponseWriter, r *http.Request) {
 }
 
 func (b *LocalBackend) handleC2NUpdate(w http.ResponseWriter, r *http.Request) {
-	// TODO(bradfitz): add some sort of semaphore that prevents two concurrent
-	// updates, or if one happened in the past 5 minutes, or something.
+	b.logf("c2n: %s /update received", r.Method)
 
 	// GET returns the current status, and POST actually begins an update.
 	if r.Method != "GET" && r.Method != "POST" {
@@ -129,19 +130,31 @@ func (b *LocalBackend) handleC2NUpdate(w http.ResponseWriter, r *http.Request) {
 	// invoke it here. For this purpose, it is ok to pass it a zero Arguments.
 	prefs := b.Prefs().AutoUpdate()
 	_, err := clientupdate.NewUpdater(clientupdate.Arguments{})
+	b.c2nUpdateStatusMu.Lock()
+	defer b.c2nUpdateStatusMu.Unlock()
 	res := tailcfg.C2NUpdateResponse{
 		Enabled:   envknob.AllowsRemoteUpdate() || prefs.Apply,
 		Supported: err == nil && !version.IsMacSysExt(),
+		Started:   b.c2nUpdateStatus.started,
 	}
 
 	defer func() {
+		resb, err := json.Marshal(res)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		b.logf("c2n: %s /update response: %s", r.Method, resb)
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(res)
+		w.Write(resb)
 	}()
 
 	if r.Method == "GET" {
 		return
 	}
+	if b.c2nUpdateStatus.started {
+		res.Err = "update already in progress"
+	}
 	if !res.Enabled {
 		res.Err = "not enabled"
 		return
@@ -173,21 +186,28 @@ func (b *LocalBackend) handleC2NUpdate(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	cmd := exec.Command(cmdTS, "update", "--yes")
+	buf := new(bytes.Buffer)
+	cmd.Stdout = buf
+	cmd.Stderr = buf
+	b.logf("c2n: running %q", strings.Join(cmd.Args, " "))
 	if err := cmd.Start(); err != nil {
 		res.Err = fmt.Sprintf("failed to start cmd/tailscale update: %v", err)
 		return
 	}
 	res.Started = true
+	b.c2nUpdateStatus.started = true
 
-	// TODO(bradfitz,andrew): There might be a race condition here on Windows:
-	// * We start the update process.
-	// * tailscale.exe copies itself and kicks off the update process
-	// * msiexec stops this process during the update before the selfCopy exits(?)
-	// * This doesn't return because the process is dead.
-	//
-	// This seems fairly unlikely, but worth checking.
-	defer cmd.Wait()
-	return
+	// Run update asynchronously and respond that it started.
+	go func() {
+		if err := cmd.Wait(); err != nil {
+			b.logf("c2n: update command failed: %v, output: %s", err, buf)
+		} else {
+			b.logf("c2n: update complete")
+		}
+		b.c2nUpdateStatusMu.Lock()
+		b.c2nUpdateStatus.started = false
+		b.c2nUpdateStatusMu.Unlock()
+	}()
 }
 
 // findCmdTailscale looks for the cmd/tailscale that corresponds to the

ipn/ipnlocal/local.go

@@ -266,6 +266,15 @@ type LocalBackend struct {
 	// at the moment that tkaSyncLock is taken).
 	tkaSyncLock sync.Mutex
 	clock       tstime.Clock
+
+	// c2nUpdateStatus is the status of c2n-triggered client update, protected
+	// by c2nUpdateStatusMu.
+	c2nUpdateStatusMu sync.Mutex
+	c2nUpdateStatus   updateStatus
+}
+
+type updateStatus struct {
+	started bool
 }
 
 // clientGen is a func that creates a control plane client.

tailcfg/c2ntypes.go

@@ -39,7 +39,7 @@ type C2NSSHUsernamesResponse struct {
 // its Tailscale installation.
 type C2NUpdateResponse struct {
 	// Err is the error message, if any.
-	Err string
+	Err string `json:",omitempty"`
 
 	// Enabled indicates whether the user has opted in to updates triggered from
 	// control.
@@ -50,5 +50,5 @@ type C2NUpdateResponse struct {
 	Supported bool
 
 	// Started indicates whether the update has started.
-	Started bool
+	Started bool `json:",omitempty"`
 }