ipn/ipnlocal: add logging and locking to c2n /update (#9290)

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>

ipn/ipnlocal/local.go

@@ -240,6 +240,8 @@ type LocalBackend struct {
 	directFileRoot          string
 	directFileDoFinalRename bool // false on macOS, true on several NAS platforms
 	componentLogUntil       map[string]componentLogState
+	// c2nUpdateStatus is the status of c2n-triggered client update.
+	c2nUpdateStatus updateStatus
 
 	// ServeConfig fields. (also guarded by mu)
 	lastServeConfJSON   mem.RO              // last JSON that was parsed into serveConfig
@@ -268,6 +270,10 @@ type LocalBackend struct {
 	clock       tstime.Clock
 }
 
+type updateStatus struct {
+	started bool
+}
+
 // clientGen is a func that creates a control plane client.
 // It's the type used by LocalBackend.SetControlClientGetterForTesting.
 type clientGen func(controlclient.Options) (controlclient.Client, error)